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A critical analysis of the thesis of the symmetry between explanation and prediction : including a case… Lee, Robert Wai-Chung 1979

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A CRITICAL ANALYSIS OF THE THESIS OF THE SYMMETRY BETV7EEN EXPLANATION AND PREDICTION: INCLUDING A CASE STUDY OF EVOLUTIONARY THEORY 9 by ROBERT WAI-CHUNG LEE B.A., The U n i v e r s i t y of Hong Kong, 1973 M..Phil., The Unive r s i t y of Hong Kong, 1976 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHILOSOPHY We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July 1979 © Robert Wai-Chung Lee, 1979 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department nf Philosophy  The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date J u l y 28, 1980. D E - 6 B P 75-51 1 E ABSTRACT One very s i g n i f i c a n t c h a r a c t e r i s t i c of Hempel's covering-law models of s c i e n t i f i c explanation, that i s , the deductive-nomological model and the i n d u c t i v e - s t a t i s t i c a l model, i s the supposed symmetry between explanation and p r e d i c t i o n . In b r i e f , the symmetry thesis asserts that explanation and p r e d i c t i o n have the same l o g i c a l structure; i n other words, i f an explanation of an. event had been taken account of i n time-, then i t could have served as a basis for p r e d i c t i n g the event i n question, and v i c e versa. The present thesis i s a c r i t i c a l analysis of the v a l i d i t y of t h i s purported symmetry between explanation and p r e d i c t i o n . The substance of the thesis begins with a defence against some common misconceptions of the symmetry t h e s i s , f o r example, the idea that the symmetry concerns statements but not arguments. S p e c i f i c a l l y , Grunbaum's i n t e r p r e t a t i o n of the symmetry thesis as pertaining to the l o g i c a l i n f e r a b i l i t y rather than the epistemological symmetry between explanation and p r e d i c t i o n i s examined. The f i r s t sub-thesis of the symmetry t h e s i s , namely that "Every adequate explanation i s a p o t e n t i a l p r e d i c t i o n , " i s then analyzed. Purported counterexamples such as evolutionary theory and the paresis case are c r i t i c a l l y i i i examined and c o n s e q u e n t l y d i s m i s s e d . S i n c e t h e r e a r e c o n f l i c t i n g v i e w s r e g a r d i n g t h e n a t u r e o f e x p l a n a t i o n and p r e d i c t i o n i n e v o l u t i o n a r y t h e o r y , a c a s e s t u d y o f t h e t h e o r y i s a l s o p r e s e n t e d . N e x t , t h e second s u b - t h e s i s o f t h e symmetry t h e s i s , namely t h a t "Every adequate p r e d i c t i o n i s a p o t e n t i a l e x p l a n a t i o n , " i s d i s c u s s e d . I n p a r t i c u l a r , t h e b a r o m e t e r c a s e i s d i s c h a r g e d as a c o u n t e r e x a m p l e t o t h e second sub-t h e s i s when t h e e x p l a n a t o r y power o f i n d i c a t o r l a w s i s p r o p e r l y u n d e r s t o o d . F i n a l l y , Salmon's c u r r e n t c a u s a l - r e l e v a n c e model o f e x p l a n a t i o n , w h i c h c l a i m s t o be an a l t e r n a t i v e t o Hempel's i n d u c t i v e - s t a t i s t i c a l model, i s c r i t i c a l l y a n a l y z e d . A m o d i f i e d i n d u c t i v e - s t a t i s t i c a l model o f e x p l a n a t i o n i s a l s o p r o p o s e d . T h i s m o d i f i e d model r e t a i n s t h e n o m o l o g i c a l i n g r e d i e n t o f Hempel's o r i g i n a l i n d u c t i v e - s t a t i s t i c a l m o d e l , b u t i t i s immune t o c r i t i c i s m s r a i s e d a g a i n s t t h e l a t t e r . I n c o n c l u s i o n , I m a i n t a i n t h a t t h e r e i s i n d e e d a symmetry between e x p l a n a t i o n and p r e d i c t i o n . B u t s i n c e d e d u c t i v e - n o m o l o g i c a l e x p l a n a t i o n and p r e d i c t i o n a r e e s s e n t i a l l y d i f f e r e n t from i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n and p r e d i c t i o n , t h e form t h e symmetry t a k e s between d e d u c t i v e - n o m o l o g i c a l e x p l a n a t i o n and p r e d i c t i o n d i f f e r s f r o m t h e form i t e x h i b i t s between i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n and p r e d i c t i o n . i v TABLE OF CONTENTS CHAPTER ONE. INTRODUCTION 1 CHAPTER TWO. HEMPEL'S TWO COVERING-LAW MODELS OF EXPLANATION AND THE SYMMETRY BETWEEN EXPLANATION AND PREDICTION . . . . . . 3 2.1 The Deductive-Nomological Model of E x p l a n a t i o n 3 2.2 The S t a t i s t i c a l Model of E x p l a n a t i o n 22 CHAPTER THREE. A DEFENCE AGAINST SOME COMMON MISCONCEPTIONS OF THE SYMMETRY THESIS 33 3.1 The Nature of E x p l a n a t i o n s and P r e d i c t i o n s 33 3.2 The Symmetry T h e s i s : L o g i c a l or E p i s t e m o l o g i c a l ? 48 3.3 E x p l a n a t i o n and R e t r o d i c t i o n . . 54 3.4 A C r i t i c a l E v a l u a t i o n of S c r i v e n ' s C r i t i c i s m s a g a i n s t Grunbaum's I n t e r p r e t a t i o n o f the Symmetry T h e s i s . 58 CHAPTER FOUR. FIRST SUB-THESIS: EVERY ADEQUATE EXPLANATION IS A POTENTIAL PREDICTION 67 4.1 E v o l u t i o n a r y Theory 67 4.2 The P a r e s i s , the B r i d g e , and the the Pneumonia Case 83 V CHAPTER FIVE. SECOND SUB-THESIS: EVERY ADEQUATE PREDICTION IS A POTENTIAL EXPLANATION 95 5.1 The Barometer Case 95 5.2 Hempel's Reply t o S c r i v e n ' s C r i t i c i s m 97 5.3 Grlinbaum's Treatment of the Barometer Case 99 5.4 An E l a b o r a t i o n o f Grlinbaum's Argument . . . . . 101 5.5 S c h e f f l e r ' s Attack on the Second Sub-thesis 106 5.6 Hempel's Reply t o S c h e f f l e r ' s C r i t i c i s m 107 CHAPTER SIX. A COMPARISON BETWEEN SALMON'S AND HEMPEL'S CONCEPTS OF EXPLANATION . . . 108 6.1 Salmon's Concept of E x p l a n a t i o n . . 10 8 6.2 A C r i t i q u e of the C-R Model o f E x p l a n a t i o n 120 6.3 A M o d i f i e d I-S Model of E x p l a n a t i o n 136 6.4 The M o d i f i e d Hempel 153 6.5 P o s t s c r i p t 156 6.6 Con c l u s i o n 159 CHAPTER SEVEN. IS EVOLUTIONARY THEORY CONSISTENT WITH HEMPEL'S COVERING-LAW MODELS? . 162 7.1 I n t r o d u c t i o n 162 7.2 Is E v o l u t i o n a r y Theory F a l s i f i a b l e ? 163 7.3 Is E v o l u t i o n a r y Theory a Met a p h y s i c a l Research Programme? 191 v i CHAPTER EIGHT. CONCLUSION 19 8 8.1 F i r s t S u b - T h e s i s — E v e r y Adequate E x p l a n a t i o n i s a P o t e n t i a l P r e d i c t i o n 19 8 8.2 Second Sub-Thesis--Every Adequate P r e d i c t i o n i s a P o t e n t i a l E x p l a n a t i o n 201 8.3 Summary 2 02 BIBLIOGRAPHY 203 v i i ACKNOWLEDGEMENT I would l i k e to express my deepest gratitude to my research supervisor, Professor John P. Stewart, f o r h i s constructive c r i t i c i s m s and constant encouragement throughout the wri t i n g of the present t h e s i s . I am also indebted to Professor Edwin Levy f o r h i s meticulous reading and c r i t i c a l comments of the d r a f t s of t h i s t h e s i s . F i n a l l y , my thanks extend to the Canadian Commonwealth Scholarship and Fellowship Committee i n granting me a Commonwealth Scholarship f o r my doctoral program. 1 CHAPTER ONE In t r o'duc t i on The aim of the present thesis i s to analyze c r i t i c a l l y Hempel's thesis of the symmetry between explanation and p r e d i c t i o n i n the context of both h i s deductive-nomological and i n d u c t i v e - s t a t i s t i c a l models of explanation. These two models of explanation, as Hempel has c l e a r l y stated, "are not meant to describe how working s c i e n t i s t s a c t u a l l y formulate t h e i r explanatory accounts. Their purpose i s rather to i n d i c a t e i n reasonably precise terms the l o g i c a l structure and the r a t i o n a l e of various ways i n which empirical science answers explanation-seeking why-questions.'V-*-' The substance of the thesis i s divided i n t o s i x chapters. Chapter Two contains an exegesis of the structure of the deductive-nomological and the i n d u c t i v e - s t a t i s t i c a l model of explanation, and an account of how the symmetry between explanation and p r e d i c t i o n comes int o the p i c t u r e . Chapter Three consists of a defence against some common misconceptions of the symmetry t h e s i s , and a study of Griinbaum's i n t e r p r e t a t i o n of the t h e s i s . Hempel (1965c), p. 412. Chapter Four i s devoted to a d i s c u s s i o n of the f i r s t sub-thesis of the symmetry t h e s i s , namely that "every adequate explanation i s a p o t e n t i a l p r e d i c t i o n . " Purported counterexamples such as evolutionary theory and the paresis case are analyzed and consequently dismissed. Chapter Five examines the v a l i d i t y of the second sub-thesis of the symmetry t h e s i s , namely that "every adequate pr e d i c t i o n i s a p o t e n t i a l explanation." S p e c i f i c a l l y , the barometer case w i l l be dismissed as a counterexample to the second sub-thesis. Chapter Six contains a c r i t i c a l study of Salmon's l a t e s t causal-relevance model of explanation which i s purported to be an a l t e r n a t i v e to Hempel's i n d u c t i v e -s t a t i s t i c a l model. A comparison of Salmon's model with my proposed modified i n d u c t i v e - s t a t i s t i c a l model i s also presented. Since there are c o n f l i c t i n g views regarding the nature of explanation and p r e d i c t i o n i n evolutionary theory. Chapter Seven w i l l be devoted to a case study of the theory. CHAPTER TWO HEMPEL'S TWO COVERING-LAW MODELS OF EXPLANATION AND THE SYMMETRY BETWEEN EXPLANATION AND PREDICTION 2.1 The Deductive-Nomological Model of Explanation 2.1.1 The Structure of the Deductive-Nomologicar Model 2.1.1.1 An I l l u s t r a t i o n A mercury thermometer i s r a p i d l y immersed i n hot water. There occurs a temporary drop of the mercury column, which i s then followed by a swift r i s e . How i s t h i s phenomenon to be explained? The increase i n temperature a f f e c t s at f i r s t mainly the glass tube of the thermometer; i t expands and thus provides a large space f o r the mercury . ins i d e whose l e v e l therefore drops. As soon as, by heat conduction, the r i s e i n temperature reaches the mercury, however, the l a t t e r expands. And since the c o e f f i c i e n t of expansion of mercury i s considerably larger than that of glass, a r i s e of the mercury l e v e l r e s u l t s . The foregoing account consists of two kinds of statements.. Those of the f i r s t kind i n d i c a t e c e r t a i n 4 c o n d i t i o n s , i n c l u d i n g p a r t i c u l a r f a c t s , o c c u r r e n c e s , o r e v e n t s , w h i c h a r e r e a l i z e d p r i o r t o , o r a t t h e same t i m e a s , t h e phenomenon t o be e x p l a i n e d . These w i l l be r e f e r r e d t o as a n t e c e d e n t o r i n i t i a l c o n d i t i o n s . I n t h e i l l u s t r a t i o n , t h e a n t e c e d e n t c o n d i t i o n s i n c l u d e , among o t h e r s , t h e f a c t t h a t t h e thermometer c o n s i s t s o f a g l a s s t u b e w h i c h i s p a r t l y f i l l e d w i t h m e r c u r y , some h o t w a t e r , and t h a t t h e thermometer i s immersed i n t o t h e h o t w a t e r . S t a t e m e n t s o f t h e second k i n d c o n s i s t o f u n i f o r m i t i e s e x p r e s s i b l e by means o f g e n e r a l o r u n i v e r s a l l a w s . By a g e n e r a l l a w , I mean a s t a t e m e n t o f u n i v e r s a l c o n d i t i o n a l f o rm w h i c h i s c a p a b l e o f b e i n g c o n f i r m e d o r d i s c o n f i r m e d by s u i t a b l e e m p i r i c a l f i n d i n g s . I n t h e p r e s e n t c a s e , t h e s e i n c l u d e t h e law s o f t h e t h e r m i c e x p a n s i o n o f mercury and o f g l a s s , and a s t a t e m e n t about t h e low t h e r m i c c o n d u c t i v i t y o f g l a s s . The above t w o ^ s e t s o f s t a t e m e n t s , i . e . t h e a n t e c e d e n t c o n d i t i o n s and t h e g e n e r a l l a w s , i f a d e q u a t e l y and c o m p l e t e l y f o r m u l a t e d , w i l l e x p l a i n t h e phenomenon under c o n s i d e r a t i o n : t h e y e n t a i l t h e consequence t h a t t h e mercury w i l l f i r s t d r o p , and t h e n r i s e . Thus, t h e e v e n t under d i s c u s s i o n i s e x p l a i n e d by subsuming i t Under g e n e r a l l a w s , i . e . by showing t h a t i t o c c u r r e d i n a c c o r d a n c e w i t h t h o s e l a w s , i n v i r t u e o f t h e r e a l i z a t i o n o f c e r t a i n s p e c i f i e d a n t e c e d e n t c o n d i t i o n s . 2 . 1 . 1 . 2 The B a s i c Schema From s c i e n t i f i c t h e p r e c e d i n g i l l u s t r a t i o n , a b a s i c schema o f e x p l a n a t i o n i n t h e form o f a d e d u c t i v e argument can be abstracted: c l ' c 2 ' • • ' ' ck (D-N) Explanans S E Explanandum-sentence Here, C\, C2t - - . * c k a r e sentences d e s c r i b i n g the antecedent conditions; L j , L 2/ . . . , L r are the general laws on which the explanation r e s t s . J o i n t l y these two subclasses of sentences w i l l be said to form the explanans S, where S may be thought of a l t e r n a t i v e l y as the c l a s s of the explanatory sentences or as t h e i r conjunction. The conclusion E of the argument i s a sentence d e s c r i b i n g the explanandum-phenomenon. E w i l l also be c a l l e d the explanandum-sentence or exp1anandum-statement. The word 'explanandum' alone w i l l be used to r e f e r e i t h e r to the explanandum-phenomenon or to the explanandum-sentence: the context w i l l show which i s meant. Hempel c a l l s t h i s kind of explanation whose l o g i c a l structure i s suggested by the D-N schema deductive-nomological explanation or D-N explanation for short, f o r i t e f f e c t s a deductive subsumption of the explanandum under p r i n c i p l e s that have the character of general laws. Thus, a D-N explanation answers the question "Why did the explanandum-phenomenon occur?" by showing that the phenomenon resu l t e d from c e r t a i n p a r t i c u l a r circumstances, s p e c i f i e d i n <Z\, C ^ F - - • , C ^ , i n accordance with laws L^, L 2 , . . . f L r . By p o i n t i n g t h i s out, the argument shows that, given the p a r t i c u l a r circumstances and the laws i n question, the occurrence of the phenomenon was to be expected; and i t i s i n t h i s sense that the explanation enables us to understand why the phenomenon occurred. Since the laws invoked i n a D-N explanation w i l l also be r e f e r r e d to as covering laws, the D-N model of explanation w i l l also be c a l l e d the covering-law model of explanation, which i s used as well to r e f e r to s t a t i s t i c a l explanations i n v o l v i n g covering laws, as w i l l be discussed l a t e r i n t h i s chapter. 2.1.1.3 Conditions of Adequacy If a proposed explanation i s to be sound, Hempel maintains, i t s constituents have to s a t i s f y c e r t a i n conditions of adequacy which may be divided i n t o l o g i c a l and empirical c o n d i t i o n s : 2 (I) L o g i c a l Conditions of Adequacy (Rl) The explanandum must be a l o g i c a l consequence of the explanans. In other words, the explanandum must be l o g i c a l l y deducible from the information contained i n the explanans; for otherwise, the explanans would not constitute adequate grounds for the explanandum. (R2) The explanans must contain general laws, and these must a c t u a l l y be required f o r the d e r i v a t i o n of the explanandum i n such a way that i f the laws were iHempel (1965c), p. 337. 2Hempel & Oppenheim (1965), pp. 247-249. deleted, the argument would no longer be v a l i d . I n t u i t i v e l y , t h i s means that r e l i a n c e on general laws i s e s s e n t i a l to t h i s type of explanation; a given phenomenon i s here explained, or accounted f o r , by showing that i t conforms to a general nomic pattern. However, i t i s not necessary for a sound explanation that the explanans must contain at l e a s t one statement which i s not a law. For, to mention j u s t one reason, one would surely want to consider as an explanation the de r i v a t i o n of the general r e g u l a r i t i e s governing the motion of double stars from the laws of c e l e s t i a l mechanics, even though a l l the statements i n the explanans are general laws. ) The explanans must have empirical content; that i s , i t must be capable, at least i n p r i n c i p l e , of t e s t by experiment or observation. This condition i s i m p l i c i t i n (RI); for since the explanandum i s assumed to describ some empirical phenomenon, i t follows from (RI) that the explanans e n t a i l s at l e a s t one consequence of empirical character, and t h i s f a c t confers upon i t t e s t a b i l i t y and empirical content. ) Empirical Condition of Adequacy ) The sentences c o n s t i t u t i n g the explanans must be true. That i n a sound explanation, the statements c o n s t i t u t i n g the explanans have to s a t i s f y some condition of f a c t u a l 8 correctness i s obvious. But i t might seem more appropriate to s t i p u l a t e that the explanans has to be highly confirmed by a l l the relevant evidence a v a i l a b l e rather than that i t should be true. This s t i p u l a t i o n , however, leads to awkard consequences. Suppose that the explanans of a given argument of the D-N form was well confirmed at a c e r t a i n e a r l i e r stage of s c i e n t i f i c research, but strongly disconfirmed by the more comprehensive evidence a v a i l a b l e at a l a t e r time, say the present. In such a case, we would have to say that o r i g i n a l l y the explanatory account was a cor r e c t explanation, but that i t ceased to be one l a t e r , when unfavourable evidence was discovered. This does not appear to accord with sound common usage which d i r e c t s us to say that on the basis of the l i m i t e d i n i t i a l evidence, the truth of the explanans, and thus the soundness of the explanation, had been quite probable;-but that ample evidence was avail a b l e makes i t h i g h l y probable that the explanans i s not true, and hence that the account i n question i s not--and never has b e e n — a correct explanation. Yet t h i s j u s t i f i c a t i o n by common usage f o r the requirement of truth i n a sound explanation, with i t s r e l i a n c e on a notion of correctness that does not appear in the proposed d e f i n i t i o n of explanation, i s surely of questionable merit, as Hempel himself recognizes i n a l a t e r a r t i c l e . ^ - For i n reference to explanations as w e l l as i n reference to statements, the vague idea of correctness can be construed i n two d i f f e r e n t ways, both of which are of i n t e r e s t and importance for the l o g i c a l analysis of science: namely, as truth i n the semantical sense which i s independent of any reference to time or to evidence, or as confirmation by the a v a i l a b l e relevant e v idence—a concept which i s c l e a r l y time dependent. We w i l l therefore d i s t i n g u i s h between true explanations which meet the requirement of t r u t h for t h e i r explanans, and explanations that are more or  les s w e l l confirmed by a given body of evidence (e.g. by the t o t a l evidence a v a i l a b l e ) . These two concepts can be introduced as follows: F i r s t , a p o t e n t i a l explanation (of D-N form) i s defined as an argument which meets a l l the l o g i c a l conditions or requirements of adequacy indicated e a r l i e r , except that the statements forming i t s explanans and explanandum need not be true. But the explanans must s t i l l contain a set of statements, L l / L 2 ' • • • * L r ' w n i c h are l a w l i k e , 2 i . e . which are l i k e laws except for possibly being f a l s e . Statements of t h i s kind w i l l also be c a l l e d nomological statements. I t i s t h i s notion of p o t e n t i a l explanation which i s involved, f o r example, when we ask whether a t e n t a t i v e l y produced but as iHempel (1962a), pp. 102-103. 2-rhe term 'lawlike statement' and the general ch a r a c t e r i z a t i o n given here of i t s intended meaning are from Goodman (1973). 10 yet untested law or theory would be able to explain c e r t a i n puzzling empirical findings; or when we say that the phlogiston theory, though now discarded, afforded an 'explanation' for c e r t a i n aspects of combustion. Next, we say that a given p o t e n t i a l explanation i s more or less highly confirmed by a given body of evidence according as i t s explanans i s more or l e s s highly confirmed by the evidence i n question. One fac t o r to be considered i n appraising the empirical soundness of a given explanation w i l l be the extent to which i t s explanans i s supported by the t o t a l relevant evidence a v a i l a b l e . F i n a l l y , by a true explanation, we understand a p o t e n t i a l explanation with true explanans, and hence also with true explanandum. 2 . 1 . 2 D-N Explanation and the Concept of Law In our ch a r a c t e r i z a t i o n of laws as lawlike statements, we s h a l l construe the cl a s s of lawlike statements as including a n a l y t i c general statements such as "A bachelor i s an unmarried male," as well as the lawlike statements i n empirical science which have empirical content. I t w i l l not be necessary to require that each lawlike statement permissible i n explanatory contexts be of the second kind. Rather, our d e f i n i t i o n of explanation w i l l be so construed as to guarantee the f a c t u a l character of the t o t a l i t y of the laws--though not of every s i n g l e one of them--which function 11 i n an explanation of an empirical f a c t . Lawlike statements can have many d i f f e r e n t l o g i c a l forms. Some paradigms of nomic statements, such as " A l l gases expand when heated under constant pressure," may be construed as having the simple universal c o n d i t i o n a l form ' (x) (Fx^Gx).' Others involve universal as well as e x i s t e n t i a l g e n e r a l i z a t i o n , as does the statement "For every metal, there e x i s t s some acid which w i l l d i s s o l v e i t " : ' (x) [(Ey) (Dyx)] . ' But lawlike statements cannot be characterized i n terms of t h e i r form alone. For instance, not a l l statements of the simple universal c o n d i t i o n a l form j u s t mentioned are lawlike; hence, even i f true, they are not laws. The statement " A l l apples i n t h i s basket are red" i l l u s t r a t e s t h i s point c l e a r l y , for we would refuse to explain, by subsumption under i t , the f a c t that a p a r t i c u l a r apple taken at random from the basket i s red. Goodman has pointed out a c h a r a c t e r i s t i c that distinguishes laws from such nonlaws: the former can, whereas the l a t t e r cannot, sustain counterfactual and subjunctive c o n d i t i o n a l statements.1 Thus, the law about the expansion of gases can serve to support statements such as "If the oxygen i n t h i s c y l i n d e r had been heated (were heated) under constant pressure, then i t would have iGoodman (1973), p a r t i c u l a r l y chaps. 3 & 4. expanded (would expand)"; whereas the statement about the basket of red apples, being an accidental g e n e r a l i z a t i o n , i s unable to sustain the counterfactual con d i t i o n a l statement " I f t h i s apple had been taken at random from t h i s basket, then i t would have been red." We might add that the two kinds of statements d i f f e r analogously i n explanatory power. The gas law, i n combination with s u i t a b l e p a r t i c u l a r data, such as that the c y l i n d e r was heated under constant pressure, can serve to explain why the volume of the gas increased. But the statement about the basket of red apples, analogously coupled with a statement such as "This apple was taken at random from t h i s basket," cannot explain why the apple i s red, though i t could well be a c t u a l l y red. Though the cha r a c t e r i z a t i o n of laws as lawlike sentences r a i s e s the important and i n t r i g u i n g problem of g i v i n g a c l e a r c h a r a c t e r i z a t i o n of lawlike sentences without, i n turn, using the concept of law, as Hempel has observed,^ - t h i s problem w i l l not be discussed i n the present t h e s i s . 2.1.3 Causal Explanation and the D-N Model One of the various modes of explanation to which the D-N model i s p a r t i c u l a r l y relevant i s the f a m i l i a r procedure of accounting for an event by pointing out i t s 'cause.' iHempel (1965c), pp. 339-343. Referring to our mercury thermometer i l l u s t r a t i o n , the temporary drop, followed by a swift r i s e , of the mercury column might be said to have been caused by the r a p i d immersion of the thermometer i n hot water. Causal a t t r i b u t i o n s of t h i s sort presuppose appropriate laws, such as that whenever metal (in our case, mercury) i s heated under constant pressure, i t expands. I t i s by reason, of t h i i m p l i c i t presupposition of laws that the D-N model i s relevant to the analysis of causal explanations. Let us examine t h i s point more c l o s e l y . Consider f i r s t the explanatory use of what may be c a l l e d general statements of causal connections, i . e . statements to the e f f e c t that an event of a given kind A causes an event of some s p e c i f i e d kind B. In the simplest case, a general statement asserting a causal connection between two kinds of events, A and B, i s tantamount to the • statement of the general law that whenever an event of kind A occurs, i t i s accompanied by an event of kind B. Most general statements of causal connection, however, c a l l f o r a more complex a n a l y s i s . Thus, the statement that i n a mammal, stoppage of the heart w i l l cause death presupposes c e r t a i n 'standard' conditions that are not e x p l i c i t l y stated, but that are surely meant to preclude, for example, the use of a heart-lung machine. "To say that X causes Y i s to say that under proper 14 conditions, an X w i l l be followed by a Y," 1 as Scriven puts i t . When t h i s kind of causal locution i s used, there u s u a l l y i s some understanding of what 'proper' or 'standard' background conditions remain indeterminate, a general statement of causal connection amounts at best to the vague claim that there are c e r t a i n further unspecified background conditions whose e x p l i c i t mention i n the given statement would y i e l d a t r u l y general law connecting the 'cause' and the ' e f f e c t ' under consideration. Next, consider statements of causal connections between i n d i v i d u a l events. For example, the statement that the death of a c e r t a i n person was caused by an overdose of phenobarbital surely presupposes a g e n e r a l i z a t i o n , namely a statement of a general causal connection between one kind of event, a person's taking an overdose of phenobarbital, and another, the death of that person. Here again, the range of a p p l i c a t i o n f o r the general causal statement i s not p r e c i s e l y stated, but a sharper s p e c i f i c a t i o n can be given by i n d i c a t i n g what c o n s t i t u t e s an overdose of phenobarbital f o r a p e r s o n — t h i s w i l l depend, among other things, on h i s weight and h i s habituation to the drug—and by adding the proviso that death w i l l r e s u l t from taking such an overdose i f the organism i s l e f t to i t s e l f , which implies, i n p a r t i c u l a r , that no countermeasures are taken. To explain the death i n question ??s having been lSc r i v e n (1958), p. 185. caused by the antecedent taking of phenobarbital i s therefore to claim that the explanandum event followed according to law upon c e r t a i n antecedent circumstances. And t h i s argument, when stated e x p l i c i t l y , conforms to the D-N model. Generally, the assertion of a causal connection between i n d i v i d u a l events seems to be u n i n t e l l i g i b l e unless i t i s taken to make, at l e a s t i m p l i c i t l y , a nomological claim to the e f f e c t that there are laws which provide the basis for the causal connection asserted. When an i n d i v i d u a l event, say b, i s said to have been caused by a c e r t a i n antecedent event or configuration of events, say a, then surely the claim i s intended that whenever 'the same cause' i s r e a l i z e d , 'the same e f f e c t ' w i l l recur.^ This claim cannot be taken to mean that whenever a recurs, then so does b, for a and b are i n d i v i d u a l events at p a r t i c u l a r s p a t i o - . temporal locations and thus occur only once. Rather, a and b should be viewed as p a r t i c u l a r events of c e r t a i n kinds (such as^ the expansion of some metal or the death of a person) of which there may be many fu r t h e r instances. And the law t a c i t l y implied by the assertion that b, as an event of kind B, was caused by a as an event of kind A, i s a general statement of causal connection t o the e f f e c t that, under s u i t a b l e circumstances, an event of kind A i s ^-Salmon, as we s h a l l see l a t e r i n chap. 6 , does not agree with t h i s . He thinks that 'causes' can be 'indeterministic' or ' s t a t i s t i c a l ' ; that i s , events of kind A may 'cause' events of kind B without the former being i n v a r i a b l y accompanied by the l a t t e r . 16 i n v a r i a b l y accompanied by an e v e n t o f k i n d B. I n most c a u s a l e x p l a n a t i o n s o f f e r e d i n o t h e r t h a n advanced s c i e n t i f i c c o n t e x t s , t h e r e q u i s i t e c i r c u m s t a n c e s a r e n o t f u l l y s t a t e d . F o r t h e s e c a s e s , t h e i m p o r t o f t h e c l a i m t h a t b, as an i n s t a n c e o f B, was c a u s e d by a may be s u g g e s t e d by t h e f o l l o w i n g a p p r o x i m a t e f o r m u l a t i o n : e v e n t b was i n f a c t p r e c e d e d by an e v e n t o f t h e k i n d A, and by c e r t a i n f u r t h e r c i r c u m s t a n c e s w h i c h , though n o t f u l l y s p e c i f i e d , were o f such a k i n d t h a t an o c c u r r e n c e o f an e v e n t o f k i n d A under s u c h c i r c u m s t a n c e s i s u n i v e r s a l l y f o l l o w e d by an e v e n t o f k i n d B. F o r example, t h e s t a t e m e n t t h a t t h e b u r n i n g (event o f k i n d B) o f a p a r t i c u l a r h a y s t a c k was caused by a l i g h t e d c i g a r e t t e dropped i n t o t h e hay ( p a r t i c u l a r e v e n t o f k i n d A) a s s e r t s , f i r s t o f a l l , t h a t t h e l a t t e r e v e n t d i d t a k e p l a c e , b u t a b u r n i n g c i g a r e t t e w i l l s e t a h a y s t a c k on f i r e o n l y i f c e r t a i n f u r t h e r c o n d i t i o n s a r e s a t i s f i e d . Thus, t h e c a u s a l a t t r i b u t i o n a t hand i m p l i e s s e c o n d l y t h a t f u r t h e r c o n d i t i o n s o f a n o t f u l l y s p e c i f i e d k i n d were r e a l i z e d , under w h i c h an e v e n t o f k i n d A i s i n v a r i a b l y f o l l o w e d by an e v e n t o f k i n d B. To t h e e x t e n t t h a t a s t a t e m e n t o f i n d i v i d u a l c a u s a t i o n l e a v e s t h e r e l e v a n t a n t e c e d e n t c o n d i t i o n s , and t h u s a l s o t h e r e q u i s i t e e x p l a n a t o r y l a w s , i n d e f i n i t e , i t i s l i k e a n o t e s a y i n g t h a t t h e r e i s a t r e a s u r e h i d d e n somewhere. ~ c s s i g n i f i c a n c e and u t i l i t y w i l l i n c r e a s e as t h e l o c a t i o n o f t h e t r e a s u r e i s narrowed down, as t h e r e l e v a n t c o n d i t i o n s and t h e c o r r e s p o n d i n g c o v e r i n g laws a r e made i n c r e a s i n g l y 17 e x p l i c i t . In some cases, such as that of the phenobarbital poisoning, t h i s can be done quite s a t i s f a c t o r i l y ; the D-N structure then emerges, and the statement of i n d i v i d u a l causal connection becomes amenable to t e s t . When, on the other hand, the relevant conditons or laws remain l a r g e l y i n d e f i n i t e , a statement of causal connection i s rather i n the nature of a program or of a sketch for an explanation i n terms of causal laws. I t might also be viewed as a 'working hypothesis' which may prove i t s worth by g i v i n g new and f r u i t f u l d i r e c t i o n to further research. The best examples of explanations conforming to the covering-law model are based on ph y s i c a l theories of deter m i n i s t i c character. B r i e f l y , a d e t e r m i n i s t i c theory deals with the changes of 'state' i n p h y s i c a l systems of some s p e c i f i e d kind. The state of such a system at any given time i s characterized by the values assumed at that time by c e r t a i n quantitative c h a r a c t e r i s t i c s of the system, the s o - c a l l e d variables of sta t e . The laws s p e c i f i e d by such a theory f o r the change of state are d e t e r m i n i s t i c i n the sense that, given the state of the system at any one time, i t s state at any other, e a r l i e r or l a t e r , time can be calc u l a t e d . For example, c l a s s i c a l mechanics o f f e r s a det e r m i n i s t i c theory for system of point masses which move under the influence of t h e i r mutual g r a v i t a t i o n a l a t t r a c t i o n alone. The state of such a system at a given time i s defined as determined by the positions and momenta of i t s component bodies at that time and does not include other 18 a s p e c t s t h a t m i g h t undergo change, such as t h e c o l o u r o r t h e c h e m i c a l c o n s t i t u t i o n o f t h e moving b o d i e s . The t h e o r y p r o v i d e s a s e t o f l a w s — e s s e n t i a l l y , t h e Newto n i a n l a w s o f g r a v i t a t i o n and o f m o t i o n — w h i c h , g i v e n t h e p o s i t i o n s and momenta o f t h e elements o f such a system a t any one t i m e , m a t h e m a t i c a l l y d e t e r m i n e t h e i r p o s i t i o n s and momenta a t any o t h e r t i m e . I n p a r t i c u l a r , t h o s e laws make i t p o s s i b l e t o o f f e r a D-N e x p l a n a t i o n o f t h e system's b e i n g i n c e r t a i n s t a t e a t a g i v e n t i m e i f t h e s t a t e o f t h e s y s t e m a t some e a r l i e r t i m e c o u l d be known. I n t h e e x p l a n a t o r y o r p r e d i c t i v e use o f a d e t e r m i n i s t i c t h e o r y , t h e n , t h e n o t i o n o f a c a u s e as a more o r l e s s n a r r o w l y c i r c u m s c r i b e d a n t e c e d e n t e v e n t has been r e p l a c e d by t h a t o f some a n t e c e d e n t s t a t e o f t h e t o t a l . sys t e m w h i c h p r o v i d e s t h e ' i n i t i a l c o n d i t i o n s ' f o r t h e c o m p u t a t i o n , by means o f t h e t h e o r y , o f t h e l a t e r s t a t e t h a t i s t o be e x p l a i n e d . I f t h e system i s n o t i s o l a t e d , i . e . i f r e l e v a n t o u t s i d e i n f l u e n c e s a c t upon t h e s y s t e m d u r i n g t h e p e r i o d o f t i m e from t h e i n i t i a l s t a t e i n v o k e d t o t h e s t a t e t o be e x p l a i n e d , t h e n t h e p a r t i c u l a r c i r c u m s t a n c e s t h a t must be s t a t e d i n t h e e x p l a n a n s i n c l u d e a l s o t h o s e o u t s i d e i n f l u e n c e s . And i t i s t h e s e 'boundary c o n d i t i o n s , ' i n c o n j u n c t i o n w i t h t h e ' i n i t i a l c o n d i t i o n s , ' w h i c h r e p l a c e t h e e v e r y d a y n o t i o n o f c a u s e , and w h i c h a r e s p e c i f i e d by t h e s t a t e m e n t s Cj_, C 2 , . . • , Cj, i n t h e s c h e m a t i c r e p r e s e n t a t i o n (D-N) o f d e d u c t i v e - n o m o l o g i c a l e x p l a n a t i o n . 19 However, causal explanation i n i t s various degrees of e x p l i c i t n e s s and p r e c i s i o n i s not, Hempel maintains, the only mode of explanation on which the D-N model has a bearing. For example, the explanation of a general law by deductive subsumption under t h e o r e t i c a l p r i n c i p l e s i s c l e a r l y not an explanation by causes.-'- But even when used to account for i n d i v i d u a l events, D-N explanations are not always causal. This i s best exemplified by the s o - c a l l e d laws of coexistence such as the pendulum law, Boyle's law, Ohm's law, etc. i n which, unlike laws of succession which concern temporal changes i n a system, a p a r t i c u l a r f a c t i s explained, not by causal antecedents, but by reference to another contemporaneous f a c t . 2 There i s yet another kind of explanation used to account for i n d i v i d u a l events which i s deductive-nomological but not causal i n character. I t i s a D-N explanation which invokes, not causal laws, but laws  of simple empirical r e g u l a r i t i e s or g e n e r a l i z a t i o n s . Unfortunately, Hempel has not touched on these explanations at a l l . I t s importance, p a r t i c u l a r l y i n i t s explanatory •'-The discussion of explanations of a general law by deductive subsumption under t h e o r e t i c a l p r i n c i p l e s i s d e l i b e r a t e l y l e f t untouched i n the present t h e s i s since, as both Scriven (1962), p. 179 and Hempel (1965c), p. 368 have observed, they do not pertain to the symmetry t h e s i s . For they cannot be spoken of as p o t e n t i a l p r e d i c t i o n s as law-statements purport to timeless u n i f o r m i t i e s and thus make no reference to any p a r t i c u l a r time, whether past, present, or future. 2 F o r a d e t a i l e d exposition of t h i s idea, see Hempel (1962a), pp. 108-110 and Hempel (1965c), pp. 352-354. 20 r o l e , w i l l be d i s c u s s e d i n C h a p t e r F i v e , S e c t i o n 5.4 o f t h e p r e s e n t t h e s i s . 2.1.4 The Symmetry o r S t r u c t u r a l I d e n t i t y between  E x p l a n a t i o n and P r e d i c t i o n I n a d e d u c t i v e - n o m o l o g i c a l e x p l a n a t i o n o f a p a r t i c u l a r p a s t e v e n t , t h e e x p l a n a n s l o g i c a l l y i m p l i e s t h e o c c u r r e n c e o f t h e explanandum e v e n t ; hence Hempel t h i n k s , we may say t h a t t h e e x p l a n a t o r y argument c o u l d a l s o have s e r v e d as a p r e d i c t i v e one i n t h e sense t h a t i t c o u l d have been used t o p r e d i c t t h e explanandum e v e n t i f t h e laws and a n t e c e d e n t c o n d i t i o n s adduced i n i t s e x p l a n a n s had been t a k e n i n t o a c c o u n t a t a s u i t a b l e e a r l i e r time.-*- P r e d i c t i v e arguments o f t h e D-N form w i l l be c a l l e d d e d u c t i v e - n o m o l o g ! c a 1  p r e d i c t i o n s . F o r c l a r i t y , we s h a l l use t h e terms ' p r e d i c t a n s ' and ' p r e d i c t a n d u m ' i n D-N p r e d i c t i o n as t h e c o u n t e r p a r t s o f 'expl a n a n s * and 'explanandum' i n D-N e x p l a n a t i o n . 2 S c i e n t i f i c e x p l a n a t i o n (of t h e D-N form) d i f f e r s from s c i e n t i f i c p r e d i c t i o n , n o t i n l o g i c a l s t r u c t u r e , b u t i n c e r t a i n p r a g m a t i c r e s p e c t s . I n one c a s e , t h e e v e n t d e s c r i b e d i n t h e c o n c l u s i o n i s known t o have o c c u r r e d , and s u i t a b l e s t a t e m e n t s o f g e n e r a l law (or laws) and a n t e c e d e n t c o n d i t i o n s 1-This remark does n o t h o l d , however, when a l l t h e laws i n v o k e d i n t h e e x p l a n a n s a r e laws o f c o e x i s t e n c e and a l l t h e a n t e c e d e n t c o n d i t i o n s adduced i n t h e e x p l a n a n s p e r t a i n t o e v e n t s t h a t a r e s i m u l t a n e o u s w i t h t h e explanandum e v e n t . ^Unknown t o me when I f i r s t i n t r o d u c e d t h e terms ' p r e d i c t a n s ' and 'predictandum' t h a t F e t z e r had a l r e a d y used t h e same t e r m i n o l o g y ; see F e t z e r ( 1 9 7 3 ) , p. 177. 21 are sought to account f o r i t . In the other case, the l a t t e r statements are given, and the statement about the event i n question i s derived from them before the time of i t s presumptive occurrence. " I t may be s a i d , therefore, that an explanation of a p a r t i c u l a r event i s not f u l l y adequate unless i t s explanans, i f taken account of i n time, could have served as a basis for p r e d i c t i n g the event i n question."-'- This conception i s often r e f e r r e d to as the th e s i s of the symmetry or s t r u c t u r a l i d e n t i t y between  explanation and p r e d i c t i o n . In essence, t h i s symmetry t h e s i s amounts to the conjunction of two sub-theses? namely (i) that every adequate explanation i s p o t e n t i a l l y a p r e d i c t i o n , and ( i i ) that conversely every adequate p r e d i c t i o n i s  p o t e n t i a l l y an explanation. Since, as Hempel maintains, s t a t i s t i c a l explanations (which w i l l be discussed i n the next section) have a s i m i l a r l o g i c a l structure as D-N explanations, except f o r the f a c t ft that they invoke covering laws which are s t a t i s t i c a l rather than u n i v e r s a l i n character, the symmetry t h e s i s i s thus applicable to s t a t i s t i c a l explanations and p r e d i c t i o n s as w e l l . Hempel & Oppenheim (1965) , p. 249. 22 2.2 The S t a t i s t i c a l Model o f E x p l a n a t i o n 2.2.1 Laws o f S t a t i s t i c a l - P r o b a b i l i s t i c Form o r  S t a t i s t i c a l Laws Our d i s c u s s i o n w i l l be m a i n l y c o n c e r n e d w i t h t h e e x p l a n a t o r y use o f s t a t i s t i c a l laws o f a v e r y s i m p l e k i n d , namely laws o f b a s i c s t a t i s t i c a l form. These a r e s t a t e m e n t s t o t h e e f f e c t t h a t t h e s t a t i s t i c a l p r o b a b i l i t y ( i . e . r o u g h l y , t h e l o n g - r u n r e l a t i v e f r e q u e n c y ) f o r an e v e n t o f k i n d F t o be a l s o o f k i n d G i s ' r , 1 o r i n s y m b o l s , p(G,F)=r. B r o a d l y s p e a k i n g , t h i s s t a t e m e n t a s s e r t s t h a t i n . t h e l o n g r u n , t h e p r o p o r t i o n o f t h o s e i n s t a n c e s o f F w h i c h a r e a l s o i n s t a n c e s o f G i s a p p r o x i m a t e l y ' r . ' J u s t l i k e t h e i r u n i v e r s a l - n o m i c c o u n t e r p a r t s , e.g. t h e gas l a w , w h i c h make g e n e r a l c l a i m s c o n c e r n i n g a c l a s s o f p o t e n t i a l l y i n f i n i t e c a s e s , t h e s t a t i s t i c a l - p r o b a b i l i s t i c laws o f say g e n e t i c s o r r a d i o a c t i v e decay a r e n o t tantamount t o d e s c r i p t i v e r e p o r t s o f t h e f r e q u e n c i e s w i t h w h i c h some k i n d o f phenomenon has been f o u n d t o o c c u r i n a f i n i t e c l a s s o f o b s e r v e d c a s e s . They a s s e r t c e r t a i n p e c u l i a r , namely p r o b a b i l i s t i c , modes o f c o n n e c t i o n between p o t e n t i a l l y i n f i n i t e c l a s s e s o f o c c u r r e n c e s . I n a s t a t i s t i c a l law o f t h e b a s i c f o r m , as d i s t i n g u i s h e d f rom a s t a t i s t i c a l d e s c r i p t i o n s p e c i f y i n g r e l a t i v e f r e q u e n c i e s i n some f i n i t e s e t , t h e ' r e f e r e n c e c l a s s ' F i s n o t assumed t o be f i n i t e . I n d e e d , we 23 m i g h t say t h a t a law o f t h e f orm 'p(G,F)=r' r e f e r s n o t o n l y t o a l l a c t u a l i n s t a n c e s o f F, b u t so t o s p e a k , t o t h e c l a s s o f a l l i t s p o t e n t i a l i n s t a n c e s . L e t us now u n d e r s t a n d a s t a t i s t i c a l e x p l a n a t i o n as any e x p l a n a t i o n t h a t makes e s s e n t i a l use o f a t l e a s t one law o f s t a t i s t i c a l form. There a r e two l o g i c a l l y d i f f e r e n t t y p e s o f s t a t i s t i c a l e x p l a n a t i o n . One o f them, namely d e d u c t i v e -s t a t i s t i c a l e x p l a n a t i o n , amounts b a s i c a l l y t o t h e d e d u c t i v e s u bsumption o f a n a r r o w e r s t a t i s t i c a l u n i f o r m i t y u n d e r more comprehensive ones. The o t h e r , namely i n d u c t i v e - s t a t i s t i c a l  e x p l a n a t i o n , i n v o l v e s t h e s u b s u m p t i o n , i n a p a r t i c u l a r n o n d e d u c t i v e s e n s e , o f a p a r t i c u l a r o c c u r r e n c e under s t a t i s t i c a l l a w s . W i t h i n t h e scope o f t h e p r e s e n t t h e s i s , s t a t i s t i c a l - p r o b a b i l i s t i c e x p l a n a t i o n w i l l a l w a y s r e f e r t o t h i s l a t t e r t y p e o f i n d u c t i v e - s t a t i s t i c a l e x p l a n t i o n , p a r t i c u l a r l y i n t h e c o n t e x t o f e v o l u t i o n a r y t h e o r y . 2.2.2 D e d u c t i v e - S t a t i s t i c a l E x p l a n a t i o n I t i s an i n s t a n c e o f t h e s o - c a l l e d g a m b l e r ' s f a l l a c y t o assume t h a t when s e v e r a l s u c c e s s i v e t o s s i n g s o f a f a i r c o i n have y i e l d e d heads, t h e n e x t t o s s w i l l more p r o b a b l y y i e l d t a i l s t h a n heads. Why t h i s i s t h e c a s e can be e x p l a i n e d by means o f two h y p o t h e s e s t h a t have t h e f o r m o f s t a t i s t i c a l l a w s . The f i r s t i s t h a t t h e random e x p e r i m e n t o f f l i p p i n g a f a i r c o i n y i e l d s heads w i t h a s t a t i s t i c a l p r o b a b i l i t y o f 1/2. The second h y p o t h e s i s i s t h a t t h e 24 outcomes of d i f f e r e n t tossings of the coin are s t a t i s t i c a l l y independent, so that the p r o b a b i l i t y of any s p e c i f i e d sequence of outcomes—such as heads twice, then t a i l s , then heads, then t a i l s three times—equals the product of the p r o b a b i l i t i e s of the constituent sing l e outcomes. These two hypotheses, i n terms of s t a t i s t i c a l p r o b a b i l i t i e s , imply deductively that the p r o b a b i l i t y f o r heads to come up a f t e r a long sequence of heads i s s t i l l 1/2. Explanations of t h i s kind are c a l l e d deductive-s t a t i s t i c a l explanations, or D-S explanations f o r short. They involve the deduction of a statement i n the form of a s t a t i s t i c a l law from an explanans that contains indispensably at l e a s t one law of s t a t i s t i c a l form. The deduction i s effected by means of the mathematical theory of s t a t i s t i c a l p r o b a b i l i t y , which makes i t possible to c a l c u l a t e c e r t a i n d e r i v a t i v e p r o b a b i l i t i e s (those r e f e r r e d to i n the explanandum) on the basis of other p r o b a b i l i t i e s ( s p e c i f i e d i n the explanans) which have been e m p i r i c a l l y ascertained or hypot h e t i c a l l y assumed. What a D-S explanation accounts f o r i s thus always a general uniformity expressed by a presumptive law of s t a t i s t i c a l form. 2.2.3 I n d u c t i v e - S t a t i s t i c a l Explanation As an explanation of why patient John Jones recovered from a streptococcus i n f e c t i o n , we might be t o l d that Jones had been given p e n i c i l l i n . But i f we t r y to amplify t h i s 25 explanatory claim by i n d i c a t i n g a general connection between p e n i c i l l i n treatment and the subsiding of a streptococcus i n f e c t i o n , we cannot j u s t i f i a b l y invoke a general law to the e f f e c t that i n a l l cases of such i n f e c t i o n , administration of p e n i c i l l i n w i l l lead to recovery. What can be asserted, and what surely i s taken f o r granted here, i s only that p e n i c i l l i n w i l l e f f e c t a cure i n a high percentage of cases, or with a high s t a t i s t i c a l p r o b a b i l i t y . This statement has the general character of a law of s t a t i s t i c a l form, and while the p r o b a b i l i t y value i s not s p e c i f i e d , the statement indicates that i t i s high. But i n contrast to cases of D-N and D-S explanation, the explanans c o n s i s t i n g of t h i s s t a t i s t i c a l law, together with the statement that the patient did receive p e n i c i l l i n , obviously does not imply the explanandum statement "the patient recovered" with deductive c e r t a i n t y , but only with high l i k e l i h o o d or near-certainty. B r i e f l y , then, the explanation amounts to t h i s argument: (a) The p a r t i c u l a r case of i l l n e s s of John J o n e s — l e t us c a l l i t 'j'—was an instance of severe streptococcus i n f e c t i o n (Sj) which was treated with large doses of p e n i c i l l i n (Pj), and the s t a t i s t i c a l p r o b a b i l i t y p(R,S.P) of recovery i n cases where S and P are present i s close to 1. Hence, the case was p r a c t i c a l l y c e r t a i n to end i n recovery (Rj). This argument might i n v i t e the following schematization: 26 (b) p(R,S.P) i s c l o s e t o 1 S j . P j ( T h e r e f o r e : ) I t i s p r a c t i c a l l y c e r t a i n ( v e r y l i k e l y ) t h a t R j I n t h e l i t e r a t u r e on i n d u c t i v e i n f e r e n c e , arguments based on s t a t i s t i c a l h y p o t h e s e s have o f t e n been, c o n s t r u e d as h a v i n g t h e above form o r a s i m i l a r one. On t h i s c o n s t r u a l , t h e c o n c l u s i o n c h a r a c t e r i s t i c a l l y c o n t a i n s a modal q u a n t i f i e r such as 'almost c e r t a i n l y , ' ' w i t h h i g h p r o b a b i l i t y , ' ' very l i k e l y , ' e t c . B u t , Hempel i n s i s t s , t h e c o n c e p t i o n o f arguments h a v i n g t h i s c h a r a c t e r i s u n t e n a b l e . F o r s e n t e n c e s o f t h e form " I t i s p r a c t i c a l l y c e r t a i n t h a t p" o r " I t i s v e r y l i k e l y t h a t p,' where t h e p l a c e o f 'p' i s o c c u p i e d by some s t a t e m e n t , a r e n o t com p l e t e s e l f - c o n t a i n e d s e n t e n c e s t h a t can be q u a l i f i e d as e i t h e r t r u e o r f a l s e . The s t a t e m e n t t h a t t a k e s t h e p l a c e o f 'p,' f o r example, ' R j , ' i s e i t h e r t r u e o r f a l s e , q u i t e i n d e p e n d e n t l y o f w h a t e v e r r e l e v a n t e v i d e n c e may be a v a i l a b l e ; b u t i t can be q u a l i f i e d as more or l e s s l i k e l y , p r o b a b l e , c e r t a i n , o r t h e l i k e o n l y r e l a t i v e  t o some body o f e v i d e n c e . One and t h e same s t a t e m e n t , s u c h as ' R j , ' w i l l be c e r t a i n , v e r y l i k e l y , n o t v e r y l i k e l y , h i g h l y u n l i k e l y , and so f o r t h , d e p e n d i n g upon what e v i d e n c e i s c o n s i d e r e d . The s e n t e n c e " I t i s a l m o s t c e r t a i n t h a t R j , " t a k e n by i t s e l f , i s t h e r e f o r e n e i t h e r t r u e n o r f a l s e , and i t can n o t be i n f e r r e d from t h e p r e m i s e s s p e c i f i e d i n (b) nor from any o t h e r s t a t e m e n t s . Thus, to present our s t a t i s t i c a l explanation i n the manner of schema (b) i s to misconstrue the function of the words 'almost c e r t a i n ' or 'very l i k e l y ' as they occur i n the formal wording of the explanation. These words c l e a r l y must be taken to i n d i c a t e that on the evidence provided by the explanans, or r e l a t i v e to that evidence, the explanandum i s p r a c t i c a l l y c e r t a i n or very l i k e l y ; that i s , that (c) 'Rj' i s p r a c t i c a l l y c e r t a i n (very l i k e l y ) r e l a t i v e to the explanans containing the sentences 'p(R,S.P) i s clos to 1' and 'Sj.Pj.* In schematized form, (c) would look l i k e t h i s : (d) p(R,S.P) i s close to 1 Sj.Pj • [ m a i c p s p r a c t i c a l l y c e r t a i n (Very Rj l i k e l y ) ] In t h i s schema, the double l i n e separating the 'premises' from the 'conclusion' i s to s i g n i f y that the r e l a t i o n of the former to the l a t t e r i s not that of deductive implication but that of inductive support, the strength of which i s indicated i n square brackets. Explanations of p a r t i c u l a r facts or events by means of s t a t i s t i c a l - p r o b a b i l i s t i c laws thus present themselves as arguments that are inductive or p r o b a b i l i s t i c i n the sense that the explanans confers upon the explanandum a high degree of inductive support; they are therefore c a l l e d 28 i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n s o r I-S e x p 1 a n a t i o n s f o r s h o r t . 2.2.4 The Requirement o f T o t a l E v i d e n c e and t h e  Requirement o f Maximal S p e c i f i c i t y B u t suppose now t h a t by way o f an e x p l a n a t i o n f o r t h e o c c u r r e n c e o f R i n a p a r t i c u l a r c a s e ' j , r we a r e o f f e r e d t h e e x p l a n a n s c o n s i s t i n g o f t h e s t a t e m e n t s *p(R,S.P) i s c l o s e t o 1' and ' S j . P j . 1 Then even i f t h i s i n f o r m a t i o n i s t r u e , i t i s s t i l l p o s s i b l e t h a t ' j ' s h o u l d a l s o happen t o s a t i s f y some f u r t h e r c o n d i t i o n s , say ' S * j . P j , e under w h i c h , a c c o r d i n g t o a n o t h e r s t a t i s t i c a l law 'p(R,S*.P) i s c l o s e t o 0,' t h e o c c u r r e n c e o f R has an e x t r e m e l y s m a l l p r o b a b i l i t y . P u t d i f f e r e n t l y , i t may w e l l be t h e c a s e t h a t *p(R,S.P) i s c l o s e t o 1,' b u t i t may a l s o be t h e c a s e t h a t 'p(R,S*.P) i s c l o s e t o 0,' o r *p(R,S*.P) i s c l o s e t o 1,' w h i c h s a y s t h a t t h e s t r e p t o c o c c u s i n f e c t i o n i n t h e case a t hand i s o f a s t r a i n t h a t i s h i g h l y r e s i s t a n t t o p e n i c i l l i n , and t h e s t a t i s t i c a l p r o b a b i l i t y f o r an i n f e c t i o n caused by t h i s s t r a i n t o r e s p o n d f a v o u r a b l y t o p e n i c i l l i n t r e a t m e n t i s e x t r e m e l y s m a l l . I n s c h e m a t i z e d f o r m , t h e argument l o o k s l i k e t h i s : (e) p(R,S*.P) i s c l o s e t o 1 S * j . P j [makes p r a c t i c a l l y c e r t a i n ] Rj I f b o t h s t a t i s t i c a l laws a r e known, t h e n a q u i c k r e c o v e r y i n t h e case a t hand s u r e l y c annot be r e a s o n a b l y e x p l a i n e d by 29 simply adducing the explanans i n (d), even though t h i s does i n f e r a high p r o b a b i l i t y upon the explanandum. This i s known as the problem of the epistemic ambiguity of s t a t i s t i c a l  explanation. Generally speaking, therefore, i f a p r o b a b i l i s t i c explanation i s to be r a t i o n a l l y acceptable, no f u r t h e r information must be av a i l a b l e whose i n c l u s i o n i n the explanans would change the p r o b a b i l i t y of the explanandum. In other words, a r a t i o n a l l y acceptable p r o b a b i l i s t i c explanation must s a t i s f y what Carnap has c a l l e d the requirement of t o t a l evidence 7'^ - that i s , i t s explanans must include a l l the a v a i l a b l e information that i s i n d u c t i v e l y relevant to the explanandum. In a D-N explanation whose explanans l o g i c a l l y implies the explanandum and thus confers upon i t the l o g i c a l p r o b a b i l i t y 1, and whose deductive conclusiveness remains unchanged i f to the explanans any further a v a i l a b l e information i s added, the requirement of 2 t o t a l evidence i s always t r i v i a l l y s a t i s f i e d . The requirement of t o t a l evidence i s not a postulate nor a theorem of inductive l o g i c ; i t i s not concerned with the formal v a l i d i t y of inductive arguments. Rather, as Carnap has stressed, i t i s a maxim for the a p p l i c a t i o n of inductive l o g i c . We might say that i t states a necessary Carnap (1962), p. 211. This point i s discussed more f u l l y and p r e c i s e l y i n Hempel (1962a), sees. 10 & 11 and Hempel (1965c), sec. 3.4. 30 condition of r a t i o n a l i t y of any such a p p l i c a t i o n i n a given 'knowledge s i t u a t i o n ' which we w i l l think of as represented by the set K of a l l statements accepted i n the s i t u a t i o n . But i n what manner should the basic idea of t h i s requirement be sought to bear upon p r o b a b i l i s t i c explanations? Surely we should not i n s i s t that the explanans must contain a l l and only the empirical information a v a i l a b l e at the time. Not a l l the a v a i l a b l e information, because otherwise a l l p r o b a b i l i s t i c explanations acceptable at time t would have to have the same explanans Kt; and not only the av a i l a b l e information, because a proffered explanation may meet the intent of the requirement i n not overlooking any relevant information a v a i l a b l e , and may nevertheless invoke some explanans statements which have not as yet been s u f f i c i e n t l y tested to be included i n Kt. The extent to which the requirement of t o t a l evidence should be imposed upon s t a t i s t i c a l explanations i s suggested by the following consideration. A prof f e r e d explanation of Jones' recovery based on the information that Jones had a streptococcus i n f e c t i o n and was treated with p e n i c i l l i n , and that the s t a t i s t i c a l p r o b a b i l i t y f o r recovery i n such cases i s very high i s unacceptable i f K includes the further information that Jones' streptococci were r e s i s t a n t to p e n i c i l l i n and that i n these reference classes the p r o b a b i l i t y of recovery i s small. Indeed, one would want an acceptable explanation to be based on a s t a t i s t i c a l 31 p r o b a b i l i t y s t a t e m e n t p e r t a i n i n g t o t h e n a r r o w e s t r e f e r e n c e class-'- o f w h i c h , a c c o r d i n g t o our t o t a l i n f o r m a t i o n , t h e p a r t i c u l a r o c c u r r e n c e under c o n s i d e r a t i o n i s a member. Thus, i f K t e l l s us n o t o n l y t h a t Jones had a s t r e p t o c o c c u s i n f e c t i o n and was t r e a t e d w i t h p e n i c i l l i n , b u t a l s o t h a t h i s s t r e p t o c o c c i were r e s i s t a n t t o p e n i c i l l i n (and i f K p r o v i d e s no i n f o r m a t i o n more s p e c i f i c t h a n t h a t ) , t h e n we w o u l d r e q u i r e t h a t an a c c e p t a b l e e x p l a n a t i o n o f J o n e s ' r e s p o n s e t o t h e t r e a t m e n t b ased on a s t a t i s t i c a l law s t a t i n g t h e p r o b a b i l i t y o f t h a t r e s p o n s e i n t h e n a r r o w e s t r e f e r e n c e c l a s s t o w h i c h o u r t o t a l i n f o r m a t i o n a s s i g n s J o n e s ' i l l n e s s , i . e . t h e c l a s s o f s t r e p t o c o c c u s i n f e c t i o n s r e s i s t a n t t o p e n i c i l l i n . The r e q u i r e m e n t o f maximal s p e c i f i c i t y f o r i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n s , as s u g g e s t e d by t h e p r e c e d i n g c o n s i d e r a t i o n , can now be s t a t e d more e x p l i c i t l y . C o n s i d e r a p r o p o s e d e x p l a n a t i o n o f t h e b a s i c s t a t i s t i c a l f o r m ( f ) p(G,F)=r Fb Gb L e t s_ be t h e c o n j u n c t i o n o f t h e p r e m i s e s , and i f K i s t h e s e t o f a l l s t a t e m e n t s a c c e p t e d a t t h e g i v e n t i m e , l e t k be a ^ F o r Hempel, a r e f e r e n c e c l a s s i s r e g a r d e d as ' n a r r o w e s t ' i f f u r t h e r s u b d i v i s i o n s o f t h e c l a s s would n o t a f f e c t t h e s t a t i s t i c a l p r o b a b i l i t y o f a s t a t e m e n t p e r t a i n i n g t o i t . See Hempel ( 1 9 6 5 c ) , p. 399. 32 sentence that i s l o g i c a l l y equivalent to K (in the sense that k i s implied by K and i n turn implies every sentence i n K). Then, to be r a t i o n a l l y acceptable i n the knowledge s i t u a t i o n represented by K, the proposed explanation (f) must meet the following condition (the requirement of maximal s p e c i f i c i t y ) : I f s_.k implies that b belongs to a cl a s s F±, and that F ^ i s a subclass of F , then s^ .k must also imply a statement specifying the s t a t i s t i c a l p r o b a b i l i t y of G i n Fi, say p ( G , F T J =r-j_. Here, r ^ must equal r unless the p r o b a b i l i t y statement just c i t e d i s simply a theorem of mathematical p r o b a b i l i t y theory. The requirement of maximal s p e c i f i c i t y i s here t e n t a t i v e l y put forward as characterizing the extent to which the requirement of t o t a l evidence properly applies to I-S explanations. The general idea thus suggested comes to t h i s : In formulating or appraising an I-S explanation, we should take into account a l l that information provided by K which i s of p o t e n t i a l explanatory relevance to the explanandum event; that i s , a l l pertinent s t a t i s t i c a l laws and such p a r t i c u l a r facts as might be connected by the s t a t i s t i c a l laws with the explanandum event. The requirement of maximal s p e c i f i c i t y disposes of the problem of epistemic ambiguity, for i t i s r e a d i l y seen that of two r i v a l s t a t i s t i c a l arguments with high associated p r o b a b i l i t i e s and with premises that a l l belong to K, at l e a s t one v i o l a t e s the requirement of maximal s p e c i f i c i t y . 33 CHAPTER THREE A DEFENCE AGAINST SOME COMMON MISCONCEPTIONS OF THE SYMMETRY THESIS 3.1 The N a t u r e o f E x p l a n a t i o n s and P r e d i c t i o n s 3.1.1 E x p l a n a t i o n s and P r e d i c t i o n s as S t a t e m e n t s o r A s s e r t i o n s I t has been p o i n t e d o u t by Scheff l e r - * - t h a t a p r e d i c t i o n i s u s u a l l y u n d e r s t o o d t o be a s t a t e m e n t o r an a s s e r t i o n r a t h e r t h a n an argument as t h e symmetry t h e s i s may s u g g e s t . More p r e c i s e l y , a p r e d i c t i o n i s a statement--token, t h a t i s ; . a c o n c r e t e u t t e r a n c e o r i n s c r i p t i o n o f a s e n t e n c e p u r p o r t i n g t o d e s c r i b e some e v e n t t h a t i s t o o c c u r a f t e r t h e p r o d u c t i o n o f t h e t o k e n . I n t h e same v e i n , S c r i v e n s t a t e s t h a t " t h e f o r e c a s t i s s i m p l y a d e s c r i p t i o n o f an e v e n t (or c o n d i t i o n , e t c . ) g i v e n p r i o r t o i t s o c c u r r e n c e and i d e n t i f i e d as r e f e r r i n g t o a f u t u r e t i m e . " 2 I n f a c t , we mi g h t add t h a t , s i m i l a r l y , an e x p l a n a t i o n i s o f t e n f o r m u l a t e d , n o t as an argument, b u t as a st a t e m e n t w h i c h t y p i c a l l y t a k e s t h e form "q because p." l - S c h e f f l e r (1957) , pp. 295-296. 2 S c r i v e n (1962) , p. 177. 34 The s i g n i f i c a n c e o f u n d e r s t a n d i n g e x p l a n a t i o n s and p r e d i c t i o n s as s t a t e m e n t s o r a s s e r t i o n s i s t h i s . A c c o r d i n g t o S c h e f f l e r , t h e dominant o r d i n a r y n o t i o n o f p r e d i c t i o n i s such t h a t any u t t e r a n c e o r i n s c r i p t i o n o f d e c l a r a t i v e , non-compound form i s a p r e d i c t i o n i f i t e x p l i c i t l y a s s e r t s s o m e t h i n g about some t i m e l a t e r t h a n t h e t i m e o f t h e u t t e r a n c e -But i t i s c l e a r l y f a l s e t h a t r e s t a t i n g e a c h s u c h p r e d i c t i o n f o l l o w i n g t h e t i m e o f i t s p r e d i c t e d o c c u r r e n c e e x p l a i n s t h i s o c c u r r e n c e even when b o t h t h e p r e d i c t i o n and r e s t a t e m e n t a r e t r u e . Thus, no i n s c r i p t i o n l i k e "Eisenhower i s e l e c t e d P r e s i d e n t on November 4, 1952" e x p l a i n s E i s e n h o w e r ' s e l e c t i o n , though e v e r y such i n s c r i p t i o n p r i o r t o November 4, 1952 i s a p r e d i c t i o n i n t h e o r d i n a r y sense. The p o i n t t h e n i s t h a t , i n t h e u s u a l sense o f ' p r e d i c t i o n , ' n o t e v e r y r e s t a t e m e n t o f a p r e d i c t i o n a f t e r t h e e v e n t i s e x p l a n a t o r y , even t h o u g h e v e r y s t a t e m e n t o f an e x p l a n a t i o n p r i o r t o t h e e v e n t i s p r e d i c t i v e . • A s i m i l a r comment i s made by S c r i v e n who c l a i m s t h a t however a c h i e v e d , a p r e d i c t i o n i s what i t i s , s i m p l y because i t i s p r o d u c e d i n advance o f t h e e v e n t i t p r e d i c t s ; i t i s i n t r i n s i c a l l y n o t h i n g b u t a b a r e d e s c r i p t i o n o f t h a t e v e n t . Whereas an e x p l a n a t i o n o f t h e e v e n t must be more t h a n t h e i d e n t i f y i n g d e s c r i p t i o n o f i t , e l s e t o r e q u e s t an e x p l a n a t i o n o f X (where "X" i s a d e s c r i p t i o n , n o t a name) i s t o g i v e an e x p l a n a t i o n o f X. E l s e w h e r e , he s a y s : What we a r e t r y i n g t o p r o v i d e when making a p r e d i c t i o n i s s i m p l y a c l a i m t h a t , a t a c e r t a i n t i m e , an e v e n t o r s t a t e o f a f f a i r s w i l l o c c u r . I n e x p l a n a t i o n we a r e l o o k i n g f o r a c a u s e , an e v e n t t h a t n o t o n l y o c c u r r e d e a r l i e r b u t s t a n d s i n a s p e c i a l r e l a t i o n t o t h e o t h e r i l b i d . event. Roughly speaking, the p r e d i c t i o n requires only a c o r r e l a t i o n , the explanation more.l Rescher too voices a s i m i l a r opinion: There i s general agreement that i t i s an e s s e n t i a l c h a r a c t e r i s t i c of explanations (in t h i s sense) that they must e s t a b l i s h t h e i r conclusion (explanans) well-nigh conclusively, i . e . i n such a manner that there i s l i t t l e , i f any, room l e f t open f o r reasonable doubt regarding i t s f a c t u a l i t y . Now t h i s requirement, i t i s c l e a r , i s f a r too stringent to be imposed l e g i t i m a t e l y upon p r e d i c t i o n s . 2 S c h e f f l e r goes further to say that neither w i l l i t do to invoke epistemology at t h i s point by asking how a p r e d i c t i o n of Eisenhower's e l e c t i o n could have been made without the use of general laws and statements of relevant antecedent conditions. He claims that the methodological genesis of a p r e d i c t i o n which does not meet r a t i o n a l or s c i e n t i f i c requirements may involve i r r a t i o n a l behaviour by i t s producer, but i s no bar to i t s predictiveness i n the ordinary sense, for "clairvoyants, prophets, and news-commentators a l l predict i n the sense under consideration, j u s t as do scientists."-^ 3.1.2 Hempelian Explanations and Predictions Hempel agrees with both Sch e f f l e r and Scriven that what i s usually c a l l e d a p r e d i c t i o n i s not an argument but a statement. But since p r e d i c t i v e statements i n empirical IScriven (1959a), p. 480. 2Rescher (1958) , p. 285. 3scheffler (1957), p. 296. 36 science are normally obtained by i n f e r e n t i a l procedures (which may be deductive or inductive i n character) on the basis of a v a i l a b l e evidence, there a r i s e s the question as to the l o g i c of p r e d i c t i v e arguments i n analogy to the problem of the l o g i c of explanatory arguments. Thus, Hempel's the s i s concerning the symmetry between s c i e n t i f i c explanation and p r e d i c t i o n should be understood as r e f e r r i n g to the l o g i c a l structure, not of explanatory or p r e d i c t i v e statements, but of explanatory or p r e d i c t i v e arguments. I t must further be noted that the symmetry thesis "makes a purely l o g i c a l {[italics minej point; i t does not depend on any empirical assumptions. S c h e f f l e r i s indeed correct i n pointing out that i n the ordinary sense of 'prediction' which takes the form of a statement, not every restatement of a p r e d i c t i o n a f t e r the event i s explanatory, even though every statement of an explanation p r i o r to the event i s p r e d i c t i v e . Yet, since Hempel i s using 'explanation' and 'prediction,' not i n the sense of statements, but i n the sense of 'explanatory and p r e d i c t i v e arguments,' i t w i l l then be true that "Every explanation, i f stated p r i o r to the event described by i t s explanandum, would be p r e d i c t i v e , while every p r e d i c t i o n , stated a f t e r the event i n question, would be explanatory." 2 In essence, Hempel's conception of 'prediction' as an iHempel (1962a), p. 114. 2 S c h e f f l e r (1957), p. 295. 37 'argument' d i f f e r s from Sc h e f f l e r ' s conception of 'prediction' as a 'statement' because Hempel incorporates the grounds for p r e d i c t i o n into the concept of 'prediction' i t s e l f . In other words/ a 'Hempelian s c i e n t i f i c p r e d i c t i o n ' includes a statement pertaining to some future event and the grounds for making such a statement. S c h e f f l e r , on the contrary, keeps a c l e a r d i s t i n c t i o n between a p r e d i c t i o n and the grounds for making i t . This would explain h i s a s s e r t i o n of an asymmetry between explanation and p r e d i c t i o n , f o r he apparently maintains that the grounds for explanation are always b u i l t i n t o an explanation since an. explanation i s more than j u s t a restatement of a p r e d i c t i o n ; whereas the grounds for p r e d i c t i o n are hot b u i l t into a p r e d i c t i o n because a p r e d i c t i o n i s merely a statement p e r t a i n i n g to some future event. Sympathetic with Hempel's idea on the nature of 'prediction' and i t s purported symmetry with 'explanation, Brodbeck comments: Of course, i f by 'prediction' we mean any prophecy, or simply "a claim that at a c e r t a i n time an event w i l l occur,"! then we may c e r t a i n l y p r e d i c t without being able to explain. On the other hand, i f by a p r e d i c t i o n we mean one for which reasons can be given, then a f t e r the event we should also be able to explain i t . I f we cannot, then we know that our premises must be f a l s e . . . . Only successful p r e d i c t i o n gives us evidence f o r accepting the premises. "Prediction" need not be of an event i n the future, only of something not as yet known.2 IScriven (1959a), p. 480. 2Brodbeck (1962), p. 253. 38 As a m a t t e r o f f a c t , s c i e n t i s t s do n o t n o r m a l l y p r e s e n t t h e i r e x p l a n a t i o n s o r p r e d i c t i o n s i n t h e way t h a t Hempel t h i n k s i t s h o u l d be done. As Brodbeck has c o r r e c t l y p o i n t e d out,-*- a p a r t from b e i n g a means o f c o m m u n i c a t i o n , language has a descr1pt1ve' f u n c t i o n t o o . F o r e x a m p l e , a p h y s i c i s t ' s p u b l i s h e d r e p o r t o f t h e t h e o r e t i c a l s i g n i f i c a n c e o f an e x p e r i m e n t and h i s l a b o r a t o r y c o n v e r s a t i o n a r e two q u i t e d i f f e r e n t t h i n g s . The l a t t e r i s e l l i p t i c a l and c o n t e x t - b o u n d i n a way h i s r e p o r t c annot be. H i s f e l l o w c o - w o r k e r w i l l ' u n d e r s t a n d ' h i s ' e x p l a n a t i o n ' o f a c e r t a i n phenomenon even when he s i m p l y m e n t i o n s some o t h e r f a c t . The i n f o r m a t i o n n e c e s s a r y t o t h i s u n d e r s t a n d i n g i s s u p p l i e d by t h e t o t a l c o n t e x t w i t h i n w h i c h t h e c o n v e r s a t i o n o c c u r s , b o t h t h e s o c i a l s i t u a t i o n i t s e l f and t h e knowledge t h e two men s h a r e . The p u b l i s h e d r e p o r t , however, w i l l have t o p r o v i d e c o n s i d e r a b l y more d e t a i l s about t h e c o n n e c t i o n between t h e e x p e r i m e n t , t h e o b s e r v a t i o n s , and what i t was p e r f o r m e d t o t e s t . M o r e o v e r , t h e terms used i n t h e r e p o r t must have c o n s t a n t meanings t h r o u g h o u t . S i n c e t h e p h y s i c i s t w r i t e s f o r o t h e r p h y s i c i s t s , h i s r e p o r t seldom, i f e v e r , e x p l i c i t l y m e n t i o n s a l l t h e i n f o r m a t i o n t h a t j u s t i f i e s c e r t a i n s t a t e m e n t s , f o r a g a i n t h e r e i s a c o n t e x t o f s h a r e d knowledge t h a t may be t a k e n f o r g r a n t e d . But t h e o b v i o u s d i f f e r e n c e s between t h e e x p l i c i t n e s s o f t h e r e p o r t and t h e b r i e f l a b o r a t o r y c o n v e r s a t i o n make c l e a r what i l b i d . , pp. 236-238. 39 a c o m p l e t e r e p o r t (one based on t h e D-N model) wo u l d be l i k e . O n l y a c o m p l e t e r e p o r t , i n c l u d i n g t h a t w h i c h i s spoken as w e l l as t h a t w h i c h i s l e f t unspoken, p r e s e n t s t h e o b s e r v a t i o n a l and l o g i c a l grounds f o r any c l a i m . O r d i n a r y c o m m u n i c a t i o n r e l i e s h e a v i l y on t h e l i n g u i s t i c and n o n - l i n g u i s t i c c o n t e x t . To examine language as d e s c r i p t i o n as p h i l o s o p h e r s do, t h a t i s , t o s t u d y "terms and s t a t e m e n t s as t h e y a r e u s e d t o make a s s e r t i o n s , no m a t t e r how we came t o a c q u i r e t h e s e uses,"-'-t h i s c o n t e x t must be s u p p l i e d . P u t d i f f e r e n t l y , e x p l a n a t i o n s o f t h e f o r m "q because p" a r e i n d e e d p a r a d i g m c a s e s o f language b e i n g used as a means o f c o m m u n i c a t i o n , b u t n o t as a f u n c t i o n o f d e s c r i p t i o n . I n e s s e n c e , Brodbeck b e l i e v e s t h a t f o r t h e p h i l o s o p h e r c o n c e r n e d w i t h language as d e s c r i p t i o n , t h e t a s k i s t o show what, so t o speak , i n t h e f a c t s t h e m s e l v e s , o r , t o be a c c u r a t e , i n t h e s t a t e m e n t s a s s e r t i n g them, r a t h e r t h a n i n t h e mind o r b e h a v i o u r o f a p a r t i c u l a r p e r s o n o r group o f p e r s o n s , makes one o r more s t a t e m e n t s a " r e a s o n " , i n a p r e c i s e l o g i c a l sense o f r e a s o n , f o r one o r more o t h e r s . What i n t h e s t a t e m e n t s t h e m s e l v e s makes one s t a t e m e n t a p r e d i c t i o n f r om o t h e r s , one c o n f i r m a g e n e r a l h y p o t h e s i s , a n o t h e r r e f u t e i t ? D e d u c t i v e e x p l a n a t i o n i s t h e o n l y answer t o t h i s q u e s t i o n , i n s o f a r as language i s used t o speak, n o t j u s t a bout how we speak, b u t about t h e w o r l d . 2 I t i s t r u e t h a t t h e p r a g m a t i c a s p e c t s o f e x p l a n a t i o n , as s u g g e s t e d by Sc r i v e n - ^ and Dr a y , 4 a r e s i g n i f i c a n t i n t h e i r ! l b i d . , p. 237. 2 I b i d . , pp. 239-240. 3 S c r i v e n ( 1 962), pp. 201-208. 4". . . as I s h a l l argue f u r t h e r i n t h i s , and i n 40 own r i g h t , b u t s c i e n t i f i c r e s e a r c h , H e m p e l m a i n t a i n s , s e e k s t o g i v e a n a c c o u n t — b o t h d e s c r i p t i v e a n d e x p l a n a t o r y — o f e m p i r i c a l p h e n o m e n a w h i c h i s o b j e c t i v e i n t h e s e n s e t h a t i t s i m p l i c a t i o n s a n d i t s e v i d e n t i a l s u p p o r t d o n o t d e p e n d e s s e n t i a l l y o n t h e i n d i v i d u a l s who h a p p e n t o a p p l y o r t e s t t h e m . T h i s i d e a l s u g g e s t s t h e p r o b l e m o f c o n s t r u c t i n g a n o n p r a g m a t i c c o n c e p t i o n o f s c i e n t i f i c e x p l a n a t i o n . . . [ w h i c h i s w h a t ] t h e t w o c o v e r i n g - l a w m o d e l s a r e c o n c e r n e d . 1 H e n c e , t o p u t f o r w a r d t h e c o v e r i n g - l a w m o d e l s i s n o t t o d e n y t h a t t h e r e a r e many o t h e r i m p o r t a n t u s e s o f t h e t e r m s ' e x p l a n a t i o n ' a n d ' p r e d i c t i o n , ' a n d e v e n l e s s i s i t t o c l a i m t h a t a l l o f t h o s e o t h e r u s e s c o n f o r m t o o n e o r o t h e r o f t h e two m o d e l s . F o r e x a m p l e , n e i t h e r m o d e l i s i n t e n d e d t o e x p l a i n t o u s t h e r u l e s o f a game o r how t o r e p a i r a c a r . T h u s , t o c o m p l a i n , a s S c r i v e n d o e s , o f t h e ' h o p e l e s s n e s s * o f t h e d e d u c t i v e m o d e l b e c a u s e i t d o e s n o t f i t t h e c a s e o f " u n d e r s t a n d i n g t h e r u l e s o f H a n o v e r i a n s u c c e s s i o n " 2 i s s i m p l y t o m i s s t h e d e c l a r e d i n t e n t o f t h e m o d e l . A n d i t i s e v e n m o r e i r r e l e v a n t f o r h i m t o a c c u s e H e m p e l a n d O p p e n h e i m ' s a n a l y s i s o f e x p l a n a t i o n o f p r e s u p p o s i n g a d e s c r i p t i v e l a n g u a g e ( w h i c h i s i n d e e d t r u e ) b y c i t i n g t h a t " t h e r e a r e c l e a r l y c a s e s w h e r e we c a n e x p l a i n w i t h o u t l a n g u a g e , e . g . , when we e x p l a i n t o t h e m e c h a n i c -; i n a Y u g o s l a v g a r a g e w h a t h a s g o n e w r o n g w i t h t h e c a r . " ^ s u c c e e d i n g c h a p t e r s , t h e r e i s a n i r r e d u c i b l e p r a g m a t i c d i m e n s i o n t o e x p l a n a t i o n . " D r a y (1957), p . 69. ^-Hempel (1963), p . 130. (1959b), p . 452. (1962), p . 192. ' S c r i v e n S c r i v e n This i s l i k e objecting to a d e f i n i t i o n of 'bank' as an establishment f o r custody of money on the ground that i t does not f i t the use of 'bank' i n " s t r o l l i n g along the bank of the r i v e r " , l e t alone 'bank' as a galley-rowers bench. In view of t h i s , Hempel i s more than w i l l i n g to concede that wordless gesticulation—however eloquent and s u c c e s s f u l — -which i s meant to i n d i c a t e to a Yugoslav garage mechanic what has gone wrong with the car does not q u a l i f y as s c i e n t i f i c explanation according to e i t h e r of the two covering-law models, but any account of s c i e n t i f i c explanation which did admit t h i s case would thereby show i t s e l f to be s e r i o u s l y inadequate. But we must bear i n mind, as Hempel reminds us, that "the two covering-law models represent the l o g i c a l structure of two important types of p r e d i c t i v e inference i n e m p i r i c a l science, but not that these are the only t y p e s . T h a t there are p r e d i c t i v e arguments that could not q u a l i f y as explanations has been asserted, for example, by S c h e f f l e r ^ and by Scriven. Hempel i s convinced by S c h e f f l e r ' s observation to the e f f e c t that s c i e n t i f i c predictions may be i n d u c t i v e l y grounded on information that includes no statements i n the form of general laws, but only a f i n i t e set of p a r t i c u l a r data, e.g. the r e s u l t s obtained i n t r y i n g out a given die or a given r o u l e t t e wheel a f i n i t e number of times for the purpose of iHempel (1963), pp. 119-120. 2 S c h e f f l e r (1957), p. 296. 3 S c r i v e n (1959a), p. 480. 42 p r e d i c t i n g l a t e r r e s u l t s . P r e d i c t i v e arguments of t h i s type — f r o m f i n i t e samples to predictions concerning as yet unobserved samples of a given p o p u l a t i o n — c l e a r l y are not of the covering-law form. What bars them from the r o l e of p o t e n t i a l explanations i s not t h e i r inductive character, f o r there are explanatory arguments of the inductive or s t a t i s t i c a l form, but the f a c t that they do not invoke any general laws, e i t h e r of s t r i c t l y u niversal or s t a t i s t i c a l form. In my opinion, Hempel i s mistaken i n t h i n k i n g that S c h e f f l e r ' s example i s s i m i l a r i n nature to Scriven's one which deals with the barometer acting as an i n d i c a t o r of storms. A d e t a i l e d analysis of the barometer case w i l l be found i n Chapter Five. For the moment, l e t me j u s t b r i e f l y point out that S c h e f f l e r ' s ball-drawing example i s q u a l i t a t i v e l y d i s t i n c t from Scriven's barometer case. For • no covering-law i s needed to explain that the 500th b a l l i s red over and above the f a c t that 499 b a l l s drawn e a r l i e r were red, whereas there i s (and i n some other cases, there could conceivably be) nomological-explanation f o r the coming of the storm which invokes the i n d i c a t o r law that "Drops i n barometric pressure are always followed by storms." In other words, Hempel's symmetry thesis appears unable to accomodate Sch e f f l e r ' s and i t s s i b l i n g cases which are types of p r e d i c t i v e inference i n empirical science whose l o g i c a l structure i s not represented by the two covering-law models,-'- but there i s ^This problem w i l l be discussed i n sec. 5.6. 43 reason to believe that Scriven's and s i m i l a r cases could be integrated i n t o the the s i s i n the l i g h t of a b e t t e r analysis of these cases. 3.1.3 Grunbaum's Interpretation of Hempel's Thesis In the Hempelian s p i r i t , Grunbaum endorses the symmetry thesis as that which concerns explanatory or p r e d i c t i v e arguments instead of explanatory or p r e d i c t i v e statements. In b r i e f , he thinks that i t i s a confusion on the part of S c h e f f l e r and Scriven i n understanding Hempel's t h e s i s to pertain to the ass e r t i b i 1 1 t y per se of the explanandum rather than to e i t h e r the deductive or inductive i n f e r a b i l i t y of the explanandum from the explanans. Before proceeding to a thorough discussion of t h i s point, i t i s e s s e n t i a l to understand f i r s t Grunbaum's assertion of a r e t r o d i c t i o n -p r e d i c t i o n asymmetry which i s deemed to have an important bearing on the symmetry between explanation and p r e d i c t i o n . Without going into d e t a i l s Grunbaum's arguments concerning entropy systems, the r e t r o d i c t i o n - p r e d i c t i o n asymmetry amounts b a s i c a l l y to t h i s : R eliable i n d i c a t o r s i n i n t e r a c t i n g systems permit only  r e t r o d i c t i v e inferences concerning the 'interactions  f o r which they vouch but no p r e d i c t i v e inferences  pertaining to corresponding l a t e r i n t e r a c t irons. 1 The bearing of t h i s r e t r o d i c t i o n - p r e d i c t i o n asymmetry on the issue of the symmetry between explanation and iGrtinbaum (1963), p. 66. 44 p r e d i c t i o n i s t h i s . F o r Hempel, t h e p a r t i c u l a r c o n d i t i o n s C i ( i = l , 2, . . '. , n) w h i c h , i n c o n j u n c t i o n w i t h t h e r e l e v a n t l a w s , a c c o u n t f o r t h e explanandum e v e n t E, may be e a r l i e r t h a n E i n b o t h e x p l a n a t i o n and p r e d i c t i o n o r t h e may be l a t e r t h a n E i n b o t h e x p l a n a t i o n and p r e d i c t i o n . Thus, a c a s e o f e x p l a n a t i o n i n w h i c h t h e would be e a r l i e r t h a n E would be one i n astronomy, f o r exam p l e , i n w h i c h a p a s t E (e.g. an e c l i p s e ) was a c c o u n t e d f o r by r e f e r e n c e t o r e l e v a n t a s t r o n o m i c a l laws and C i (e. g . some s t a t e m e n t s d e s c r i b i n g some r e l e v a n t c o n f i g u r a t i o n o f h e a v e n l y b o d i e s ) t h a t were s t i l l f u r t h e r i n t h e p a s t t h a n E. These a s s e r t i o n s h o l d , Grunbaum s t i p u l a t e s , because Hempel's c r i t e r i o n f o r an e x p l a n a t i o n as opposed t o a p r e d i c t i o n i s t h a t E b e l o n g s t o th e s c i e n t i s t ' s p a s t when he o f f e r s h i s a c c o u n t o f i t , and h i s c r i t e r i o n f o r a c o r r e s p o n d i n g p r e d i c t i o n i s t h a t E b e l o n g s t o t h e s c i e n t i s t ' s f u t u r e when i t i s made. S h a r i n g Grunbaum's s t i p u l a t i o n i s Rudner who s a y s : I t s h o u l d be c l e a r t h a t t h e p r o - d i c t i v i t y o r r e t r o -d i c t i v i t y o f an e x p l a n a t o r y a r g u m e n t — w h i c h depends upon t e m p o r a l r e l a t i o n s between explanandum and some s t a t e m e n t s o f t h e e x p l a n a n s — d o e s n o t d e t e r m i n e w h e t h e r a g i v e n use i s a p r e d i c t i v e o r an e x p l a n a t i o n a l use o f t h a t e x p l a n a t o r y argument; f o r t h i s l a t t e r f a c t depends o n l y on a t e m p o r a l r e l a t i o n between t h e explanandum and t h e d a t e on w h i c h t h e use o c c u r s ( r e g a r d l e s s o f t h e d a t e s o f t h e e x p l a n a n s e v e n t s ) . On t h e o t h e r hand, i n t h e r e t r o d i c t i o n - p r e d i c t i o n a n t i t h e s i s , a r e t r o d i c t i o n i s c h a r a c t e r i z e d by t h e f a c t t h a t t h e C i a r e l a t e r t h a n E, w h i l e t h e C i a r e e a r l i e r J-Rudner (1966) , p. 64. 45 than E i n the kind of p r e d i c t i o n which i s a n t i t h e t i c a l to r e t r o d i c t i o n but hot i d e n t i c a l with Hempelian p r e d i c t i o n . In the following diagram, the i , k, 1, m may each range over the values 1, 2, . . . , n. Cfc may coincide C i may coincide with the Now or with the Now or even succeed i t even precede i t -prediction-* - ^ r e t r o d i c t i o n — — p r e d i c t i o n - ^ < - r e t r o d i c t i o n — • •—•—— • — | — • © — o o > +t C i E i E2 Ck Now C i E 3 E 4 C m v v > V V , ' H-explanation H-prediction If the p r e f i x *H' i s used as an abbreviation for 'Hempelian', then two consequences are apparent. F i r s t , a r e t r o d i c t i o n as well as a p r e d i c t i o n can be an H-prediction, and a p r e d i c t i o n as well as a r e t r o d i c t i o n can be an H-explanation. Second, being an H-prediction rather than an H-explanation, or conversely, depends on the t r a n s i e n t homocentric 'now*, but there i s no such 'now'-dependence i n the case of being a r e t r o d i c t i o n instead of a p r e d i c t i o n , or conversely. With the help of t h i s d i s t i n c t i o n , Hempel's symmetry thes i s between H-explanation and H-prediction i s now formulated by Grunbaum as follows: Any p r e d i c t i o n which q u a l i f i e s l o g i c a l l y and methodologically as an H-explanation a l s o q u a l i f i e s as an H-prediction, provided that the s c i e n t i s t i s i n possession of the information concerning the Ci p r i o r to the occurrence of E, and conversely. And any r e t r o d i c t i o n which q u a l i f i e s l o g i c a l l y and methodologically as an H-explanation a l s o q u a l i f i e s as an H-prediction, provided that the information 46 concerning the relevant C i i s a v a i l a b l e at an appropriate time, and conversely.1 I t i s Grtinbaum b e l i e f that Hempel's a f f i r m a t i o n of symmetry does not pertain to the a s s e r t i b i 1 1 t y per se of the explanandum but to e i t h e r the deductive or inductive i n f e r a b i l i t y of the explanandum from the explanans. What Hempel and Popper assert i s that to the extent that there i s ever explanatory i n f e r a b i l i t y , there i s a l s o p r e d i c t i v e i n f e r a b i l i t y and v i c e versa. They do not claim that every time you are e n t i t l e d to assert, on some grounds or other, that a c e r t a i n kind of event d i d occur i n the past, you are also e n t i t l e d to say that the same kind of event w i l l occur i n the future. Being concerned with s c i e n t i f i c understanding, Hempel and Popper hold that there i s temporal symmetry, not of a s s e r t i b i l i t y per se, but of a s s e r t i b i l i t y on the strength  of the explanans. The s c i e n t i f i c relevance of dealing with p r e d i c t i v e arguments rather than mere p r e d i c t i v e assertions can hardly be contested by Scriven's claim that a p r e d i c t i o n i s i n t r i n s i c a l l y nothing but a bare d e s c r i p t i o n of the event i t p r e d i c t s . For surely a soothsayer's unsupported prophecy that there w i l l not be a t h i r d world war i s not of s c i e n t i f i c s i g n i f i c a n c e and ought not command any s c i e n t i f i c i n t e r e s t p r e c i s e l y because of the unreasoned manner of i t s achievement. Hence a s c i e n t i f i c a l l y warranted p r e d i c t i o n iGrtinbaum (1963) , p. 73. 47 o f a n e v e n t m u s t b e m o r e t h a n a m e r e p r e - a s s e r t i o n o f t h e e v e n t . O n c e t h i s d i s t i n c t i o n b e t w e e n a s s e r t i b i l i t y a n d i n f e r a b i l i t y i s m a d e , t h e i s s u e o f t h e a d e q u a c y o f t h e s y m m e t r y t h e s i s w i l l t h u s r e v o l v e a r o u n d w h e t h e r t h e r e i s t e m p o r a l s y m m e t r y w i t h r e g a r d t o t h e d e g r e e o f e n t a i l m e n t , a s i t w e r e , c h a r a c t e r i z i n g t h e l o g i c a l l i n k b e t w e e n t h e e x p l a n a n s a n d t h e e x p l a n a n d u m . S p e c i f i c a l l y , we s h a l l n e e d t o a n s w e r t w o q u e s t i o n s : (1) W o u l d t h e t y p e o f a r g u m e n t w h i c h y i e l d s a p r e d i c t i o n o f a f u t u r e e x p l a n a n d u m e v e n t n o t f u r n i s h p r e c i s e l y t h e same a m o u n t o f s c i e n t i f i c u n d e r s t a n d i n g o f a c o r r e s p o n d i n g p a s t e v e n t ? (2) D o e s a n e x p l a n a n s e x p l a i n a n e x p l a n a n d u m r e f e r r i n g t o a p a s t e v e n t a n y m o r e c o n c l u s i v e l y t h a n t h i s same k i n d o f e x p l a n a n s p r e d i c t i v e l y i m p l i e s t h e e x p l a n a n d u m p e r t a i n i n g t o t h e c o r r e s p o n d i n g f u t u r e e v e n t ? 48 3.2 The Symmetry Thesis: L o g i c a l or Epistemblogical? 3.2.1 Can There be False Explanations? S c h e f f l e r i s convinced that there i s a d e f i n i t e asymmetry between explanation and p r e d i c t i o n f o r he thinks that some p r e d i c t i v e restatements a f t e r a c e r t a i n event are not explanatory, since p r e d i c t i v e success involves the p o s s i b i l i t y of p r e d i c t i v e f a i l u r e , i . e . f a l s e p r e d i c t i o n s . But no explanation i s f a l s e , since i t c o n s i s t s of an explanans, which by (R4) must be true, and an explanandum which, being a l o g i c a l consequence of the l a t t e r , cannot be f a l s e e i t h e r . ! In h i s opinion, t h i s divergence i s r e l a t e d to the use of s c i e n t i f i c p redictions i n t e s t i n g the body of assumptions at a given time. For such testings to occur, i t must make sence to judge a derived p r e d i c t i o n f a l s e , thereby f o r c i n g a r e v i s i o n i n i t s ground-premises. Even Hempel has to admit that S c h e f f l e r ' s remark i s quite c o r r e c t , but he considers i t "to i n d i c a t e , not that there i s a basic discrepancy between explanation and p r e d i c t i o n , but that the requirement of tr u t h f o r s c i e n t i f i c explanations i s unduly r e s t r i c t i v e . " 2 He maintains, and I think c o r r e c t l y , 1 S c h e f f l e r (1957), 2Hempel (1962), p. p. 297. 118. 49 t h a t t h i s r e s t r i c t i o n i s a v o i d e d by h i s d i s t i n c t i o n between p o t e n t i a l e x p l a n a t i o n s and t r u e e x p I a n a t i o n s w h i c h e n a b l e s us t o speak o f e x p l a n a t i o n s no l e s s t h a n p r e d i c t i o n s as b e i n g p o s s i b l y f a l s e , and as b e i n g more o r l e s s w e l l c o n f i r m e d by t h e e m p i r i c a l e v i d e n c e a t hand.^ I n o t h e r w o rds, i n c o n t r a s t t o a c o r r e c t o r t r u e e x p l a n a t i o n i n w h i c h r e q u i r e m e n t ( R 4 ) , i . e . t h e e x p l a n a n s must be t r u e , i s a l w a y s s a t i s f i e d , t h e e x p l a n a n s o f a p o t e n t i a l e x p l a n a t i o n needs o n l y be more o r l e s s s t r o n g l y s u p p o r t e d o r c o n f i r m e d by a g i v e n body o f e v i d e n c e . A p o t e n t i a l e x p l a n a t i o n i s u s e d , f o r example, when we say t h a t t h e p h l o g i s t o n t h e o r y , t h o u g h now d i s c a r d e d as f a l s e , a f f o r d e d an e x p l a n a t i o n f o r c e r t a i n a s p e c t s o f c o m b u s t i o n . As a m a t t e r o f f a c t , a p a r t from showing t h a t S c h e f f l e r ' s remark does n o t s e r v e t o i n d i c a t e t h a t t h e r e i s a b a s i c d i s c r e p a n c y between e x p l a n a t i o n and p r e d i c t i o n s i n c e t h e r e can be f a l s e e x p l a n a t i o n s as w e l l as f a l s e p r e d i c t i o n s , t h e u n d e r s t a n d i n g o f e x p l a n a t i o n s i n t h e sense o f p o t e n t i a l e x p l a n a t i o n s has a d e f i n i t e advantage o v e r u n d e r s t a n d i n g i t i n t h e sense o f t r u e e x p l a n a t i o n s . F o r t h e c o n c e p t o f a t r u e e x p l a n a n s has no p l a c e i n Hempel's i n d u c t i v e - s t a t i s t i c a l model o f e x p l a n a t i o n . I ndeed, i f we a c c e p t Hempel's e a r l i e r r e q u i r e m e n t t h a t t h e s t a t e m e n t s c o n t a i n e d i n t h e e x p l a n a n s be t r u e , t h e n t h e r e i s such a t h i n g as a g e n u i n e e x p l a n a t i o n o b j e c t i v e l y and q u i t e a p a r t f rom o u r knowledge s i t u a t i o n . To •'•See s e c . 2.1.1.3. 50 be sure, the state of our knowledge determines what we take to be explanations, but no mention i s made of our knowledge s i t u a t i o n i n the c r i t e r i a f o r genuine explanations. But i f we s e t t l e f o r Hempel's l a t e r requirement that the explanans be highly confirmed, then the very concept of a genuine explanation involves a reference to our knowledge s i t u a t i o n , and there i s no longer a thing as a correct explanation s i m p l i c i t e r . In view of Hempel's lengthy attempt to argue that i n d u c t i v e - s t a t i s t i c a l explanation must be r e l a t i v i z e d to the knowledge situation-*-—a c h a r a c t e r i s t i c which he r e f e r s to as the "epistemic r e l a t i v i t y of s t a t i s t i c a l e x p l a n a t i o n " 2 - - i t c e r t a i n l y appears that he has abandoned h i s e a r l i e r requirement that the explanans of an adequate explanation must be true. That i s to say, " i n the analysis of the l o g i c a l structure of explanatory arguments, . . . the requirement [ v i z . (R4)] may be disregarded." 3 In the absence of (R4), the symmetry thes i s should thus be properly understood as the t h e s i s of the symmetry between e x p l a n a t i o n — i n the sense of p o t e n t i a l but not true explanation—and p r e d i c t i o n . In the l i g h t of t h i s modification, Scriven's complaint that Hempel and Oppenheim's analysis i s merely an analysis !see Hempel (1965c), Sec. 3.4.2, pp. 397-403. 2Hempel (1965c), p. 402. 3Hempel and Oppenheim (1965), n. 3 (added i n 1964), p. 249. of 'correct explanation' rather than of 'explanation* i s no longer j u s t i f i e d , since Hempel would now agree with Scriven that "'explanations,' or 'an explanation,' or 'his explanation,' or 'a possible explanation,* do not always have to be true (or of the appropriate type, or adequate); they only need high confirmation, at some stage. 3.2.2 Epistemological Asymmetry In agreement with S c h e f f l e r ' s focus of attack, Rescher r e j e c t s Hempel's symmetry thesis on the reasoning that there i s an epistemolog i c a1 asymmetry between explanation and p r e d i c t i o n . He accuses Hempel's thesis of being inconsistent with s c i e n t i f i c custom and usage regarding the concepts of explanation and p r e d i c t i o n , f o r among other things, "only true statements are proper objects f o r explanation, but c l e a r l y not so with p r e d i c t i o n . " 2 In support of h i s claim of an epistemological asymmetry, he points to a large number of cases i n which we have v i r t u a l l y c e r t a i n knowledge of the past on the basis of traces found i n the present. As regards p r e d i c t i v e knowledge, we usually have only fragmentary and i n general merely probable knowledge of the future on the basis of knowledge of the present and/or the past, although i n c e r t a i n f i e l d s (e.g. astronomy) our knowledge of natural laws does a f f o r d v i r t u a l l y c e r t a i n p r e d i c t i v e knowledge.3 ••-Scriven (1962), p. 191. 2Rescher (1958), n. 1, p. 282. 3 I b i d . , p. 284. 52 The p l a u s i b i l i t y of Rescher's argument depends on the answers to two questions: f i r s t , "Does an epistemological asymmetry ever e x i s t ? " , and second, "Even i f i t d i d e x i s t , does i t thereby refute Hempel's thesis?" Before answering ei t h e r question, i t i s fundamentally important to d i s t i n g u i s h , as Grunbaum has r i g h t l y pointed out, between the following two sets of ideas: (1) an asymmetry between H-explanation and H-prediction both i n regard to the grounds on which we clai m to know that the explanandum i s true and c o r r e l a t i v e l y i n regard to the degree of our confidence i n the supposed t r u t h of the explanandum, and (2) an asymmetry, i f any, between H-explanation and H-prediction with respect to the l o g i c a l r e l a t i o n obtaining between the explanans and the explanandum.1 For convenience, l e t us r e f e r to the f i r s t asymmetry as pertaining to the ' a s s e r t i b i l i t y ' of the explanandum while speaking of the second as an asymmetry i n the ' i n f e r a b i l i t y ' or 'why' of the explanandum. In the l i g h t of t h i s d i s t i n c t i o n , l e t us now examine the v a l i d i t y of Rescher's argument. In short, the f i r s t question w i l l get an aff i r m a t i v e answer whereas the second question a negative one. If understood as pertaining to the a s s e r t i b i l i t y of the explanandum., Rescher's contention of the existence of an epistemological asymmetry i s indeed c o r r e c t . For, as argued e a r l i e r by Grunbaum2, there are, i n some cases, h i g h l y  r e l i a b l e records of past i n t e r a c t i o n s but no spontaneously •'"Grunbaum (1963), pp. 76-77. 2See sec. 3.1.4. 53 produced records of corresponding future i n t e r a c t i o n s . This implies that while we can c e r t i f y the a s s e r t i b i l i t y or t r u t h of an explanandum r e f e r r i n g to a past i n t e r a c t i o n on the basis of a record without invoking the supposed t r u t h of any (usual) explanans thereof, generally no p r e - i n d i c a t o r but only the supposed t r u t h of an appropriate explanans can be c a l l e d upon to uphold the a s s e r t i b i l i t y or t r u t h of the explanandum pertaining to a future i n t e r a c t i o n . Thus there i s a very large c l a s s of cases i n which an epistemological asymmetry does obtain with respect to the a s s e r t i b i l i t y of the explanandum. But t h i s asymmetry of a s s e r t i b i l i t y cannot, as Grunbaum has convincingly argued, undermine Hempel's symmetry t h e s i s which states that to the same extent to which an exp1anandum r e f e r r i n g to the past can be post-asserted oh the strength of  i t s explanans i n an H-explanation, a corresponding explanandum r e f e r r i n g to the future can be preasserted on the strength of the same explanans i n an H-prediction. In other words, you can post-assert an explanandum on the strength of i t s explanans no better than you can preassert i t . l iGrtlnbaum (19-63) , pp. 77-78. 54 3.3 E x p l a n a t i o n and R e t r o d i c t i o n 3.3.1 R e s c h e r ' s C o n t e n t i o n o f an Asymmetry T h e s i s R e s c h e r ' s d i s c o n t e n t w i t h t h e symmetry t h e s i s i s founded on h i s v i e w t h a t t h e l a t t e r " r e s t s upon a t a c i t b u t u n w a r r a n t e d a s s u m p t i o n as t o t h e n a t u r e o f t h e p h y s i c a l universe."-'- The b a s i c r e a s o n adduced f o r t h i s c o n t e n t i o n i s t h a t " t h e e x p l a n a t i o n o f e v e n t s i s o r i e n t e d ( i n t h e main) towards t h e p a s t , w h i l e p r e d i c t i o n i s o r i e n t e d t o w a r d s t h e f u t u r e . " 2 T h i s b e i n g t h e c a s e , b e f o r e we can d e c i d e w h e t h e r ( d e d u c t i v e - n o m o l o g i c a l ) e x p l a n a t i o n and p r e d i c t i o n have t h e same l o g i c a l s t r u c t u r e , we have t o a s c e r t a i n w h e t h e r t h e n a t u r a l l a w s o f our w o r l d do i n f a c t p e r m i t . r e f e r e n c e s f r o m t h e p r e s e n t t o t h e f u t u r e as w e l l as from t h e p r e s e n t t o th e p a s t . I n R e s c h e r ' s o p i n i o n , t h e s t a t u s o f p r e d i c t i o n o f f u t u r e s t a t e s i n terms o f t h e p r e s e n t , and o f e x p l a n a t i o n o f p a s t s t a t e s i n terms o f p r e c e d i n g ones as i n f e r r e d f r o m t h e p r e s e n t , would be w h o l l y a n a l o g o u s o n l y i f t h e u n i v e r s e w h i c h p h y s i c s u n v e i l s were o f t h e Type I system i n w h i c h i t i s p o s s i b l e b o t h t o make i n f e r e n c e s r e g a r d i n g p a s t s t a t e s and t o make p r e d i c t i o n s r e g a r d i n g f u t u r e s t a t e s . B u t i R e s c h e r (1958), p. 282. 2 I b i d . , p. 286. 3 A d e t a i l e d a n a l y s i s o f t h e f o u r t y p e s o f p h y s i c a l systems can be found i n Resc h e r (1958), pp. 282-284. 55 i t i s a matter of f a c t , and not of l o g i c a l a n a l y s i s , to e s t a b l i s h whether t h i s c l a s s i f i c a t i o n , or some other, i s correct. To profess the equivalence of explanation and p r e d i c t i o n upon t h e o r e t i c a l grounds i s to pre-empt an empirical question, by basing the l o g i c a l analysis upon a hidden assumption as to the character of p h y s i c a l laws, an assumption that properly requires f a c t u a l ( i . e . e x t r a - l o g i c a l ) warrant. There can be no t h e o r e t i c a l j u s t i f i c a t i o n f o r c l o s i n g t h i s question of the nature of the p h y s i c a l universe (viewed In to to as a closed system) by f i a t i n the context of an analysis of explanation and prediction.1 3.3.2 Hempel's Reply: Reseller's Conflation of Explanation with Retrodiction Hempel agrees with Rescher that a given system might, well be governed by laws which permit deductive inferences concerning the future, but not concerning the past, or conversely. As a schematic i l l u s t r a t i o n , 2 he introduces for consideration a model 'world' which c o n s i s t s simply of a sequence of colours, namely Blue (B), Green (G), Red (R), and Yellow (Y), which appear on a screen during successive one-second i n t e r v a l s i i , 12' ^3' - • • • L e t the succession of colours be governed by three laws: (Li) -B i s always followed by G. ( L 2 ) G and R are always followed by Y. 3 ( L 3 ) Y i s always followed by R. iRescher (1958), p. 284. 2Hempel (1962a), pp. 114-115. ^This law i s a l i t t l e misleading. The 'and' i n i t should not be interpreted as a conjunction sign i n l o g i c . In f a c t , 56 Then, g i v e n t h e c o l o u r o f t h e s c r e e n f o r a c e r t a i n i n t e r v a l , say i 3 , t h e s e laws u n e q u i v o c a l l y d e t e r m i n e t h e ' s t a t e o f t h e w o r l d , ' i . e . , t h e s c r e e n c o l o u r , f o r a l l l a t e r i n t e r v a l s , b u t n o t f o r a l l e a r l i e r ones. F o r example, g i v e n t h e i n f o r m a t i o n t h a t d u r i n g i 3 t h e s c r e e n i s Y, t h e l a w s p r e d i c t t h e c o l o u r s f o r t h e subsequent i n t e r v a l s u n i q u e l y as RYRYRY . . .. , b u t f o r t h e p r e c e d i n g s t a t e s i ^ and ±2/ t h e y y i e l d no u n i q u e i n f o r m a t i o n s i n c e t h e y a l l o w h e r e two p o s s i b i l i t i e s : BG and YR. Thus, i t i s p o s s i b l e t h a t a s e t o f laws g o v e r n i n g a c e r t a i n s ystem s h o u l d p e r m i t u n i q u e d e d u c t i v e p r e d i c t i o n s o f l a t e r s t a t e s f rom a g i v e n one, and y e t n o t y i e l d u n i q u e d e d u c t i v e r e t r o d i c t i o n s c o n c e r n i n g e a r l i e r s t a t e s ; c o n v e r s e l y , a s e t o f law s may p e r m i t u n i q u e r e t r o d i c t i o n , b u t no u n i q u e p r e d i c t i o n . B u t — a n d h e r e l i e s what Hempel t h i n k s i s t h e f l a w i n R e s c h e r ' s a r g u m e n t — t h i s i s by no means t h e same t h i n g as t o say t h a t such l a w s , w h i l e p e r m i t t i n g d e d u c t i v e p r e d i c t i o n o f l a t e r s t a t e s f r o m a g i v e n one, do n o t p e r m i t e x p l a n a t i o n ; o r , i n t h e c o n v e r s e c a s e , t h a t w h i l e p e r m i t t i n g e x p l a n a t i o n , t h e y do n o t p e r m i t p r e d i c t i o n . To i l l u s t r a t e by r e f e r e n c e t o t h e above s i m p l e model w o r l d : Suppose t h a t d u r i n g i 3 we f i n d t h e s c r e e n t o be Y, and t h a t we seek t o e x p l a i n t h i s f a c t . T h i s can be done i f we can a s c e r t a i n , f o r example, i t s h o u l d be d i v i d e d i n t o two s u b l a w s : ( L 2 Q ) G i s alw a y s f o l l o w e d by Y. ( L ^ ) R i s alw a y s f o l l o w e d by Y. 57 t h a t the c o l o u r f o r had been B. For from the statement of t h i s p a r t i c u l a r antecedent f a c t , we can i n f e r , by means of L j , t h a t the c o l o u r f o r ±2 must have been G and hence, by 1>2, t h a t the c o l o u r f o r i3 had t o be Y. E v i d e n t l y , the same argument, used b e f o r e i 3 , c o u l d serve t o p r e d i c t u n i q u e l y the c o l o u r f o r i 3 on the b a s i s of t h a t f o r Indeed, q u i t e g e n e r a l l y , any p r e d i c t i v e argument made p o s s i b l e by the laws f o r our model world can a l s o be used f o r e x p l a n a t o r y purposes and v i c e v e r s a . And t h i s i s so a l t h o u g h those laws, w h i l e p e r m i t t i n g unique p r e d i c t i o n s , do not always p e r m i t unique r e t r o d i c t i o n s . Thus, R e s e l l e r ' s o b j e c t i o n misses i t s p o i n t because i t t a c i t l y confounds e x p l a n a t i o n w i t h r e t r o d i c t i o n . The n o t i o n o f s c i e n t i f i c r e t r o d i c t i o n , however, i s o f i n t e r e s t i n i t s own r i g h t , and, as i n the case o f e x p l a n a t i o n and p r e d i c t i o n , one important v a r i e t y o f i t i s the d e d u c t i v e -n o m o l o g i c a l one. I t has the D-N form, but w i t h i t s antecedent c o n d i t i o n s C]_, C2 / • • • , r e f e r r i n g t o c i r c u m s t a n c e s which occur l a t e r than the event s p e c i f i e d i n the explanandum E. In astronomy, an i n f e r e n c e l e a d i n g , by means of laws of c e l e s t r i a l mechanics, from" d a t a c o n c e r n i n g the p r e s e n t p o s i t i o n s and movements o f the sun, t h e e a r t h , and Mars t o a statement of the d i s t a n c e between' e a r t h and Mars a year l a t e r or a year e a r l i e r i l l u s t r a t e s d e d u c t i v e - n o m o l o g i c a l p r e d i c t i o n and r e t r o d i c t i o n r e s p e c t i v e l y . In t h i s case, t h e same laws can be used f o r both purposes because the p r o c e s s e s are r e v e r s i b l e . 58 3.4 A C r i t i c a l Evaluation of Scriven's C r i t i c i s m s against  Grunbaum's Interpretation of the Symmetry Thesis Against Grunbaum's i n t e r p r e t a t i o n of the Hempelian t h e s i s , ^ Scriven o f f e r s the following c r i t i c i s m s . 3.4.1 The Predictiori-Retrodiction Asymmetry Scriven complains that neither does he agree with a l l of Grunbaum's very abstract account of the conditions which make the p r e d i c t i o n - r e t r o d i c t i o n asymmetry p o s s i b l e i n terms of entropy, nor i s he c l e a r how much of i t i l l u m i n a t e s the l o g i c a l issue. On t h i s , I tend to agree with Scriven for I too f i n d the entropy account much more d i f f i c u l t t o understand than the l o g i c a l issue i t s e l f . The t r u t h i s that the footprint-on-the-beach example can be properly understood as a t y p i c a l case of an asymmetry between p r e d i c t i o n and r e t r o d i c t i o n without n e c e s s a r i l y involving the concept of entropy at a l l . Following Scriven, the l o g i c a l i s sue can thus be stated simply as follows: If we have laws of the form: E's are always preceded by P's (A) connecting v a r i a b l e s f o r which a law of the form: E's are always succeeded by P's (B) does not hold, then r e t r o d i c t i o n w i l l be possible when p r e d i c t i o n i s not. 2 •^Cf. sec. 3.1.3. 2 S c r i v e n (1963), p. 99. 59 3.4.2 I s Grunbaum's I n t e r p r e t a t i o n o f t h e H e m p e l i a n T h e s i s  a L e g i t i m a t e One? I n S c r i v e n ' s o p i n i o n , Grunbaum's i n t e r p r e t a t i o n o f t h e Hempelian t h e s i s as a symmetry between H - e x p l a n a t i o n s and H - p r e d i c t i o n s , and n o t between e x p l a n a t i o n s and p r e d i c t i o n s s i m p l i c i t e r , i s n o t l e g i t i m a t e . He t h i n k s t h a t " s i n c e i n Hempel's a c c o u n t o f p r e d i c t i o n s some r e t r o d i c t i o n s a r e p r e d i c t i o n s , " 1 t h i s i s t o be a r e a s o n f o r r e j e c t i n g t h e a c c o u n t r a t h e r t h a n a ground f o r i n t r o d u c i n g t h e t e r m 'Hempelian p r e d i c t i o n " and s a y i n g t h a t Hempel's c l a i m i s t h a t some r e t r o d i c t i o n s a r e H - p r e d i c t i o n s , n o t p r e d i c t i o n s . I n t h e same v e i n b u t i n more d e t a i l , F e t z e r a r g u e s : U n f o r t u n a t e l y , Hempel p r o v i d e s no s u p p o r t f o r Grunbaum's i n c o r p o r a t i o n o f r e t r o d i c t i o n i n f o t h e symmetry t h e s i s . On t h e c o n t r a r y , Hempel e x p l i c i t l y r e p u d i a t e s t h e i d e n t i f i c a t i o n o f r e t r o d i c t i o n w i t h e x p l a n a t i o n , e m p h a s i z i n g t h a t t h e t e m p o r a l d i r e c t i o n o f b o t h e x p l a n a t o r y and p r e d i c t i v e i n f e r e n c e i s t h e same: "namely, f r o m s t a t e m e n t s c o n c e r n i n g c e r t a i n i n i t i a l and b o undary c o n d i t i o n s t o a s t a t e m e n t c o n c e r n i n g t h e s u b s e q u e n t o c c u r r e n c e o f t h e explanandum-event."-^ C o n s e q u e n t l y , Grunbaum's e x t e n s i o n o f t h e symmetry t h e s i s t o encompass r e t r o d i c t i o n i s u n w a r r a n t e d when advanced as a c l a r i f i c a t i o n o f t h e H - t h e s i s . 2 15Hempel, ' D e d u c t i v e - N o m o l o g i c a l v s . S t a t i s t i c a l E x p l a n a t i o n ' , pp. 115-6 ( e s p e c i a l l y f n . 1 9 ) . S c r i v e n s t i p u l a t e s f u r t h e r t h a t i f H - p r e d i c t i o n d i d n o t i n c l u d e r e t r o d i c t i o n s b u t i n c l u d e what he c a l l s ' p o s t d i c t i o n s , ' t h e n he would r a i s e v i r t u a l l y no s e r i o u s o b j e c t i o n . A p o s t d i c t i o n , i n h i s s e n s e , ! l b i d . , p. 100. 2 F e t z e r (1974) , pp. 178 -179. 60 i s an inference from the data of tn to a l a t e r occurrence at t^t made at t>, ( t _ 2 > t i ) - E-9'/> w e now i n f e r that an ec l i p s e occurred i n B.C. 307 from Babylonian data of an e a r l i e r period. This i s not p r e d i c t i o n , but i t does appear to be i d e n t i c a l i n every respect that i s relevant to a discussion of inference. No such claim can be made about r e t r o d i c t i o n because the law used there are, Of  necessity, temporally reversed compared to those used i n pr e d i c t i o n . That i s obviously a relevant and serious difference i f we are t a l k i n g about temporal symmetries.^ Ultimately, Scriven aims at demonstrating that Grunbaum's attempt to save Hempel's the s i s by bringi n g i n H-explanations and H-predictions cannot dismiss the counterexamples raised against the symmetry by showing that they are not examples of H-explanations or H-predictions, fo r Scriven's examples are hot attacks on the symmetry between H-explanations and H-predictions. What the attempt does show i s only the weakness of the H-analysis of explanations and. pred i c t i o n s . I am not at a l l persuaded by the foregoing c r i t i c i s m s which I am going to answer r e s p e c t i v e l y . To begin with, nowhere i n Hempel's account of pre d i c t i o n can I f i n d the assertion that some r e t r o d i c t i o n s are p r e d i c t i o n s . What I do f i n d are passages which strongly hint at, i f not p l a i n l y e s t a b l i s h , the f a c t that r e t r o d i c t i o n s and predictions are two c l e a r l y d i f f e r e n t things. For example: In astronomy, an inference leading, by means of the laws of c e l e s t i a l mechanics, from data concerning the present positions and movements of the sun, the earth, and Mars IScriven (1963), p. 101. 61 to a statement of the distance between earth and Mars a year l a t e r or a year e a r l i e r i l l u s t r a t e s deductive-nomological p r e d i c t i o n and r e t r o d i c t i o n r e s p e c t i v e l y ; the same laws can be used for both purposes because the processes involved are r e v e r s i b l e . 1 This kind of account of further phenomena which i s made possible by a nomological explanation i s not l i m i t e d to future events; i t may r e f e r to the past as w e l l . For example, given c e r t a i n information about the present locations and v e l o c i t i e s of the c e l e s t i a l bodies involved, the p r i n c i p l e s of Newtonian mechanics and of o p t i c s y i e l d not only predictions about the future s o l a r and lunar e c l i p s e s , but also "postdictions," or " r e t r o d i c t i o n s , " about past ones.2 In f a c t , the only way I can make sense of the a s s e r t i o n that some r e t r o d i c t i o n s are p r e d i c t i o n s , i f Hempel did make such an assertion, i s to i n t e r p r e t 'predictions' as 'H-predictions,' i n which case they w i l l include r e t r o d i c t i o n s . My reply to the c r i t i c i s m that Grunbaum's extension of the symmetry thesis to encompass r e t r o d i c t i o n i s unwarranted when advanced as a c l a r i f i c a t i o n of Hempel's t h e s i s goes as follows. Three very important facts concerning the i n t e r -r e l a t i o n s h i p s between explanation, p r e d i c t i o n , and r e t r o d i c t i o n must be c l e a r l y borne i n mind. F i r s t , Hempel i s e x p l i c i t l y aware that predictions and r e t r o d i c t i o n s are not always symmetrical (Fact 1), for . . . i t i s possible that a set of laws governing a given system should permit unique deductive p r e d i c t i o n s of l a t e r states from a given one, and yet not y i e l d unique deductive r e t r o d i c t i o h s concerning e a r l i e r s tates; conversely, a set of laws may permit unique r e t r o d i c t i o n , but no unique p r e d i c t i o n . 3 l l b i d . , p.' 116. 2 H empel (1965b), p. 303. 3Hempel (1962a), p. 115. 62 Second, t o a s s e r t F a c t 1 i s by no means t h e same t h i n g as t o s a y t h a t such l a w s , w h i l e p e r m i t t i n g d e d u c t i v e p r e d i c t i o n o f l a t e r s t a t e s from a g i v e n one, do n o t p e r m i t e x p l a n a t i o n ; o r , i n t h e c o n v e r s e c a s e , t h a t w h i l e p e r m i t t i n g e x p l a n a t i o n , t h e y do n o t p e r m i t p r e d i c t i o n . . . .. I n d e e d , q u i t e g e n e r a l l y , any p r e d i c t i v e argument made p o s s i b l e by t h e laws f o r our model w o r l d can a l s o be used f o r e x p l a n a t o r y p u r p o s e s and v i c e v e r s a . And t h i s i s so a l t h o u g h t h o s e l a w s , w h i l e p e r m i t t i n g u n i q u e p r e d i c t i o n s , do n o t a l w a y s p e r m i t u n i q u e r e t r o d i c t i o n s . 1 ( F a c t 2) I n o t h e r words. F a c t 2 s t a t e s t h a t e x p l a n a t i o n i s s y m m e t r i c a l w i t h p r e d i c t i o n . And t h i r d l y , as F e t z e r has a l s o n o t e d , Hempel e x p l i c i t l y r e p u d i a t e s t h e i d e n t i f i c a t i o n 2 o f r e t r o d i c t i o n w i t h e x p l a n a t i o n ( F a c t 3 ) : "To r e e m p h a s i z e t h e p o i n t a t i s s u e : p o s t d i c t i o n , o r r e t r o d i c t i o n , i s n o t t h e same t h i n g as e x p l a n a t i o n . " 3 Does t h e a s s e r t i o n o f F a c t 3 e s t a b l i s h , as F e t z e r has a r g u e d , t h a t t h e r e i s no p l a c e f o r r e t r o d i c t i o n i n Hempel's symmetry t h e s i s ? Not n e c e s s a r i l y , f o r i t does n o t e x c l u d e • t h e p o s s i b i l i t y t h a t r e t r o d i c t i o n can be i n c o r p o r a t e d i n t o e x p l a n a t i o n . And i t i s t h i s p o s s i b i l i t y , i n my o p i n i o n , t h a t p r o v i d e s a leeway i n w h i c h Grunbaum's i n t e r p r e t a t i o n o f t h e symmetry t h e s i s i s b u i l t on. A c u r s o r y e x a m i n a t i o n o f t h e above t h r e e f a c t s does n o t appear t o p r o v i d e any s u b s t a n t i a l grounds i n s u p p o r t o f i l b i d . 2 ' I d e n t i t y ' here means s t r u c t u r a l i d e n t i t y o r s t r u c t u r a l symmetry as I have been u s i n g t h r o u g h o u t t h e t h e s i s , b u t n o t s t r i c t i d e n t i t y i n t h e l o g i c a l s e n s e . 3Hempel (1962a), n. 19, p.116. 63 Grunbaum's i n t e r p r e t a t i o n of Hempel's t h e s i s . But we have overlooked one equally important f a c t 7 namely that there are many other cases i n which p r e d i c t i o n and r e t r o d i c t i o n are symmetrical (Fact l ' ) ; t h a t i s to say, cases (e.g. i n astronomy) i n which a set of laws which permit unique deductive predictions of l a t e r states also y i e l d unique deductive r e t r o d i c t i o n s concerning e a r l i e r states and v i c e versa. "For example, given c e r t a i n information about the present locations and v e l o c i t i e s of the c e l e s t i a l bodies involved, the p r i n c i p l e s of Newtonian mechanics and of o p t i c s y i e l d not only predictions about the future s o l a r and lunar e c l i p s e s , but also 'predictions,' or ' r e t r o d i c t i o n s , ' about past ones."^ In the l i g h t of t h i s newly established Fact 1', l e t us re-examine the whole s i t u a t i o n . I f Hempel holds both Fact 1' and Fact 2, then he cannot also hold Fact 3, f o r the former two f a c t s , i n conjunction, e n t a i l that r e t r o d i c t i o n i s symmetrical with explanation which i s i n c o n s i s t e n t with Fact 3. But Hempel does e x p l i c i t l y hold Fact 3. To preserve consistency, the best option open to him i s to d i f f e r e n t i a t e the sense of 'prediction' i n Fact 1' from the sense of 'prediction' i n Fact 2. This i s probably what induces Grunbaum to introduce a symmetry between H-explanation and H-prediction (corresponding to Fact 2) as d i s t i n g u i s h e d from the symmetry and sometimes asymmetry between p r e d i c t i o n and r e t r o d i c t i o n (corresponding to Fact 1 and Fact 1' r e s p e c t i v e l y ) . 3-Ibid. , p. 115. 64 But i s Grunbaum's i n t e r p r e t a t i o n of the Hempelian the s i s g u i l t y of moving too f a r away from Hempel's o r i g i n a l intention? I don't think so f o r two good reasons. F i r s t , i n h i s o r i g i n a l exposition of the symmetry t h e s i s , ^ Hempel maintains that we speak of an explanation of the explanandum event i f the l a t t e r i s given and the explanans i s provided  afterwards. On the other hand, we speak of a p r e d i c t i o n when the predictans i s given and. the predictandum event i s derived p r i o r to the occurrence of the event. I t may be stretching too f a r to say that these two statements confirm beyond doubt Grunbaum's contention that "Hempel's c r i t e r i o n for an explanation as opposed to a p r e d i c t i o n i s that E belong to the s c i e n t i s t ' s past when he o f f e r s h i s account of i t , and h i s c r i t e r i o n for a corresponding p r e d i c t i o n i s that E belong to the s c i e n t i s t ' s future when i t i s made."2 But at l e a s t i t i s not s t r i c t l y s p e c i f i e d that i n explanation, . the explanandum event must occur a f t e r the explanans, and i n p r e d i c t i o n , the predictans must occur before the predictandum event. Modestly put, Grunbaum's i n t e r p r e t a t i o n of the symmetry thesis i s at le a s t not in c o n s i s t e n t with Hempel's o r i g i n a l account of i t . Secondly, i n response to Rescher's c r i t i c i s m that the symmetry thesis rests upon a t a c i t but unwarranted assumption as to the nature of the phy s i c a l universe, Hempel r e p l i e s iHempel and Oppenheim (1965), p. 249. 2Grunbaum (1963), p. 72. 65 t h a t t h e t h e s i s "makes a p u r e l y l o g i c a l p o i n t ; i t does n o t depend on any e m p i r i c a l assumptions."''- T h i s d e n i e s R e s e l l e r ' s c o n t e n t i o n t h a t t h e e x p l a n a t i o n o f e v e n t s i s o r i e n t e d t owards t h e p a s t w h i l e p r e d i c t i o n i s o r i e n t e d t o w a r d s t h e f u t u r e , i n w h i c h c a s e p h y s i c a l laws do p l a y an e s s e n t i a l p a r t . B u t s i n c e Hempel r e b u t s t h i s , i t i s n a t u r a l and r e a s o n a b l e t o assume t h a t h i s c o n c e p t o f e x p l a n a t i o n i s n o t s t r i c t l y o r i e n t e d towards t h e p a s t whereas h i s c o n c e p t o f p r e d i c t i o n i s n o t s o l e l y o r i e n t e d towards t h e f u t u r e . T h i s i s ample e v i d e n c e i n s u p p o r t o f Grunbaum's c o n t e n t i o n t h a t Hempelian e x p l a n a t i o n and p r e d i c t i o n a r e c o n c e r n e d w i t h t h e s c i e n t i s t ' s p a s t and f u t u r e r e s p e c t i v e l y . C o n s e q u e n t l y , p r e d i c t i o n and r e t r o d i c t i o n can e x i s t i n b o t h H - e x p l a n a t i o n and H - p r e d i c t i o n . I n r e p l y t o S c r i v e n ' s s u g g e s t i o n o f u s i n g p o s t d i c t i o n s i n p l a c e o f r e t r o d i c t i o n s , I can o n l y say t h a t h i s s u g g e s t i o n i s n o t a good one. To b e g i n w i t h , t h e f a c t t h a t r e t r o d i c t i o n u ses laws w h i c h a r e t e m p o r a l l y r e v e r s e d compared t o t h o s e used i n p r e d i c t i o n has no a d v e r s e b e a r i n g a t a l l on Hempel's t h e s i s w h i c h o n l y c o n c e r n s t h e t e m p o r a l symmetry between e x p l a n a t i o n and p r e d i c t i o n . As a m a t t e r o f f a c t , a s Grunbaum has shown, r e t r o d i c t i o n and p r e d i c t i o n might sometimes even be a s y m m e t r i c a l . We w i l l t u r n n e x t t o a more s e r i o u s o b j e c t i o n . I f ' p o s t d i c t i o n s ' a r e t o r e p l a c e r e t r o d i c t i o n s i n t h e Grunbaumian 1Hempel (1962a), p. 114. 66 schema, they are a disastrous f a i l u r e . Judging by t h e i r very d e f i n i t i o n , they correspond only to 'prediction' i n H-explanation and not even 'prediction' i n H-prediction, say alone of ' r e t r o d i c t i o n ' i n eit h e r H-explanation and H-prediction. This being the case, they can hardly claim to be a better replacement than r e t r o d i c t i o n s . What I hope to have demonstrated by the foregoing discussion i s Scriven's f a i l u r e to weaken the p l a u s i b i l i t y of Grunbaum's i n t e r p r e t a t i o n of the symmetry t h e s i s by showing i t to be non-Hempelian. Granting t h i s , I think Grunbaum i s more than j u s t i f i e d to refute those counterexamples of the symmetry thesis when they are not examples of H-explanation or H-prediction. 67 CHAPTER FOUR FIRST SUB-THESIS: EVERY ADEQUATE EXPLANATION IS A POTENTIAL PREDICTION 4.1: E v o l u t i o n a r y Theory- 1 S h a r i n g Mayr's c o n v i c t i o n t h a t " t h e t h e o r y o f n a t u r a l s e l e c t i o n can d e s c r i b e and e x p l a i n phenomena w i t h c o n s i d e r a b l e p r e c i s i o n , b u t i t c a n n o t make r e l i a b l e p r e d i c t i o n s , " 2 S c r i v e n m a i n t a i n s t h a t t h e most i m p o r t a n t l e s s o n t o be l e a r n e d f r o m e v o l u t i o n a r y t h e o r y i s t h a t " s a t i s f a c t o r y e x p l a n a t i o n o f t h e p a s t i s p o s s i b l e even when p r e d i c t i o n o f t h e f u t u r e i s i m p o s s i b l e . " 3 I n s u p p o r t o f t h i s p o i n t , he t e l l s us t h a t a c a r e f u l d i s s e c t i o n o f t h e p r i n c i p l e o f " t h e s u r v i v a l o f t h e f i t t e s t " w i l l i n v a r i a b l y l e a d us t o s uch a c o n c l u s i o n : I t i s f a i r l y o b v i o u s t h a t no c h a r a c t e r i s t i c s c a n be i d e n t i f i e d as c o n t r i b u t i n g t o ' f i t n e s s * i n a l l e n v i r o n m e n t s . . . . £l]t i s q u i t e c l e a r t h a t we c a n n o t p r e d i c t w h i c h o r g a n i s m s w i l l s u r v i v e e x c e p t i n so f a r as we can p r e d i c t t h e e n v i r o n m e n t a l changes. B u t we a r e p o o r l y e q u i p p e d t o do t h i s w i t h much p r e c i s i o n s i n c e v a r i a t i o n s i n t h e sun's o u t p u t and even i n t e r s t e l l a r i n f l u e n c e s have s u b s t a n t i a l e f f e c t s , q u i t e a p a r t f r om t h e l o c a l i r r e g u l a r i t i e s o f g e o l o g y and c l i m a t e . However, l l n t h e p r e s e n t t h e s i s , ' e v o l u t i o n a r y t h e o r y ' i s t o be u n d e r s t o o d as r e f e r r i n g o n l y t o t h e t h e o r y o f n a t u r a l s e l e c t i o n , b u t n o t as a g e n e r a l term w h i c h can be u s e d t o r e f e r t o o t h e r t h e o r i e s o f e v o l u t i o n . 2 M a y r (1961), p. 1504. 3 S c r i v e n (1959a), s u b t i t l e o f t h e a r t i c l e , p. 477. 68 these d i f f i c u l t i e s of p r e d i c t i o n do not mean that the idea of f i t n e s s as a factor i n s u r v i v a l loses a l l i t s explanatory power. I t i s not only true but obvious that animals which happen to be able to swim are b e t t e r f i t t e d for surviving a sudden and unprecedented inundation of t h e i r a r i d habitat, and i n some such cases i t i s j u s t t h i s factor which explains t h e i r s u r v i v a l . N a t u r a l l y we could have said i n advance that i f a f l o o d occurred, they would be l i k e l y to survive; l e t us c a l l t h i s a hypothetical p r o b a b i l i t y p r e d i c t i o n . But hypothetical p r e d i c t i o n s do not have any value for actual p r e d i c t i o n except i n so far as the conditions mentioned in the hypothesis are predictable or experimentally producible: hence there w i l l be cases where we can explain why c e r t a i n animals and plants survived even when we could not have predicted  that they would.1 Equipped with t h i s supposedly proven u n t e n a b i l i t y of hypothetical p r o b a b i l i t y predictions whose v a l i d i t y r e l i e s on a "somewhat unhelpful sense of ' i n p r i n c i p l e ' , " 2 Scriven proceeds to e s t a b l i s h the existence of an asymmetry damaging to Hempel's thesis on the basis of the account of a case of nonsurvival given i n evolutionary theory: There are good grounds for saying we cannot even i n  p r i n c i p l e p r e d i c t everything (uncertainty p r i n c i p l e , c l a s s i c a l u n p r e d i c t a b i l i t y of a computer's s t a t e ) ; hence, good grounds for saying that even i n p r i n c i p l e explanation and p r e d i c t i o n do not have the same form. F i n a l l y , i t i s not i n general possible to l i s t a l l the exceptions to a claim about, for example, the f a t a l e f f e c t s of a lava flow, so we have to leave i t i n p r o b a b i l i t y form; t h i s has the r e s u l t of e l i m i n a t i n g the very degree of c e r t a i n t y from the p r e d i c t i o n that the explanation has, when we f i n d the f o s s i l s i n the l a v a . 3 I f Scriven was r i g h t i n claiming that evolutionary theory could only explain but not p r e d i c t , i t would appear that Darwin was not a s c i e n t i s t i n the c l a s s of Newton 1 I b i d . , p. 478. 2 I b i d . , p. 479. 3 I b i d . , p. 480. 69 whose theory c o u l d do b oth. In s a l v a g i n g the s c i e n t i f i c s t a t u s o f Darwin, S c r i v e n promptly p o i n t s out t h a t Darwin was o p e r a t i n g i n a f i e l d of a w h o l l y d i f f e r e n t k i n d from N ewton's—a f i e l d t h a t belongs to the group of what he c a l l s ' i r r e g u l a r s u b j e c t s . ' Thus, he urges t h a t " i n p l a c e of t h e s o c i a l s c i e n t i s t s ' f a v o r i t e Myth of the Second Coming (of Newton), we s h o u l d r e c o g n i z e the R e a l i t y o f the A l r e a d y -A r r i v e d (Darwin); the paradigm of the e x p l a n a t o r y but n o n p r e d i c t i v e s c i e n t i s t . In a s i m i l a r frame o f mind speaks Toulmin who makes r e f e r e n c e t o Darwin's t h e o r y , e x p l a i n i n g the o r i g i n o f s p e c i e s by v a r i a t i o n and n a t u r a l s e l e c t i o n . No s c i e n t i s t has e v e r used t h i s t h e o r y to f o r e t e l l the c o m i n g - i n t o - e x i s t e n c e o f c r e a t u r e s o f a n o v e l s p e c i e s , s t i l l l e s s v e r i f i e d h i s f o r e c a s t . Yet many competent s c i e n t i s t s have a c c e p t e d Darwin's t h e o r y as h a v i n g g r e a t e x p l a n a t o r y power.2 4.1.1 Reply t o C r i t i c i s m s 4.1.1.1 Reply t o S c r i v e n S c r i v e n i s c o m p l e t e l y j u s t i f i e d i n a s s e r t i n g t h a t t h e r e are cases of p o s t - e x p l a i n i n g s u r v i v a l on the b a s i s o f f i t n e s s i n which we can e x p l a i n why but not p r e d i c t t h a t , f o r f u t u r e f i t n e s s and s u r v i v a l depend on f u t u r e i n t e r a c t i o n s which cannot be p r e d i c t e d from g i v e n i n f o r m a t i o n , whereas p o s t l l b i d . , p. 477. 2 T o u l m i n (1961), pp. 24-25. 70 f i t n e s s and s u r v i v a l depend on p a s t i n t e r a c t i o n s w h i c h can be r e t r o d i c t e d from t h a t same i n f o r m a t i o n . I n t y p i c a l Grunbaumian f a s h i o n , t h i s i s tantamount t o a s s e r t i n g t h a t " t h e r e i s an e p i s t e m o l o g i c a l asymmetry between H - e x p l a n a t i o n and H - p r e d i c t i o n i n r e g a r d t o t h e a s s e r t i b i l i t y b o t h o f t h e a n t e c e d e n t f i t n e s s a f f i r m e d i n t h e e x p l a n a n s and o f t h e explanandum c l a i m i n g s u r v i v a l . B u t as soon as S c r i v e n v e n t u r e s f u r t h e r t o c l a i m t h a t such c a s e s a r e h a r m f u l t o Hempel's symmetry t h e s i s , he i s no l o n g e r on s a f e g r o unds. F o r , as Grunbaum s a y s , t h e s c i e n t i f i c i n f e r a b i l i t y from a c a u s e , and hence o u r u n d e r s t a n d i n g o f t h e why o f s u r v i v a l f u r n i s h e d by an e x p l a n a n s w h i c h does c o n t a i n t h e a n t e c e d e n t c o n d i t i o n t h a t t h e g i v e n a n i m a l s a r e a b l e t o swim d u r i n g a sudden, u n p r e c e d e n t e d i n u n d a t i o n o f t h e i r a r i d h a b i t a t , i s h o t one i o t a more p r o b a b i l i s t i c ( i . e . , l e s s c o n c l u s i v e ) i n t h e c a s e o f a p a s t one. F o r i f t h e l o g i c a l n e r v e o f i n t e l l i g i b i l i t y l i n k i n g t h e e x p l a n a n s ( f i t n e s s u nder s p e c i f i e d k i n d s o f i n u n d a t i o n a l c o n d i t i o n s ) w i t h t h e explanandum ( s u r v i v a l ) i s o n l y p r o b a b i l i s t i c i n t h e f u t u r e c a s e , how c o u l d i t p o s s i b l y be any l e s s p r o b a b i l i s t i c i n t h e p a s t c a s e ? I t i s e v i d e n t t h a t p o s t e x p l a n a t p r y i n d u c t i v e i n f e r a b i l i t y i s e n t i r e l y on a p a r h e r e w i t h p r e d i c t i v e i n f e r a b i l i t y f r o m f i t n e s s as a cause.2 I n c i d e n t a l l y , S c r i v e n i s m i s t a k e n i n t h i n k i n g t h a t we can e x p l a i n why c e r t a i n a n i m a l s have s u r v i v e d when we c o u l d n o t have p r e d i c t e d t h a t t h e y w o uld. F o r even i f we c o u l d have p r e d i c t e d t h e f l o o d , we c o u l d n o t have p r e d i c t e d a n y t h i n g about i n d i v i d u a l a n i m a l s . We c o u l d have p r e d i c t e d o n l y t h a t more a n i m a l s w h i c h can swim would s u r v i v e t h a n t h o s e w h i c h J-Grttnbaum (19 63) , p. 84. 2 I b i d . , p. 85. 71 c o u l d n o t . By t h e same t o k e n , we ca n n o t e x p l a i n why a p a r t i c u l a r a n i m a l happened t o s u r v i v e , f o r t h e r e a r e many o t h e r f a c t o r s b e s i d e s swimming a b i l i t y t h a t c o n t r i b u t e t o s u r v i v a l . T h a t i s i n d e e d why we can o n l y s t a t e a ' h y p o t h e t i c a l p r o b a b i l i t y p r e d i c t i o n ' s i n c e o u r knowledge o f t h e s i t u a t i o n i s i n c o m p l e t e . I f t h a t i s t h e c a s e , t h e n S c r i v e n ' s a t t e m p t t o e s t a b l i s h t h e e x i s t e n c e , v i a t h e l a v a c a s e , o f an asymmetry h a r m f u l t o Hempel's t h e s i s b r e a k s down, f o r t h e l a v a c a s e o n l y e n t i t l e s h im t o c o n c l u d e t h a t t h e m e r e l y p r o b a b i l i s t i c c o n n e c t i o n between t h e o c c u r r e n c e o f a l a v a f l o w and t h e e x t i n c t i o n o f c e r t a i n o r g a n i s m s has t h e r e s u l t o f d e p r i v i n g p r e a s s e r t i b i 1 1 t y o f t h e v e r y degree o f c e r t a i n t y p o s s e s s e d by p o s t a s s e r t i o n s , h e r e . However, he i s n o t a t a l l j u s t i f i e d i n s u p p o s i n g t h a t p r e d i c t i v e i n f e r a b i l i t y i n t h i s c a s e l a c k s t h e c e r t a i n t y t h a t can be a s c r i b e d t o t h e c o r r e s p o n d i n g p o s t e x p l a n a t o r y i n f e r a b i l i t y . F o r t h e g r e a t e r degree o f c e r t a i n t y o f t h e l a t t e r r e s i d e s o n l y i n t h e a s s e r t i b i l i t y o f t h e explanandum, b u t n o t i n t h e l o g i c a l r e l a t i o n between t h e e x p l a n a n s ( t h e l a v a f l o w ) and t h e explanandum ( f a t a l i t i e s on t h e p a r t o f c e r t a i n o r g a n i s m s ) . Thus, we s h o u l d now r e a l i z e t h a t S c r i v e n ' s c o n t e n t i o n o f an asymmetry i n t h e c e r t a i n t y o f e x p l a n a t i o n and p r e d i c t i o n i s i n i t i a t e d by a c o n f u s i o n between t h e f o l l o w i n g two r a d i c a l l y d i s t i n c t k i n d s o f asymmetry: ( i ) a d i f f e r e n c e i n t h e degree o f c e r t a i n t y ( c a t e g o r i c i t y ) o f our knowledge o f t h e t r u t h o f t h e explanandum and o f t h e c l a i m o f e n v i r o n m e n t a l u n f i t n e s s made by t h e e x p l a n a n s , and ( i i ) a difference i n the "degree of entailment" as i t were, l i n k i n g the explanandum to the explanans. (This section should be read i n conjunction with Section 6.2.2 because important concepts mentioned here, e.g. hypothetical p r o b a b i l i t y p r e d i c t i o n s , w i l l be discussed i n greater d e t a i l i n the l a t e r section. The same goes f o r Section 4.1.1.2 below.) 4.1.1.2 Reply to Toulmin Toulmin's c r i t i c i s m deserves close s c r u t i n y because i t t y p i f i e s some common misinterpretations of Darwin's theory by philosophers who are too p h i l o s o p h i c a l l y minded to bother to read the relevant s c i e n t i f i c texts on the subject. F i r s t and foremost, as Mayr t e l l s us, Darwin f a i l e d to solve the problem indicated by the t i t l e of h i s masterpiece. Although he demonstrated the modification of species i n the time dimension, he never s e r i o u s l y attempted a rigorous analysis of the problem of the m u l t i p l i c a t i o n of species and of the s p l i t t i n g of one species in t o two. His f a i l u r e to deal with the problem of the o r i g i n of species was mainly due to h i s lack of understanding of the nature of species. Secondly, i t is' simply f a c t u a l l y i n c o r r e c t to say that no s c i e n t i s t has ever used t h i s theory to f o r e t e l l the coming-into-existence of creatures of a novel species, s t i l l l l b i d . , p. 83. 2See Mayr (1963), p. 12. 73 l e s s v e r i f i e d h i s f o r e c a s t . As T o u l m i n h i m s e l f has t o l d u s , when A u s t r a l i a n s used myxomatosis t o c o n t r o l t h e r a b b i t p o p u l a t i o n , i t was f o r e c a s t on t h e b e s t D a r w i n i a n p r i n c i p l e s t h a t a new s t r a i n o f r a b b i t s w o u l d become dominant, whose c o n s t i t u t i o n s were more r e s i s t a n t t o t h e d i s e a s e t h a n t h e a v e r a g e members o f t h e o r i g i n a l p o p u l a t i o n . The c o r r e c t n e s s o f t h i s p r e d i c t i o n has h e l p e d t o c o n f i r m t h e m e r i t s o f t h e D a r w i n i a n t h e o r y . And t h e same t h i n g has happened i n o t h e r s m a l l - s c a l e c a s e s i n v o l v i n g m e l anism i n moths, t h e r e a c t i o n s o f i n f e c t i v e m i c r o - o r g a n i s m s t o a n t i b i o t i c s , and so on.^ The f a c t t h a t t h e s e h a p p e n i n g s a r e s m a l l - s c a l e i n n a t u r e s h o u l d c o n s t i t u t e no q u a l i t a t i v e p r o b l e m s i n c e many t h e o r i e s , p a r t i c u l a r l y t h o s e o f a comprehensive n a t u r e l i k e Newton's o r E i n s t e i n ' s t h e o r y , do n o t a l w a y s g e t f u l l - s c a l e d i r e c t t e s t s . S i n c e Darwin's t h e o r y i s u n d o u b t e d l y v e r y c o m p r e h e n s i v e i n s c o p e , s m a l l - s c a l e t e s t s o f i t a r e t o be e x p e c t e d . I n f a c t , t e s t s c o n d u c t e d i n l a b o r a t o r i e s a r e .always s m a l l - s c a l e i n n a t u r e . . F u r t h e r m o r e , i t i s a c o n s e n s u s among modern e v o l u t i o n i s t s ( w i t h p e r h a p s t h e e x c e p t i o n o f t h e s a l t a t i o n i s t s l e d by G o l d s c h m i d t ) t h a t answers t o q u e s t i o n s -about l a r g e - s c a l e e v o l u t i o n a r y changes ( o f t e n c a l l e d t r a n s p e c i f i c e v o l u t i o n s o r m a c r o e v o l u t i o n s ) a r e t o be f o u n d i n o u r knowledge o f s m a l l - s c a l e e v o l u t i o n a r y changes ( o f t e n c a l l e d m i c r o e v o l u t i o n s ) . I n Mayr's words, " t r a n s p e c i f i c e v o l u t i o n i s n o t h i n g b u t an e x t r a p o l a t i o n and m a g n i f i c a t i o n o f t h e e v e n t s t h a t t a k e p l a c e w i t h i n p o p u l a t i o n s and s p e c i e s . " S h a r i n g t h e same v i e w i s Dobzhansky who w r i t e s : E x p e r i e n c e shows, however, t h a t t h e r e i s no way t o w a r d u n d e r s t a n d i n g o f t h e mechanisms o f m a c r o e v o l u t i o n a r y 1 T o u l m i n (1961), p. 25. 2 M a y r (1963), p. 586. 74 changes, w h i c h r e q u i r e t i m e on g e o l o g i c a l s c a l e s , o t h e r t h a n t h r o u g h u n d e r s t a n d i n g o f m i c r o e v o l u t i o n a r y p r o c e s s e s o b s e r v a b l e w i t h i n t h e span of a human l i f e t i m e , o f t e n c o n t r o l l e d by man's w i l l , and sometimes r e p r o d u c i b l e i n l a b o r a t o r y e x p e r i m e n t s . 1 T o u l m i n ' s r e a c t i o n t o t h e s e s m a l l - s c a l e c a s e s goes l i k e t h i s : I t h a s , i n f a c t , been p o s s i b l e o n l y i n t h e l a s t few y e a r s t o make p o s i t i v e , v e r i f i a b l e f o r e c a s t s on t h e b a s i s o f t h e t h e o r y - — e v e n on a s m a l l s c a l e . Y e t , r e g a r d e d as a t h e o r y , t h e m e r i t s o f D a r w i n ' s i d e a s have been a c t i v e l y d i s c u s s e d f o r a f u l l c e n t u r y ; and f o r most o f t h i s t i m e t h e i r e x p l a n a t o r y power has been u n q u e s t i o n e d . A c t u a l f o r e c a s t i n g became p o s s i b l e o n l y w i t h t h e development o f modern e c o l o g y and g e n e t i c s , y e t men d i d n o t w a i t f o r t h i s b e f o r e r e c o g n i z i n g t h e e x p l a n a t o r y m e r i t s o f t h e t h e o r y o f n a t u r a l s e l e c t i o n . 2 My r e p l y t o t h i s argument i s t w o - f o l d . F i r s t , t h e c l a i m t h a t t h e e x p l a n a t o r y power o f D a r w i n ' s t h e o r y has been u n q u e s t i o n e d i s s i m p l y an o v e r s t a t e m e n t . I t i s t r u e t h a t Darwin's t h e o r y , once p u b l i s h e d , c r e a t e d a g r e a t o u t b u r s t among b o t h men o f s c i e n c e and laymen. But t h i s came s b o u t more so because o f i t s r e l i g i o u s i m p l i c a t i o n s t h a n i t s e x p l a n a t o r y m e r i t s . 3 I t g a i n e d i n s t a n t r e c o g n i t i o n b ecause t h e t i m e was r i p e f o r such a t h e o r y ; i d e a s l i k e t h o s e o f M a l t h u s on p o p u l a t i o n , L y e l l on t h e c o n c e p t o f u n l i m i t e d <a t i m e , and De Beer on a n a l o g y and homology, a l l p aved t h e way f o r i t s i n s t a n t r e c o g n i t i o n . But such a r e c o g n i t i o n i s n o t tantamount t o s c i e n t i f i c a c c e p t a n c e , as i s e v i d e n t f rom t h e d o b z h a n s k y (1951), p. 16. 2 T o u l m i n (1961), pp. 25-26. 3 S e e , f o r example, Manser (1965), p. 20-23 and H i m m e l f a r b (1959). yearly report of 1858 made by the President of the Linnaean S o c i e t y — a f t e r Darwin and Wallace had j o i n t l y presented t h e i r ideas on natural s e l e c t i o n at a meeting of the so c i e t y on July 1—which stated that no s c i e n t i f i c a l l y s i g n i f i c a n t event had occurred during the year.! As a matter of f a c t , Darwin's theory, when f i r s t published, drew many more c r i t i c s than sympathizers among fellow s c i e n t i s t s , 2 even h i s mentor L y e l l held back f o r ten years before f i n a l l y accepting Darwin's theory i n f u l l . Furthermore, the explanatory power of t h i s theory was great l y hampered by Darwin's lack of knowledge of the causes of hereditary v a r i a t i o n , and h i s opinions on t h i s subject were neither c l e a r nor consistent. At times he accepted, and i n other writings, he rejected Lamarck's notion of the inheritance of adaptations acquired during the l i f e t i m e of an i n d i v i d u a l . Along with most p r a c t i c a l animal breeders and students of human heredity of his days, Darwin regarded the hereditary substances as f l u i d i n nature, and the observed intermediate nature of hybrids between races or breeds as r e s u l t i n g from a mixture of parental f l u i d s i n t h e i r bodies — t h e theory of blending inheritance. One of Darwin's severest c r i t i c s , Fleeming Jenkin, pointed out that s e l e c t i o n could not sort out superior f l u i d s from a mixture, and so, ^P r e s i d e n t i a l Address, Proceedings of the Linnaean Society, 1858. 2See H u l l , Tessner & Diamond (1978). For an ex c e l l e n t account of the various kinds of c r i t i c i s m s r a i s e d against Darwin's theory by his contemporaries, see H u l l (1973). 76 a c c o r d i n g t o t h e p r e v a i l i n g t h e o r y , c o u l d n o t b e t h e means b y w h i c h s u p e r i o r t y p e s c o u l d be d e r i v e d f r o m c r o s s e s b e t w e e n b r e e d s . B e c a u s e he knew s o l i t t l e a b o u t h e r e d i t y , D a r w i n c o u l d n o t p r o v i d e a s a t i s f a c t o r y a n s w e r t o t h i s c r i t i c i s m . ( I n c i d e n t a l l y , t h e t h e o r y was f u r t h e r w e a k e n e d a t t h i s t i m e b y L o r d K e l v i n ' s c a l c u l a t i o n t h a t t h e e a r t h was f a r y o u n g e r t h a n L y e l l h a d p o s t u l a t e d , t h u s s t r i p p i n g t h e t h e o r y o f t h e g e o l o g i c a l t i m e s p a n n e c e s s a r y f o r m i n u t e a n d g r a d u a l e v o l u t i o n a r y c h a n g e s . ) I t was n o t u n t i l t h e r e d i s c o v e r y o f M e n d e l ' s l a w s o f h e r e d i t y i n 1900 t h a t t h i s p r o b l e m c o u l d be s o l v e d . E v e n t h e r e d i s c o v e r y o f M e n d e l i a n g e n e t i c s d i d n o t i m m e d i a t e l y r e i n f o r c e D a r w i n ' s t h e o r y ; o n t h e c o n t r a r y , i t p l a c e d t h e l a t t e r i n t e m p o r a r y j e o p a r d y . T h i s was b e c a u s e t h e M e n d e l i a n r a t i o s w e r e n o t a l w a y s a p p l i c a b l e i n n a t u r a l p o p u l a t i o n s . F o r e x a m p l e , when we c r o s s a t a l l human b e i n g -w i t h a s h o r t o n e , we f i n d t h a t no s i m p l e M e n d e l i a n r a t i o o f t h r e e t o o n e c a n b e f o u n d i n t h e s e c o n d h y b r i d g e n e r a t i o n , s i n c e t h e d i f f e r e n c e b e t w e e n t a l l n e s s a n d s h o r t n e s s i n humans i s g o v e r n e d b y many g e n e s , e a c h one o f w h i c h h a s o n l y a s l i g h t e f f e c t o n s i z e . D a r v / i n ' s t h e o r y was b a s e d u p o n h i s s o u n d o b s e r v a t i o n t h a t m o s t d i f f e r e n c e s b e t w e e n n a t u r a l p o p u l a t i o n s a r e q u a n t i t a t i v e i n n a t u r e , b u t n e i t h e r h e n o r t h e e a r l i e r M e n d e l i a n g e n e t i c i s t s l i k e H u g o d e V r i e s a n d W. J o h a n n s e n u n d e r s t o o d why t h i s i s s o . I n f a c t , a f e w y e a r s l a t e r , t h e s e g e n e t i c i s t s b e g a n t o a d v o c a t e t h e m u t a t i o n  t h e o r y w h i c h s t i p u l a t e s t h a t e v o l u t i o n t a k e s p l a c e t h r o u g h 77 the spontaneous o r i g i n of new types which d i f f e r from t h e i r parents i n several c h a r a c t e r i s t i c s . Natural s e l e c t i o n , according to them, has only the negative function of eliminating those types which are u n f i t to survive. This mutation theory has not held up f o r a number of reasons. To begin with, the mutations which de V r i e s discovered i n the evening primrose were l a t e r found to be not the r e s u l t of new genetic v a r i a t i o n s at a l l , but merely a p e c u l i a r type of genetic segregation due to the f a c t that t h i s plant has a very anomalous type of chromosome behaviour. Later on, T. H. Morgan and h i s associates found true mutations i n the f r u i t f l y Drosophila which are a c t u a l l y spontaneous a l t e r a t i o n s of genes, and they have since been found i n a large number of d i f f e r e n t organisms. But nearly a l l mutations which produce large, conspicuous changes also make the organisms bearing them weaker and unable to compete with t h e i r unchanged associates. As a r e s u l t of the f a i l u r e of the mutation theory, Darwin's theory became the theory at the time. But t h i s by no means implies that the theory had thereby gained tremendous explanatory power since t h i s v i c t o r y was b u i l t on i t s r i v a l ' s defeat. In actual f a c t , the true t e s t s of the theory's explanatory power had to wait f o r the a r r i v a l of people l i k e J . B. S. Haldane, Sewall Wright, and i n p a r t i c u l a r R. A. Fisher, i n the f i r s t h a l f of t h i s century who were successful in synthesizing genetics with natural 78 s e l e c t i o n to produce a sound theory of e v o l u t i o n . In t h i s s o - c a l l e d neo-Darwinian synthetic theory of e v o l u t i o n , natural s e l e c t i o n has then the p o s i t i v e creative function of s o r t i n g out a few adaptive gene combinations from the i n f i n i t e number of p o s s i b i l i t i e s inherent i n the gene pool. In t h i s evolutionary framework, most of the adaptively important mutations do not produce e n t i r e l y new types, but merely add q u a n t i t a t i v e l y to the gene pool of already e x i s t i n g v a r i a t i o n s . The foregoing h i s t o r i c a l sketch of the development of Darwin's theory should s u f f i c e to show that Toulmin's s t i p u l a t i o n that the explanatory power of t h i s theory has been unquestioned for nearly a f u l l century before the emergence of modern ecology and genetics i s an overstatement. As a matter of f a c t , I would argue that i t s explanatory power becomes eminent a f t e r the emergence of modern ecology and genetics, that i s , a f t e r i t has metamorphosed i n t o the neo-Darwinian synthetic theory of evolution. There i s no doubt that between i t s f i r s t p u b l i c a t i o n and i t s l a t e r metamorphosis, most s c i e n t i s t s 'accepted' t h a t the theory had great explanatory power. But i n my opinion, t h i s 'acceptance' arose more out of f a i t h or ' s c i e n t i f i c i n t u i t i o n ' that i n time the gaps i n the theory would eventually be f i l l e d . For i n the absence of the knowledge of hereditary v a r i a t i o n s , Darwin's theory simply lacked explanatory power, since the explanation that c e r t a i n organisms survived because they were f i t t e s t to survive i s tautologous and thus devoid of 7 9 e x p l a n a t o r y power when ' f i t t e s t ' i s d e f i n e d as t h o s e t h a t s u r v i v e d . Thus, t h e e x p l a n a t o r y power o f t h e t h e o r y i s a c t u a l l y dependent upon a.n e o - D a r w i n i a n r e v i s i o n o f t h e d e f i n i t i o n o f t h e ' f i t t e r ' o r g a nisms as t h o s e w h i c h p o s s e s s a more v a r i a t e d gene p o o l t h a n i t s f e l l o w o r g a n i s m s . I n e s s e n c e , t h i s s i t u a t i o n i s a n a l o g o u s t o t h a t o f t h e a c c e p t a n c e o f t h e C o p e r n i c a n t h e o r y . Some s c i e n t i s t s t e n d t o t h i n k t h a t t h e C o p e r n i c a n t h e o r y was r i g h t w h i l e t h e P t o l e m a i c t h e o r y was wrong, and t h a t K e p l e r ' p r o v e d ' t h a t t h i s was so. But t h i s i s o v e r s i m p l i f y i n g t h e r e a l p i c t u r e . What a c t u a l l y happened was t h a t t h e P t o l e m a i c t h e o r y was as good as (or as bad as) t h e C o p e r n i c a n t h e o r y , t h a t i s t o s a y , t h e y had m o r e - o r - l e s s t h e same e x p l a n a t o r y power."'" But K e p l e r chose t o b u i l d h i s t h e o r y ( m o s t l y o u t o f a e s t h e t i c a p p e a l ) on t h e C o p e r n i c a n model. I t i s t h e s u c c e s s o f K e p l e r ' s t h e o r y t h a t g r e a t l y i n f l u e n c e s o u r r e t r o s p e c t i v e e v a l u a t i o n o f t h e C o p e r n i c a n t h e o r y as an i n t r i n s i c a l l y b e t t e r e x p l a n a t o r y t h e o r y t h a n i t s P t o l e m a i c c o u n t e r p a r t . I n l i k e manner, I would argue t h a t o u r c o n f i d e n c e i n t h e e x p l a n a t o r y power o f Darwin's t h e o r y i s more a d i r e c t r e s u l t o f t h e s u c c e s s e n j o y e d by t h e n e o - D a r w i n i a n s y n t h e t i c t h e o r y o f e v o l u t i o n t h a n o f t h e i n i t i a l e x p l a n a t o r y power o f t h e o r i g i n a l D a r w i n i a n t h e o r y i t s e l f . x N e i t h e r was t h e C o p e r n i c a n t h e o r y s i m p l e r f o r i t i n v o l v e d as many e p i c y c l e s as t h e P t o l e m a i c t h e o r y . F o r a d e t a i l e d a n a l y s i s o f t h i s p o i n t , see Kuhn (1 9 5 7 ) . My second r e a c t i o n t o Toulmin's argument c o n c e r n s the p r e d i c t i v e power of Darwin's t h e o r y . The f a c t t h a t a c t u a l p r e d i c t i o n s o f the t h e o r y were not p o s s i b l e u n t i l t h e development o f modern ecology and g e n e t i c s does n o t imply t h a t the t h e o r y i s p r e d i c t i v e l y impotent. To say t h i s i s t o confuse actua1 predic11ons o f a t h e o r y w i t h i t s p r e d i c t a b i l i t y ; the former always i m p l i e s t h e l a t t e r , but not always c o n v e r s e l y . T h i s i s where the concept o f ' h y p o t h e t i c a l p r e d i c t i o n s ' comes i n . A t h e o r y p o s s e s s e s p r e d i c t a b i l i t y i f i t can be used t o make ' h y p o t h e t i c a l p r e d i c t i o n s , ' but a c t u a l p r e d i c t i o n s a r e p o s s i b l e o n l y i f , i n a d d i t i o n , the ' i f - c l a u s e ' o f the ' h y p o t h e t i c a l p r e d i c t i o n i s s a t i s f i e d . T h i s ' i f - c l a u s e ' c o n t a i n s sometimes n a t u r a l and sometimes t e c h n o l o g i c a l c o n d i t i o n s which need t o be i r o n e d out f i r s t b e f o r e any a c t u a l p r e d i c t i o n i s p o s s i b l e . The e a r l y D a r w i n i s t s d i d not make any s p e c i f i c a c t u a l p r e d i c t i o n s because they were handicapped by b o t h n a t u r a l and t e c h n o l o g i c a l d i f f i c u l t i e s (the most o b v i o u s one b e i n g the l a c k o f knowledge o f h e r e d i t a r y v a r i a t i o n ) which made such p r e d i c t i o n s a p r a c t i c a l i m p o s s i b i l i t y . Moreover, they were so f a s c i n a t e d by the apparent, tremendous e x p l a n a t o r y power o f the t h e o r y t h a t they tended t o devote most, i f not a l l , o f t h e i r a t t e n t i o n t o i t s e x p l a n a t o r y a s p e c t , t h e r e b y n e g l e c t i n g i t s p r e d i c t i v e power. A t h i r d and f i n a l c r i t i c i s m o f Toulmin's argument i s propounded by Hempel who urges us t o d i s t i n g u i s h c a r e f u l l y between "what might be c a l l e d the story of evolution from the theory of the underlying, mechanism of mutation and natural s e l e c t i o n . " ^ In other words, we must not confuse phylogenetic descriptions with evolutionary theory. 2 The * story of evolution, as a hypothesis about the gradual development of various types of organisms, and about the subsequent e x t i n c t i o n of many of these, has the character of a hypothetical h i s t o r i c a l narrative describing the putative stages of the evolutionary process; i t i s the associated evolutionary theory that provides what explanatory  i n s i g h t we have into t h i s process. For example, the story of evolution might t e l l us that dinosaurs made t h e i r appearance in the T r i a s s i c period and that they died out i n the Cretaceous period. Such a narrative account does not, of course, explain why they came into existence; nor does i t explain why they became e x t i n c t . Indeed, even the associated, theory of mutation and natural s e l e c t i o n does not answer the f i r s t of the two questions, though i t might be held to shed l i g h t on the l a t t e r . Yet, even to account f o r t h e i r e x t i n c t i o n , we need a vast array of a d d i t i o n a l hypotheses about t h e i r p h y s i c a l , b i o t i c , and populational environments. But i f we have hypotheses of t h i s kind that are s p e c i f i c enough to provide, i n combination with the theory of natural s e l e c t i o n , at l e a s t a p r o b a b i l i s t i c explanation f o r t h e i r e x t i n c t i o n , then c l e a r l y the explanans adduced i s also iHempel (1965c), p. 370. 2See Hu l l (1974), p. 50. 82 q u a l i f i e d a s a b a s i s f o r a p o t e n t i a l p r o b a b i l i s t i c p r e d i c t i o n . I n H e m p e l ' s o p i n i o n , t h e a p p a r e n t p e r s u a s i v e n e s s o f T o u l m i n ' s a r g u m e n t seems t o d e r i v e f r o m two s o u r c e s : f i r s t , a w i d e s p r e a d t e n d e n c y t o c o n f u s e t h e d e s c r i p t i v e s t o r y o f e v o l u t i o n w i t h t h e e x p l a n a t i o n o f t h e v a r i o u s s t a t e s o f t h e p r o c e s s ; s e c o n d , a n e q u a l l y w i d e s p r e a d t e n d e n c y t o o v e r e s t i m a t e t h e e x t e n t t o w h i c h e v e n t h e t h e o r y o f m u t a t i o n a n d n a t u r a l s e l e c t i o n c a n a c c o u n t f o r t h e d e t a i l s o f t h e e v o l u t i o n a r y s e q u e n c e . 83 4.2 The Paresis, the Bridge, and the Pneumonia Case The p r i n c i p a l message conveyed by the p a r e s i s and the bridge case i s Scriven's i n s i s t e n c e that we can explain i n d i v i d u a l events without recourse to laws at a l l . In p a r t i c u l a r , an appeal to laws i s held to be 'wholly unnecessary' f o r i d e n t i f y i n g the cause of an event. But since "without the universal law, i t i s not p o s s i b l e to make p r e d i c t i o n s , i t follows that an asymmetry e x i s t s between explanation and p r e d i c t i o n . In Scriven's words: We can explain but not p r e d i c t , whenever we have a proposition of the form 'The only cause of X i s A' (I) — f o r example, 'The only cause of paresis i s s y p h i l i s . ' Notice that t h i s i s p e r f e c t l y compatible with the statement that A i s often not followed by X — i n f a c t , very few s y p h i l i t i c s develop pa r e s i s . Hence, when A i s observed, we can p r e d i c t that X i s more l i k e l y to occur than without A, but s t i l l extremely u n l i k e l y . So we must, on the evidence, s t i l l p r e d i c t that i t w i l l not occur. But i f i t does, we can appeal to (I) to provide and guarantee our explanation.2 In the bridge case, Scriven considers an explanation to the e f f e c t that the collapse of a bridge was caused by metal fatigue. This account, he argues, might be supported by pointing out that the f a i l u r e could have been caused only by an excessive load, by external damage, or by metal fa t i g u e . The f i r s t two f a c t o r s were not present i n the case at hand whereas there i s evidence of metal f a t i g u e . Given the 1 S c r i v e n (1959a), p. 481. 2 I b i d . , p. 480. information that the bridge d i d i n fac t c o l l a p s e , t h i s would e s t a b l i s h not only that metal fatigue was at f a u l t but that i t was strong enough to cause the f a i l u r e . ^ Analogously, Barker has argued that i t can be correct to speak of explanation i n many cases where s p e c i f i c p r e d i c t i o n i s not p o s s i b l e . Thus, f o r instance, i f the patient shows a l l the symptoms of pneumonia, sickens and dies, I can then explain h i s d e a t h — I know what k i l l e d him—but I could not have d e f i n i t e l y predicted i n advance that he was going to die; f o r usually pneumonia f a i l s to be f a t a l . 2 Since the paresis case i s e s s e n t i a l l y s i m i l a r to the pneumonia case, c r i t i c i s m s r a i sed against the former w i l l automatically apply to the l a t t e r which w i l l therefore not be treated i n d i v i d u a l l y i n the subsequent d i s c u s s i o n . 4.2.1 Scriven on Cause and Explanation I t i s evident from Scriven's own examples that 'cause plays an e s s e n t i a l r o l e i n h i s schema of explanation. To be able to locate or i d e n t i f y the cause of any event or phenomenon i s tantamount to having given an explanation of i t . This being the case, an analysis or unpacking of the notion of 'cause' i s of utmost importance i n the proper understanding of Scriven's concept of explanation which i s pronouncedly d i f f e r e n t from the Hempelian model. Unfortunately, Scriven'has not offered a p o s i t i v e analysis of the concept of 'cause'; he maintains that i t i s ISee Scriven (1962), pp. 182-185. 2Barker (1961), p. 271. an unanalyzable concept that everyone j u s t n a t u r a l l y understands. Fortunately, he does say what he thinks 'causes' are not: People have sometimes argued that i f A r e a l l y i s the cause of X, i t must always be followed by X^ . This i s to confuse causes with s u f f i c i e n t conditions, and p r a c t i c a l l y to abolish them from the applied sciences, since there are almost no absolutely r e l i a b l e statements of s u f f i c i e n t conditions a v a i l a b l e here. Causes are not necessary conditions e i t h e r ; t h e i r l o g i c a l nature i s complex, though there i s r e l a t i v e l y l i t t l e d i f f i c u l t y i n using the term 'cause' correctly.1 I f 'causes' are not simply s u f f i c i e n t conditions and neither are they necessary conditions, what exactly are they The c l o s e s t attempt to being a d e f i n i t i o n of 'cause' that Scriven has given i s t h i s : Speaking loosely, we could say that a cause i s a nonredundant member of a set of conditions j o i n t l y s u f f i c i e n t for the e f f e c t ( i . e . the remaining set i s [si not i t s e l f s u f f i c i e n t ) , the choice between the s e v e r a l candidates that usually meet t h i s requirement being based on considerations of context. . . . [P] rob ably the best view of a cause i s that i t i s any p h y s i c a l  explanation [ i t a l i c s mine] which involves reference to only one state or event (or a few) other than the e f f e c t , which i s independently v a r i a b l e . 2 4.2.2 C i r c u l a r i t y of the D e f i n i t i o n What have we learned from Scriven on explanation? To begin with, to give an explanation of a p a r t i c u l a r event or phenomenon i s to locate or i d e n t i f y i t s cause. But then a cause i s , i n turn, "any physical explanation [ i t a l i c s mine] •"•Scriven (1959a), n. 11, p. 482. 2 S c r i v e n (1962), p. 215. 86 w h i c h i n v o l v e s r e f e r e n c e t o o n l y o n e s t a t e o r e v e n t ( o r a few) o t h e r t h a n t h e e f f e c t . T h i s i s a p l a i n l y c i r c u l a r d e f i n i t i o n w h i c h f a i l s t o p r o v i d e a p r o p e r u n d e r s t a n d i n g , i f a n y , o f S c r i v e n ' s c o n c e p t o f e x p l a n a t i o n . 4 . 2 . 3 A R e v i s e d L o o k a t S c r i v e n ' s C o m p l a i n t o f a n A s y m m e t r y  b e t w e e n E x p l a n a t i o n a n d P r e d i c t i o n L e t u s r e c o n s t r u c t S c r i v e n ' s g e n e r a l a r g u m e n t i n t h e p a r e s i s c a s e i n t h e l i g h t o f h i s d e f i n i t i o n o f ' c a u s e ' a s "a n o n r e d u n d a n t member o f a s e t o f c o n d i t i o n s j o i n t l y s u f f i c i e n t f o r t h e e f f e c t ( i . e . t h e r e m a i n i n g s e t i s [Vic] n o t i t s e l f s u f f i c i e n t . " 2 L e t X s t a n d f o r p a r e s i s , A f o r s y p h i l i s , a n d B , C , D , e t c . f o r t h e o t h e r m e m b e r s o f t h e s e t o f c o n d i t i o n s ( o f w h i c h A i s a member) t h a t i s j o i n t l y s u f f i c i e n t f o r X -T o s a y "The o n l y c a u s e o f X i s A " i s a t l e a s t t o a f f i r m t h e l a w t h a t X d o e s n o t o c c u r w i t h o u t A ; i n o t h e r w o r d s , A i s a n e c e s s a r y c o n d i t i o n f o r X o r " W h e n e v e r we h a v e X , t h e n we a l s o h a v e A . " I t i s a l s o , h o w e v e r , t o s a y m o r e , n a m e l y t h a t t h e r e i s a s e t o f c o n d i t i o n s , o f w h i c h A i s a l w a y s a m e m b e r , u n d e r w h i c h X o c c u r s ; t h a t i s , c e r t a i n o t h e r f a c t o r s , B , C , D , e t c . a n d A a r e j o i n t l y s u f f i c i e n t f o r X . I n o t h e r w o r d s , A i s a n e c e s s a r y c o n d i t i o n a n d a l s o one o f t h e s e v e r a l j o i n t l y s u f f i c i e n t c o n d i t i o n s . T h i s i s i n d e e d t h e k i n d o f ! l b i d . 2 I b i d . 87 s i t u a t i o n i n w h i c h S c r i v e n s p e a k s o f o n e e v e n t a s t h e ' c a u s e ' o f a n o t h e r i n a c c o r d a n c e w i t h h i s a b o v e d e f i n i t i o n o f ' c a u s e . * By h y p o t h e s i s , we do n o t know t h e o t h e r s u f f i c i e n t c o n d i t i o n s . I f we knew t h e m , t h e n d e d u c t i v e e x p l a n a t i o n a n d p r e d i c t i o n w o u l d f o l l o w d i r e c t l y f r o m a s t a t e m e n t o f t h e n e c e s s a r y a n d s u f f i c i e n t c o n d i t i o n s . H o w e v e r , s i n c e we d o n o t know B , C , D , e t c . , how do we a c c o u n t f o r t h e f a c t t h a t a c t u a l l y we w o u l d n o r m a l l y e x p l a i n X b y A ? A s f a r a s I c a n s e e , t h e o n l y j u s t i f i c a t i o n , a n d I t h i n k S c r i v e n h a s t o a g r e e , i s b y i m p l i c i t l y a d d i n g t o o u r k n o w l e d g e o f t h e n e c e s s a r y - c o n d i t i o n , o u r ' g u e s s ' a b o u t t h e s u f f i c i e n t - ' c o n d i t i o n s . " K n o w i n g t h a t b o t h X a n d A h a v e o c c u r r e d , we a s s u m e t h e p r e s e n c e o f t h e u n s p e c i f i a b l e B , C , D , e t c . T h e e x p l a n a t i o n o f X t h e n f o l l o w s d e d u c t i v e l y . T h a t i s why we a c c e p t A a s t h e e x p l a n a t i o n o f X . I n o r d e r t o p r e d i c t X f r o m A , t h e unknown c o n d i t i o n s m u s t a l s o b e s p e c i f i e d , a n d t h i s we a r e u n a b l e t o d o . T h u s , t h e a s y m m e t r y e x i s t s i n p r a c t i c e , b u t n o t i n p r i n c i p l e . N o r i s t h i s a n " u n h e l p f u l s e n s e o f ' i n p r i n c i p l e ' , " - 1 - f o r o n l y b y e x h i b i t i n g t h e f o r m o f t h e a r g u m e n t t h a t w o u l d , i f we knew B , C , D , e t c . , p e r m i t t h e p r e d i c t i o n , c a n we c l a r i f y why t h e p u r p o r t e d e x p l a n a t i o n r e a l l y d o e s s t a t e ' w h y ' X o c c u r r e d . I n t h e same s p i r i t a r g u e s G r u n b a u m : I n s o f a r a s a p a s t o c c u r r e n c e o f p a r e s i s c a n b e i n d u c t i v e l y i n f e r r e d f r o m p r i o r s y p h i l i s , s o a l s o a f u t u r e o c c u r r e n c e o f p a r e s i s c a n b e . F o r t h e c a u s a l r e l a t i o n o r c o n n e c t i o n ^ • S c r i v e n ( 1 9 5 9 a ) , p . 4 7 9 . 88 between s y p h i l i s and paresis i s incontestably time symmetric: P r e c i s e l y i n the way and t o the extent that s y p h i l i s was a necessary condition f o r p a r e s i s , i t also wi11 be. Hence the only bona f i d e asymmetry here i s the record-based but innocuous one i n the a s s e r t i b i l i t y of the explanandum per se, but there i s no asymmetry of i n f e r a b i l i t y of paresis from s y p h i l i s . . . . For Hempel and Oppenheim did not maintain that an explanandum which can be postasserted can always also be preasserted; what they did maintain was only that the explanans never postexplains any better or more co n c l u s i v e l y than i t implies p r e d i c t i v e l y , there being complete symmetry between postexplanatory i n f e r a b i l i t y and p r e d i c t i v e i n f e r a b i l i t y from a given explanans.1 The kind of explanation we have been di s c u s s i n g always i m p l i c i t l y assumes that c e r t a i n u n s p e c i f i a b l e events had occurred. More commonly, perhaps, are explanations i n which the i m p l i c i t premise asserts that c e r t a i n e a s i l y s p e c i f i a b l e events did not occur, as i s evident i n the bridge case. We explain the collapse of the bridge as due to metal fa t i g u e because we know a law to the e f f e c t that i f a bridge collapsed, i t was caused by metal fatigue or an excessive load or external damage. But we cannot p r e d i c t the collapse of the bridge as caused by metal fatigue from the above law and the f a c t that the bridge did collapse, for we do not know f o r sure that i t was metal fatigue but not an excessive load or external damage that had caused the c o l l a p s e . Yet, i f we had known beforehand that neither an excessive load nor external damage was responsible for the collapse, we could have predicted the collapse of the bridge as caused by metal fatigue which follows deductively from the law, the i n i t i a l condition that the bridge had collapsed, and the d e n i a l of iGrunbaum (1963), pp. 87-88. 89 t h e o t h e r two p o s s i b l e c a u s e s . The e x p l a n a t i o n o f t h e c o l l a p s e i s l o g i c a l l y c o n c l u s i v e b e c a u s e , g i v e n t h e e x p l i c i t and i m p l i c i t p r e m i s e s , i t must have o c c u r r e d . C l e a r l y , i f we knew t h e i m p l i c i t p r e m i s e ( i . e . t h e d e n i a l o f t h e o t h e r two p o s s i b l e c a u s e s . o r w h a t e v e r o t h e r c a u s e s t h e r e may be) b e f o r e t h e e v e n t , i t c o u l d have been p r e d i c t e d . Thus, i n t h e s e c a s e s where i m p l i c i t p r e m i s e s a r e i n v o l v e d , t h e symmetry t h e s i s r e m a i n s i n t a c t . I n sum, i t appears t h a t i f S c r i v e n were t o d e f i n e 'cause' as a nonredundant member o f a s e t o f c o n d i t i o n s j o i n t l y s u f f i c i e n t f o r t h e e f f e c t , t h e n n e i t h e r t h e p a r e s i s n o r t h e b r i d g e c a s e c o n s t i t u t e s a c o u n t e r e x a m p l e t o t h e symmetry t h e s i s , f o r t h e y o n l y e s t a b l i s h t h a t an asymmetry e x i s t s i n p r a c t i c e , b u t n o t i n p r i n c i p l e . 4 .2.4 S c r i v e n ' s I n d i r e c t A p p r o a c h a t C a u s a l E x p l a n a t i o n I n o p p o s i t i o n t o Hempel's d e d u c t i v e - n o m o l o g i c a l a p p r o a c h a t c a u s a l e x p l a n a t i o n , S c r i v e n p r o p o s e s t h a t an i n d i r e c t a p p r o a c h may be j u s t as e f f e c t i v e . W i t h r e f e r e n c e t o t h e b r i d g e c a s e , i t amounts t o showing m e t a l f a t i g u e (C^) t o be a n e c e s s a r y c o n d i t i o n f o r t h e c o l l a p s e o f t h e b r i d g e (X) under t h e c i r c u m s t a n c e s ( C 2, . . . , C p ) : T h i s would i n v o l v e a p p e a l i n g t o a p r o p o s i t i o n o f t h e f o r m " I f X o c c u r s , t h e n e i t h e r A^, B^, o r C i c a u s e d i t " and showing t h a t C 2 , • • • , Cp r u l e o u t A i and B i . The main t r o u b l e w i t h such laws f o r t h e t h e s i s o f Hempel and Oppenheim i s t h a t t h e y do n o t p e r m i t any p r e d i c t i o n s o f X s i n c e t h e o c c u r r e n c e o f X i s r e q u i r e d f o r t h e i r a p p l i c a t i o n . Nor can such laws be f o r m u l a t e d f o r p r e d i c t i v e u s e , f o r t h e y a r e q u i t e d i f f e r e n t f r o m 90 "If A i or B i or C i occur, they w i l l produce X," not j u s t because one states necessary and the other s u f f i c i e n t conditions, but because the f i r s t does not and the second does require quantitative formulation i f i t i s to be t r u e — f o r i t i s obviously f a l s e that any degree of fatigue produces f a i l u r e . ^ Scriven i s making two d i s t i n c t objections here: f i r s t , that h i s i n d i r e c t approach at causal explanation i s detrimental to Hempel's thesis for i t does not permit any predictions of X whose occurrence i s required for i t s a p p l i c a t i o n ; second, that such i n d i r e c t causal laws cannot be formulated for p r e d i c t i v e use. The f i r s t o bjection w i l l be dealt with i n Section 4.2.5. Let us consider the second objection here. In short, I cannot agree with t h i s objection f o r I do not f i n d the proposition " I f X occurs, then e i t h e r A i , B i , or caused i t " very much d i f f e r e n t from the proposition " I f A i or B i or C i occur, they w i l l produce X." The s e n t e n t i a l proposition " I f X occurs, then e i t h e r A i , B i , or C i caused i t " — ( 1 ) can be rewritten i n a c a t e g o r i c a l but l o g i c a l l y equivalent form, v i z . " A l l X's are caused by either A i , B i , or C i " — ( 2 ) . This, i n turn, can be rewritten i n the active voice as " E i t h e r A i , B i , or C i causes ( a l l cases of) X " — ( 3 ) . Since Scriven asserts that the proposition " I f A i , or B i , or C i occur, they w i l l produce X"—(4) states s u f f i c i e n t conditions, I cannot see how he could possibly deny that (3) also states s u f f i c i e n t conditions. !Scriven (1962), p. 187. 91 And i f (3) does state s u f f i c i e n t conditions, so does (1) which i s equivalent to (3), v i a (2). The element of necessity i n (1) i s not provided by i t s form, but by the a d d i t i o n a l i n i t i a l conditions C2 / • • • / Cp which r u l e out A j and B]_. Scriven's objection would stand only i f the comparison i s between (1) plus the i n i t i a l conditions and (4) only ( i . e . , not i n c l u d i n g the i n i t i a l c o n ditions). But t h i s i s u n f a i r to Hempel's concept of causal explanation i n which i n i t i a l conditions play a v i t a l part. I n c i d e n t a l l y , Scriven cannot say that (1) states only necessary conditions i f he were to con s i s t e n t l y hold at the same time that a cause i s a nonredundant member of a set of conditions j o i n t l y s u f f i c i e n t for the e f f e c t . With regard to the c r i t i c i s m that (4) but not (1) requires quantitative formulation i f i t i s to be true-— for i t i s obviously f a l s e that any degree of fatigue produces f a i l u r e , my reply i s simply t h i s . I f (4) requires q u a n t i t a t i v e formulation, so does (3). And since (1) i s l o g i c a l l y equivalent to (3), v i a (2), i t follows that (1) also requires quantitative formulation. Thus the c r i t i c i s m i s untenable. 4.2.5 The Significance of Self-Evidencing Explanations A common objection to the f i r s t sub-thesis of the symmetry thesis i s based on the observation that sometimes the only ground we have for asserting some e s s e n t i a l statement i n the explanans l i e s i n the knowledge that the explanandum event d i d i n f a c t occur.. In such cases, the explanatory argument c l e a r l y could not have been used to pre d i c t that event. Such i s the case i n the bridge example. Given the information that the bridge d i d i n f a c t c o l l a p s e , t h i s would e s t a b l i s h not only that metal fatigue was at f a u l t (since the other two possible causes, i . e . an excessive load or external damage, were absent), but that i t was severe enough to cause the f a i l u r e . This case exemplifies an explanatory account one of whose constituent hypotheses i s supported only by the occurrence of the event to be explained — s o that the l a t t e r could not have been predicted by means of the explanatory argument. However, as Hempel has accurately observed, . . . the point thus i l l u s t r a t e d does not a f f e c t at a l l the con d i t i o n a l thesis that an adequate explanatory, argument must be such that i t could have served to ' pre d i c t the explanandum event if_ the information included i n the explanans had been known and taken i n t o account before the occurrence of that event. What Scriven's cases show i s that sometimes we do not know independently of the occurrence of the explanandum event that a l l the conditions l i s t e d i n the explanans are r e a l i z e d . However, t h i s means only that i n such cases our con d i t i o n a l thesis i s counterfactual, i . e . , that i t s i f -clause i s not s a t i s f i e d , but not that the t h e s i s i t s e l f i s f a l s e . Moreover, Scriven's argument does not even show that i n the kind of case he mentions i t i s l o g i c a l l y or nomologically impossible (impossible by reason of the laws of l o g i c or the laws of nature) f o r us to know the c r i t i c a l explanatory factor before, or independently of, the occurrence of the explanandum-event; the i m p o s s i b i l i t y appears to be rather a p r a c t i c a l and perhaps temporary one, r e f l e c t i n g present l i m i t a t i o n s of knowledge or technology. iflempel (1965c), p. 371. 93 L e t u s c a l l e x p l a n a t i o n s o f t h i s k i n d i n w h i c h t h e o c c u r r e n c e o f t h e e x p l a n a n d u m e v e n t a f f o r d s t h e o n l y a v a i l a b l e e v i d e n t i a l s u p p o r t f o r o n e o f t h e e x p l a n a n s s t a t e m e n t s , s a y Ci, ' s e l f - e v i d e n c i n g e x p l a n a t i o n s . I n H e m p e l ' s o p i n i o n , a n e x p l a n a t o r y a r g u m e n t o f t h e D - N f o r m w h i c h i s s e l f - e v i d e n c i n g i s n o t f o r t h a t r e a s o n c i r c u l a r o r p o i n t l e s s . T o b e s u r e , i f t h e same a r g u m e n t w e r e a d d u c e d i n s u p p o r t o f t h e a s s e r t i o n , t h a t t h e e x p l a n a n d u m e v e n t d i d o c c u r ( o r , t h a t E i s t r u e ) , t h e n i t w o u l d b e o p e n t o t h e c h a r g e o f e p i s t e m i c c i r c u l a r i t y . I f t h e a r g u m e n t i s t o a c h i e v e i t s o b j e c t i v e , t h e n a l l t h e g r o u n d s i t a d d u c e s i n s u p p o r t o f E - - i . e . i n i t i a l c o n d i t i o n s c l / C-2, • • • , Cj^; l a w s L ^ , I>2, . . . , L r — w o u l d h a v e t o b e e s t a b l i s h e d i n d e p e n d e n t l y o f E . A n d t h i s c o n d i t i o n i s v i o l a t e d h e r e s i n c e t h e o n l y g r o u n d we h a v e f o r b e l i e v i n g o r a s s e r t i n g C x i n c l u d e s t h e a s s u m p t i o n t h a t E i s t r u e . B u t when t h e same a r g u m e n t i s u s e d f o r e x p l a n a t o r y p u r p o s e s , i t . d o e s n o t c l a i m t o e s t a b l i s h t h a t E i s t r u e ; t h i s i s p r e s u p p o s e d b y t h e q u e s t i o n "Why d i d t h e e v e n t d e s c r i b e d b y E o c c u r ? " N o r n e e d a s e l f - e v i d e n c i n g e x p l a n a t i o n i n v o l v e a n e x p l a n a t o r y c i r c l e . T h e i n f o r m a t i o n t h a t t h e e x p l a n a n d u m e v e n t h a s o c c u r r e d i s n o t i n c l u d e d i n t h e e x p l a n a n s ( so t h a t t h e o c c u r r e n c e o f t h e e v e n t i s n o t ' e x p l a i n e d b y i t s e l f ) ; r a t h e r i t s e r v e s , q u i t e o u t s i d e t h e e x p l a n a t o r y c o n t e x t , a s e v i d e n c e s u p p o r t i n g one o f t h e e x p l a n a n s s t a t e m e n t s . T h u s , a n a c c e p t a b l e s e l f - e v i d e n c i n g e x p l a n a t i o n b e n e f i t s , a s i t w e r e , b y t h e w i s d o m o f h i n d s i g h t d e r i v e d f r o m t h e i n f o r m a t i o n 94 t h a t t h e e x p l a n a n d u m e v e n t h a s o c c u r r e d , b u t i t d o e s n o t m i s u s e t h a t i n f o r m a t i o n s o a s t o p r o d u c e a c i r c u l a r e x p l a n a t i o n . CHAPTER FIVE SECOND SUB-THESIS: EVERY ADEQUATE PREDICTION IS A POTENTIAL EXPLANATION 5.1 The Barometer Case In c r i t i c i z i n g the second s u b - t h e s i s o f the symmetry t h e s i s , S c r i v e n s t a t e s : What we are t r y i n g t o p r o v i d e when making a p r e d i c t i o n i s simply a c l a i m t h a t , a t a c e r t a i n time, an event o r s t a t e of a f f a i r s w i l l o c c u r . In e x p l a n a t i o n we a r e l o o k i n g f o r a cause, an event t h a t not o n l y o c c u r r e d e a r l i e r but stands i n a s p e c i a l r e l a t i o n t o t h e other, event. Roughly speaking, the p r e d i c t i o n r e q u i r e s o n l y a c o r r e l a t i o n , the e x p l a n a t i o n more. T h i s d i f f e r e n c e has as one consequence the p o s s i b i l i t y of making p r e d i c t i o n s from i n d i c a t o r s o t h e r than c a u s e s — f o r example, p r e d i c t i n g a storm from a sudden drop i n the b a r o m e t r i c p r e s s u r e . C l e a r l y we c o u l d not say t h a t the drop i n p r e s s u r e i n our house caused the storm: i t merely presaged i t . So we can sometimes p r e d i c t what we cannot e x p l a i n . 1 Elsewhere, i n the same frame of mind, he argues t h a t t h e r e c e r t a i n l y seem to be o c c a s i o n s when we can p r e d i c t some phenomenon w i t h the g r e a t e s t s u c c e s s , b u t cannot p r o v i d e any e x p l a n a t i o n of i t . F o r example, we may d i s c o v e r t h a t whenever cows l i e down i n the open f i e l d s by day, i t always r a i n s w i t h i n a few hours. We are i n an e x c e l l e n t p o s i t i o n f o r p r e d i c t i o n , but we c o u l d s c a r c e l y o f f e r the e a r l i e r event as an e x p l a n a t i o n o f the l a t t e r . I t appears t h a t e x p l a n a t i o n r e q u i r e s something "more than" p r e d i c t i o n ; and my s u g g e s t i o n would be t h a t , whereas an u n d e r s t a n d i n g of a phenomenon I S c r i v e n (1959a), p. 480. 96 o f t e n e n a b l e s u s t o f o r e c a s t i t , t h e a b i l i t y t o f o r e c a s t i t d o e s n o t c o n s t i t u t e a n u n d e r s t a n d i n g o f a p h e n o m e n o n . T h e d i s t i n g u i s h i n g f e a t u r e o f c a s e s o f t h e b a r o m e t e r t y p e f r o m o r d i n a r y c a u s a l p r e d i c t i o n s i s t h i s . When we make a p r e d i c t i v e i n f e r e n c e o f a s t o r m f r o m a s u d d e n b a r o m e t r i c d r o p , we a r e i n f e r r i n g a n e f f e c t ( i . e . t h e s t o r m ) o f a p a r t i c u l a r c a u s e ( i . e . w h a t e v e r p a r t i c u l a r w e a t h e r c o n d i t i o n s t h a t b r i n g a b o u t a s u d d e n d r o p o f t h e a t m o s p h e r i c p r e s s u r e a n d s u b s e q u e n t l y a s t o r m ) f r o m a n o t h e r ( e a r l i e r ) e f f e c t ( i . e . a s u d d e n d r o p i n b a r o m e t r i c p r e s s u r e ) o f t h a t same c a u s e . H e n c e , t h e i n f e r e n c e t o t h e s t o r m i s n o t f r o m a c a u s e o f t h e s t o r m b u t o n l y f r o m a n i n d i c a t o r o f i t . A n d t h e l a w c o n n e c t i n g s u d d e n b a r o m e t r i c d r o p s t o s t o r m s i s t h e r e f o r e a l a w a f f i r m i n g o n l y a n i n d i c a t o r - t y p e o f c o n n e c t i o n r a t h e r t h a n a c a u s a l c o n n e c t i o n . T h e a b o v e r e l a t i o n s h i p s b e t w e e n t h e c a u s e a n d t h e two e f f e c t s c a n be b e t t e r u n d e r s t o o d i n a s i m p l e s c h e m a t i z e d f o r m : C 3 C a u s a l Law C => E 2 C a u s a l Law E i => E 2 I n d i c a t o r Law w h e r e C = T h e r e e x i s t s t h e p a r t i c u l a r w e a t h e r c o n d i t i o n s , E i = T h e r e i s a s u d d e n d r o p i n b a r o m e t r i c p r e s s u r e , E2= A s t o r m o c c u r s . i S c r i v e n ( 1 9 6 2 ) , p p . 1 7 6 - 1 7 7 . 97 5.2 Hempel's R e p l y t o S c r i v e n ' s C r i t i c i s m I n d e f e n c e o f t h e second s u b - t h e s i s a g a i n s t S c r i v e n ' s c r i t i c i s m , Hempel c o n s i d e r s t h e f o l l o w i n g example. One o f t h e e a r l y symptoms o f meas l e s i s t h e appearance o f s m a l l w h i t i s h s p o t s , known as K o p l i k s p o t s , on t h e mucous l i n i n g s o f t h e c h e e k s . Hempel concedes t h a t someone l i k e S c r i v e n may be r e l u c t a n t t o say t h a t t h e e x p l a n a t i o n o f a p e r s o n ' s h a v i n g d e v e l o p e d t h e l a t e r m a n i f e s t a t i o n s o f t h e m e a s l e s i s p r o v i d e d by a c o n j u n c t i o n o f t h e u n i v e r s a l s t a t e m e n t t h a t t h e appearance o f K o p l i k s p o t s i s al w a y s f o l l o w e d by t h e l a t e r m a n i f e s t a t i o n s o f t h e m e a s l e s , and t h e s i n g u l a r s t a t e m e n t t h a t t h e p e r s o n i n q u e s t i o n showed K o p l i k s p o t s a t a s h o r t l y e a r l i e r d a t e . I n o t h e r words, S c r i v e n w o u l d . n o t want t o s a y t h a t t h e p e r s o n i n q u e s t i o n had d e v e l o p e d h i g h f e v e r and o t h e r symptoms o f t h e m e a s l e s because he had p r e v i o u s l y had K o p l i k s p o t s . B u t , i n Hempel's o p i n i o n , t h i s c a s e — a n d o t h e r s s i m i l a r t o i t — d o e s n o t c o n s t i t u t e a d e c i s i v e o b j e c t i o n t o t h e second s u b - t h e s i s , f o r such r e l u c t a n c e may w e l l r e f l e c t d o u b t s as t o wh e t h e r , as a m a t t e r o f u n i v e r s a l l a w, t h o s e s p o t s a r e al w a y s f o l l o w e d by t h e l a t e r m a n i f e s t a t i o n s o f m e a s l e s . P e r h a p s a l o c a l i n o c u l a t i o n w i t h a s m a l l amount o f m e a s l e s v i r u s w o u l d produce t h e s p o t s w i t h o u t l e a d i n g t o a f u l l - b l o w n c a s e o f the m e a s l e s . I f t h i s were s o , t h e appearance o f t h e s p o t s would s t i l l a f f o r d a u s u a l l y r e l i a b l e b a s i s f o r 98 p r e d i c t i n g the o c c u r r e n c e of f u r t h e r symptoms, s i n c e e x c e p t i o n a l c o n d i t i o n s of the k i n d j u s t mentioned would be extremely r a r e ; but the g e n e r a l i z a t i o n t h a t K o p l i k spots are always f o l l o w e d by l a t e r symtoms o f the measles would not express a law and thus c o u l d not p r o p e r l y support a c o r r e s p o n d i n g D-N e x p l a n a t i o n . 1 In b r i e f , Hempel i s s u g g e s t i n g t h a t the b a s i s o f the r e l u c t a n c e t o r e g a r d the g e n e r a l i z a t i o n c o n c e r n i n g the K o p l i k spots as e x p l a n a t o r y i s the f a c t t h a t t h i s g e n e r a l i z a t i o n may not concern a u n i v e r s a l r e g u l a r i t y . A l though I endorse the second s u b - t h e s i s , I have g r e a t r e s e r v a t i o n over Hempel's f o r e g o i n g defence of i t . F o r t o d i s m i s s S c r i v e n ' s c r i t i c i s m s o l e l y on the b a s i s t h a t t h e r e i s no u n i v e r s a l r e g u l a r i t y between the appearance o f K o p l i k spots and the l a t e r m a n i f e s t a t i o n s o f the measles, and thus no law t o support a d e d u c t i v e - n o m o l o g i c a l e x p l a n a t i o n , i s a v e r y poor s t r a t e g i c move s i n c e the same c r i t i c i s m would c r e e p up a g a i n i n i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n . And i t would be v e r y hard f o r Hempel to deny t h a t the g e n e r a l i z a t i o n about the K o p l i k spots i s a t y p i c a l case of a s t a t i s t i c a l law s i n c e he says e x p l i c i t l y t h a t "the appearance o f the s p o t s would s t i l l a f f o r d a u s u a l l y r e l i a b l e b a s i s f o r p r e d i c t i n g the o c c u r r e n c e o f f u r t h e r symptoms." 2 -•-Hempel 2 I b i d . (1965c), p. 375. 99 5.3 Grunbaum's T r e a t m e n t o f t h e B a r o m e t e r Case I n r e b u t t i n g S c r i v e n ' s c r i t i c i s m t h a t s u b s u m p t i o n under i n d i c a t o r laws p r o v i d e s no s c i e n t i f i c u n d e r s t a n d i n g a t a l l whereas subsumption under c a u s a l laws d o e s , Grunbaum s t i p u l a t e s t h a t t h e r e i s no d e c i s i v e d i f f e r e n c e r e g a r d i n g t h e e x p l a n a t o r y power between c a u s a l and i n d i c a t o r l a w s . What i s d e c i s i v e , he c l a i m s , i s t h e i r s i m i l a r i t y as r e g a r d s c o n t i n g e n c y . F o r . . . a c a u s a l law w h i c h i s used i n an e x p l a n a n s and i s n o t i t s e l f d e r i v e d f rom some w i d e r c a u s a l law i s f u l l y as l o g i c a l l y c o n t i n g e n t as a mere i n d i c a t o r law w h i c h i s l i k e w i s e n o t d e r i v e d from a c a u s a l law b u t i s used as a p r e m i s e f o r t h e d e d u c t i o n o f an explanandum ( e i t h e r p r e d i c t i v e l y o r p o s t d i c t i v e l y , i . e . , H - e x p l a n a t o r i l y ) . Why t h e n p r e f e r ( p r e d i c t i v e o r p o s t d i c t i v e ) s u b s u m p t i o n o f an explanandum under a c a u s a l law t o s u b s u m p t i o n under a mere i n d i c a t o r law? The j u s t i f i c a t i o n f o r t h i s p r e f e r e n c e would seem t o l i e n o t m e r e l y i n t h e g r e a t e r  g e n e r a l i t y [ i t a l i c s mine] o f t h e c a u s a l l a w ; i t a l s o r e s t s on t h e much l a r g e r v a r i e t y o f e m p i r i c a l c o n t i n g e n c i e s w h i c h must be r u l e d out i n t h e c e t e r u s p a r i b u s c l a u s e s p e c i f y i n g t h e r e l e v a n t c o n d i t i o n s under w h i c h t h e i n d i c a t o r law h o l d s , as compared t o t h e v a r i e t y o f such c o n t i n g e n c i e s p e r t a i n i n g t o t h e c o r r e s p o n d i n g c a u s a l l a w . 1 But t h i s d i f f e r e n c e b o t h i n g e n e r a l i t y and i n t h e v a r i e t y o f c o n t i n g e n c i e s , he c o n c l u d e s , does n o t show t h a t t h e i n d i c a t o r l a w p r o v i d e s no s c i e n t i f i c u n d e r s t a n d i n g o f p a r t i c u l a r phenomena subsumable under i t ; i t shows o n l y , so f a r as I can s e e , t h a t one m i g h t s i g n i f i c a n t l y speak o f d e g r e e s o f s c i e n t i f i c u n d e r s t a n d i n g . And t h i s c o n c l u s i o n i s e n t i r e l y c o m p a t i b l e w i t h t h e c o n t e n t i o n r e q u i r e d by t h e symmetry t h e s i s t h a t t h e b arometer i n d i c a t o r l a w f u r n i s h e s t h e same p o s i t i v e amount o f s c i e n t i f i c l-Grunbaum (1963), p. 93. understanding of a past storm as of a future one predicted by i t . 1 1 I b i d . 101 5.4 Ah Elaboration of Grunbaum's Argument I t i s understandable to have the i n t u i t i v e f e e l i n g that subsumption under i n d i c a t o r laws does not provide any s c i e n t i f i c understanding whereas subsumption under causal laws does. In other words, one might f e e l that the former type of explanation does not r e a l l y explain whereas the l a t t e r type does. But t h i s f e e l i n g , I claim, i s merely psychological, though i t has pragmatic i m p l i c a t i o n s . Our d i s s a t i s f a c t i o n with indicator-type explanations i s probably due to the f a c t that these explanations, u n l i k e causal ones, do not provide us with an answer as to 'how* the explanandum event occurred. In the barometer case, f o r example, subsumption of the explanandum event, i . e . the occurrence of a storm^ under the i n d i c a t o r law that "Sudden drops i n the barometric pressure are always followed by storms" does not t e l l us 'how' the storm came about. Without knowledge of the 'how,' we do f e e l r e l u ctant to say that a genuine explanation has been given. Our quest for the 'how' i n explanation has indeed a pragmatic ancestry. Take the case of the Koplik spots. An indicator-type explanation of a c h i l d developing measles by pointing to the fa c t that he has Koplik spots and that "The appearance of Koplik spots i s always followed by measles" does not provide u s with any clue as to 'how' h i s measles 102 came about, and consequently we would not be able to t r e a t i t . But i f we discover that measles i s caused by measles v i r u s , we can then develop methods to cure the i l l n e s s . A causal explanation i n t h i s case has obvious pragmatic . importance, and d i f f e r e n t kinds of pragmatic considerations enter i n t o d i f f e r e n t kinds of causal explanations. In view of the importance of pragmatic considerations which are p a r t i c u l a r l y s i g n i f i c a n t i n the sciences, we n a t u r a l l y tend to regard a s c i e n t i f i c explanation as genuine only i f i t provides an answer to the 'how' question, that i s , only i f i t i s causal i n nature. But, i n my opinion, t h i s i s to confuse the 'why' with the *how' aspect of explanation. I have no grudge against causal explanations. In f a c t , I hold them to be 'better' explanations than indicator-type explanations because of t h e i r 'greater generality.' Yet I r e s i s t i n a t t r i b u t i n g the status of genuine explanations only to causal explanations. On the contrary, I agree with Grunbaum that indicator-type explanations are also genuinely explanatory, though they do not answer the 'how' question and consequently cannot match causal explanations i n terms of pragmatic r a m i f i c a t i o n s . But to ask an 'explanation-seeking why-question' i s to ask "Why the explanandum event was to be expected?",-*- and an indicator-type explanation answers t h i s question s a t i s f a c t o r i l y . Of course, i f we knew what causes the •^ -Cf. sec. 6.3.1. 103 explanandum event to occur, we would have knowledge of 'why' and also 'how' the event was to be expected. As f a r as a genuine explanation goes, however, an indicator-type explanation w i l l s u f f i c e . As a matter of f a c t , my observation i s that laws of the form of simple empirical r e g u l a r i t i e s or generalizations such as "The sun r i s e s everyday" are i n roughly the same boat as i n d i c a t o r laws i n terms of explanatory power. For i t seems that no explanation of why the sun r i s e s today has been given by simply c i t i n g the empirical r e g u l a r i t y that the sun r i s e s everyday. Yet, i f we replace the above r e g u l a r i t y by the causal lav? s t a t i n g that "The sun r i s e s everyday because the Earth completes a f u l l r o t a t i o n on i t s axis every twenty-four hours, thus making day and night," then an explanation seems i n place. Put d i f f e r e n t l y , i t appears that subsumption under simple empirical g e n e r a l i z a t i o n s , j u s t l i k e t h e i r i n d i c a t o r r e l a t i v e , does not explain at a l l whereas subsumption under causal laws does. Consider the following example of Mr. Friday l i v i n g on a deserted i s l a n d with his master and teacher Robinson Crusoe. Suppose our Mr. Friday i s an excellent observer. He observes that the sun r i s e s everyday. When Crusoe asks him the question "Why does the sun r i s e today?", the only r a t i o n a l answer he can give i s that the sun has r i s e n everyday and today i s no exception. But suppose he now learns from Crusoe that the sun r i s e s everyday because the Earth completes a f u l l , r o t a t i o n on i t s axis every twenty-four hours, thus 104 making day and n i g h t - I f c o n f r o n t e d w i t h t h e same q u e s t i o n as why t h e sun r i s e s t o d a y , h i s e x p l a n a t i o n w o u l d no l o n g e r be "Because i t has r i s e n e v e r y d a y , " b u t "Because t h e E a r t h c o m p l e t e s a f u l l r o t a t i o n on i t s a x i s e v e r y t w e n t y - f o u r h o u r s , t h u s making day and n i g h t . " I n v i e w o f t h e c a u s a l n a t u r e o f t h e l a t t e r e x p l a n a t i o n , i t i s v e r y t e m p t i n g t o c o n c l u d e t h a t F r i d a y ' s e a r l i e r e x p l a n a t i o n w h i c h i n v o l v e s a s i m p l e e m p i r i c a l g e n e r a l i z a t i o n i s r e a l l y n o t a g e n u i n e e x p l a n a t i o n a t a l l , whereas h i s l a t e r e x p l a n a t i o n w h i c h i n v o l v e s a c a u s a l law i s . I c a n n o t agree w i t h t h e f o r e g o i n g c o n c l u s i o n . R a t h e r , I would m a i n t a i n t h a t b o t h e x p l a n a t i o n s a r e g e n u i n e l y e x p l a n a t o r y , though i t i s t r u e t h a t t h e e a r l i e r e x p l a n a t i o n has l e s s e x p l a n a t o r y power, due t o t h e f a c t t h a t i t i s l e s s e r i n g e n e r a l i t y . Our i n t u i t i v e f e e l i n g t h a t t h e e a r l i e r e x p l a n a t i o n does n o t e x p l a i n a t a l l whereas t h e l a t e r e x p l a n a t i o n does m e r e l y r e f l e c t s , I t h i n k , a d i f f e r e n c e i n t h e r e s p e c t i v e s t a t e o f knowledge when t h e two e x p l a n a t i o n s a r e made. A t t h e t i m e when t h e e a r l i e r e x p l a n a t i o n was .made, i t was t h e o n l y p o s s i b l e e x p l a n a t i o n s i n c e no knowledge o f a c a u s a l l a w, b u t o n l y an e m p i r i c a l r e g u l a r i t y , was a v a i l a b l e . The s i t u a t i o n changes, however, when t h e l a t e r e x p l a n a t i o n i s made. A c a u s a l e x p l a n a t i o n i s a v a i l a b l e s i n c e we now p o s s e s s t h e knowledge o f a c a u s a l law o v e r and above t h e knowledge o f a mere e m p i r i c a l r e g u l a r i t y . T h i s l a t e r c a u s a l e x p l a n a t i o n i s i n d e e d a b e t t e r e x p l a n a t i o n t h a n t h e e a r l i e r e x p l a n a t i o n because i t i s g r e a t e r i n g e n e r a l i t y . Y e t , i t by no means follows that the e a r l i e r explanation has never possessed any explanatory power. What follows i s only that i t i s no longer used since a better explanation i s at our dis p o s a l . And i f the causal explanation i s very well established, i t i s not unnatural to have the f e e l i n g that i t i s the only genuine explanation, whereas the e a r l i e r explanation which involves a mere empirical r e g u l a r i t y i s not. 106 5.5 S c h e f f l e r ' s Attack on the Second Sub-T.hesis S c h e f f l e r has argued that i n many instances, evidence statements which provide adequate grounds f o r a p r e d i c t i o n do not always provide an adequate basis f o r explanation. In b r i e f , the argument i s that an adequate predictans may cons i s t simply of statements of i n i t i a l c onditions or previous events, but no laws. Yet an "explanation seems to require appeal to general p r i n c i p l e s , whether u n i v e r s a l or s t a t i s t i c a l i n form, serving to connect events i n pattern."1 For instance,a l i s t of r e s u l t s obtained i n a long s e r i e s of drawings with replacement, say 1,000, from an urn known to contain red and black b a l l s may provide a good ba s i s for pr e d i c t i n g the percentage of red and black b a l l s to be expected i n the next 1,000 drawings, but t h i s l i s t of data provides no explanation for the subsequent r e s u l t s . Cases l i k e these r a i s e the question of whethrr there are sound modes of s c i e n t i f i c p r e d i c t i o n that proceed from f i n i t e observed samples to f i n i t e unobserved samples without i n v o l v i n g any general law which i s required by Hempel f o r any adequate explanation. I S c h e f f l e r (1963), p. 43. 107 5.6 Hempel's R e p l y t o S c h e f f l e r ' s C r i t i c i s m Hempel concedes t o S c h e f f l e r t h a t t h e r e i s a l i m i t t o t h e scope o f h i s two c o v e r i n g - l a w models; t h a t t h e y " r e p r e s e n t t h e l o g i c a l s t r u c t u r e o f two i m p o r t a n t t y p e s o f p r e d i c t i v e i n f e r e n c e i n e m p i r i c a l s c i e n c e , b u t n o t t h a t t h e s e a r e t h e o n l y t y p e s . " 1 He a d m i t s t h a t S c h e f f l e r ' s argument t o t h e e f f e c t t h a t s c i e n t i f i c p r e d i c t i o n may be i n d u c t i v e l y grounded on i n f o r m a t i o n t h a t i n c l u d e s no g e n e r a l l a w s i s i n d e e d w e l l -founded. F o r on some c u r r e n t t h e o r i e s o f p r o b a b i l i s t i c i n f e r e n c e , such as Carnap's t h e o r y o f i n d u c t i v e l o g i c , s u c h i n d u c t i v e i n f e r e n c e s a r e p o s s i b l e on p u r e l y l o g i c a l g r o u n d s ; t h e i n f o r m a t i o n about t h e g i v e n sample c o n f e r s a d e f i n i t e l o g i c a l p r o b a b i l i t y upon any pr o p o s e d p r e d i c t i o n c o n c e r n i n g an as y e t unobserved sample. -But Hempel a l s o reminds us t h a t i n o p p o s i t i o n t o t h e r e l e v a n t t h e o r i e s o f i n d u c t i v e l o g i c , c e r t a i n s t a t i s t i c a l t h e o r i e s o f p r o b a b i l i s t i c i n f e r e n c e eschew t h e n o t i o n o f p u r e l y l o g i c a l p r o b a b i l i t i e s and q u a l i f y p r e d i c t i o n s o f t h e k i n d h e r e c o n s i d e r e d as sound o n l y on t h e f u r t h e r a s s u m p t i o n t h a t t h e s e l e c t i o n o f i n d i v i d u a l c a s e s from t o t a l p o p u l a t i o n has t h e c h a r a c t e r o f a random e x p e r i m e n t w i t h c e r t a i n g e n e r a l s t a t i s t i c a l c h a r a c t e r i s t i c s . But t h a t a s s u m p t i o n , when e x p l i c i t l y s p e l l e d o u t , has t h e form o f a g e n e r a l l a w o f s t a t i s t i c a l - p r o b a b i l i s t i c form; hence t h e p r e d i c t i o n s a r e e f f e c t e d by means o f c o v e r i n g laws a f t e r a l l . . . . Thus c o n s t r u e d , even t h e p r e d i c t i o n s h e r e under d i s c u s s i o n t u r n o u t t o be ( i n c o m p l e t e l y f o r m a u l a t e d ) p o t e n t i a l e x p l a n a t i o n s . Hempel Hempel (1963) , p. 119. (19 6 5 c ) , p. 376. CHAPTER SIX A COMPARISON BETWEEN SALMON'S AND HEMPEL'S CONCEPTS OF EXPLANATION 6.1 Salmon's Concept of Explanation In what follows, I s h a l l present not only Salmon's o r i g i n a l s t a t i s t i c a l - r e l e v a n c e (S-R for short) model of explanation, but also h i s l a t e s t causal-relevance (C-R fo r short) model of explanation. Since I w i l l be concentrating on the general framework rather than the de t a i l e d set-up of these two models, both presentations w i l l be kept as simple, straightforward, and non-technical as possible. 6.1.1 The Stat i s t i c a l - R e l e v a n c e (S-R) Model of Explanation In short, according to the S-R model, "an explanation i s an assembly of facts s t a t i s t i c a l l y relevant to the explanandum, regardless of the degree of p r o b a b i l i t y that results."-'- The proper form of explanatory questions should not be "Why does t h i s thing disappear?", but should be "Why does t h i s thing which i s a streptococcus i n f e c t i o n , 1Salmon (1971), p. 11. 109 disappear?" In other words, the question should take the form "Why i s t h i s x which i s A also B?", and the answer then takes the form "Because t h i s x i s also C." C must be a property that i s s t a t i s t i c a l l y relevant to but l o g i c a l l y independent of the a t t r i b u t e c l a s s B within the reference c l a s s A. A set of mutually exclusive and exhaustive subclass of a c l a s s i s a p a r t i t i o n of that c l a s s . And a property C, which p a r t i t i o n s the c l a s s A i n t o two subclasses, A.C and A.C, i s said to be s t a t i s t i c a l l y relevant to B within A i f and only i f P(A.C,B) f P ^ B ) . 1 If every property that determines a place s e l e c t i o n ^ i s s t a t i s t i c a l l y i r r e l e v a n t to B i n A, then A i s said to be a homogeneous reference c l a s s for B. A reference c l a s s xs homogeneous i f there i s no way, even i n p r i n c i p l e , to e f f e c t a s t a t i s t i c a l l y relevant p a r t i t i o n without already knowing which elements have the a t t r i b u t e i n question and which do not. To avoid ambiguity, I s h a l l follow Salmon i n adopting the Reichenbachian non-standard notation f o r p r o b a b i l i t y statements i n which P(A,B) stands f o r the p r o b a b i l i t y from A to B, i . e . the p r o b a b i l i t y , given A, of B. •^Salmon adopts von Mises' notion of a place s e l e c t i o n which e f f e c t s a p a r t i t i o n of a reference c l a s s i n t o two subclasses, elements of the place s e l e c t i o n and elements not included i n the place s e l e c t i o n . A place s e l e c t i o n may or may not be s t a t i s t i c a l l y relevant to a given a t t r i b u t e i n a given reference c l a s s . I f the place s e l e c t i o n i s s t a t i s t i c a l l y i r r e l e v a n t to an a t t r i b u t e within a reference c l a s s , the p r o b a b i l i t y of that a t t r i b u t e within the subclass determined by the place s e l e c t i o n i s equal to the p r o b a b i l i t y of that a t t r i b u t e within the e n t i r e o r i g i n a l reference c l a s s . I f every place s e l e c t i o n i s i r r e l e v a n t to a given a t t r i b u t e i n a given sequence, the sequence i s said to be random. 110 For Salmon, the aim i n s e l e c t i n g a r e f e r e n c e c l a s s t o which t o a s s i g n a s i n g l e case i s not t o s e l e c t t h e narrowest, but the br o a d e s t , a v a i l a b l e c l a s s . However, the r e f e r e n c e c l a s s s h o u l d be homogeneous, and a c h i e v i n g homogeneity r e q u i r e s making the r e f e r e n c e c l a s s narrower i f i t was not a l r e a d y homogeneous. In o t h e r words, i t i s d e s i r a b l e t o narrow the r e f e r e n c e c l a s s i n s t a t i s t i c a l l y r e l e v a n t ways, b u t not i n s t a t i s t i c a l l y i r r e l e v a n t ways. In s h o r t , Salmon's r e f e r e n c e  c l a s s r u l e s t a t e s t h a t we must choose the b r o a d e s t homo gen e o u s  r e f e r e n c e c l a s s t o which the s i n g l e event b e l o n g s . Now a g e n e r a l c h a r a c t e r i z a t i o n o f e x p l a n a t i o n o f p a r t i c u l a r events l o o k s l i k e t h i s . The answer t o the q u e s t i o n "Why does t h i s x which i s a member o f A have the a t t r i b u t e B" c o n s i s t s o f a p a r t i t i o n of the r e f e r e n c e c l a s s A i n t o a number of s u b c l a s s e s , a l l o f which are homogeneous w i t h r e s p e c t t o B, al o n g w i t h the p r o b a b i l i t i e s o f B w i t h i n each o f these s u b c l a s s e s . 1 In a d d i t i o n , we must say which o f t h e members of the p a r t i t i o n c o n t a i n s our p a r t i c u l a r x. More f o r m a l l y , an e x p l a n a t i o n o f the f a c t t h a t x, a member of A, i s a member of B would go as f o l l o w s : P(A.C 1,B) = Pi P(A.C 2,B) = p 2 P(A.C n,B) = p n i T h e reason f o r doing t h i s i s e x p l a i n e d by Salmon i n the s e c t i o n c a l l e d " M u l t i p l e Homogeneity" i n h i s (1971), pp. 58-62. I l l where A.Ci, A.C 2, . . • , A.C n i s a homogeneous p a r t i t i o n of A with respect to B, P i = Pj only i f i = j , and x £ A.Cfc. In sum, an S-R explanation i s a set of p r o b a b i l i t y statements, q u a l i f i e d by c e r t a i n provisos, plus a statement s p e c i f y i n g the compartment to which the explanandum event belongs. Within t h i s S-R model of explanation, low p r o b a b i l i t y events become amenable to explanation. We w i l l i l l u s t r a t e t h i s by the well-known paresis case. The p r i o r p r o b a b i l i t y of a person contracting paresis i s very low, and the reference c l a s s of people i n general i s inhomogeneous. Thus, we can make a s t a t i s t i c a l l y relevant p a r t i t i o n i n t o people who have untreated l a t e n t s y p h i l i s and those who do not. The p r o b a b i l i t y that a person with untreated l a t e n t s y p h i l i s w i l l contract p a r e s i s , i . e . the p o s t e r i o r p r o b a b i l i t y , i s s t i l l low, but i t i s considerably higher than the p r i o r p r o b a b i l i t y of paresis among people i n general. To c i t e untreated latent s y p h i l i s as an explanation of paresis i s correct, f o r i t does provide a p a r t i t i o n of the general reference c l a s s which y i e l d s a more homogeneous reference class and a higher p o s t e r i o r p r o b a b i l i t y f o r the explanandum event. 112 6 . 1 . 2 The Causal-Relevance (C-R) Model of Explanation This i s the model of explanation which Salmon now prefers to h i s o r i g i n a l S-R model. B r i e f l y , the C-R model states that we have an explanation of an event i f we have a complete set of s t a t i s t i c a l l y relevant f a c t o r s , the pertinent p r o b a b i l i t y values, and causal explanations of the ( s t a t i s t i c a l ) r e g u l a r i t i e s involved. The s i g n i f i c a n c e of causal r e l a t i o n s , however, has long been observed by Salmon i n h i s discussion of the barometer case. The barometer case poses an objection to the o r i g i n a l S-R model i n the following way. There i s a c o r r e l a t i o n between the behaviour of the barometer and the occurrence of storms. I f we take the general reference c l a s s of days i n the v i c i n i t y of my house and ask for the p r o b a b i l i t y of a storm, we get a rather low p r i o r p r o b a b i l i t y . I f we p a r t i t i o n that reference c l a s s i n t o two subclasses, namely days on which there i s a sudden drop i n the barometer and days on which there i s not, we have a p o s t e r i o r p r o b a b i l i t y of a storm i n the former c l a s s that i s much higher than the p r i o r p r o b a b i l i t y . The new reference c l a s s is f a r more homogeneous than the old one. Thus, according to the S-R model of explanation, the drop i n barometer reading would seem to explain the storm. But, as Scriven has argued, the barometer reading i s only an i n d i c a t o r ; i t does not cause the storm and therefore i t does not explain i t s 1 S c r i v e n ( 1 9 5 9 a ) , p. 480. 113 o c c u r r e n c e . The s t o r m i s caused by c e r t a i n w i d e s p r e a d a t m o s p h e r i c c o n d i t i o n s , and t h e b e h a v i o u r o f t h e b a r o m e t e r i s m e r e l y symptomatic o f them. Salmon's r e p l y t o t h i s o b j e c t i o n r u n s l i k e t h i s . He i s w i l l i n g t o a d m i t t h a t symptomatic e x p l a n a t i o n s seem t o have g e n u i n e e x p l a n a t o r y v a l u e i n t h e a bsence o f knowledge o f c a u s a l r e l a t i o n s , t h a t i s , as l o n g as we do n o t know t h a t we a r e d e a l i n g o n l y w i t h symptoms. However, c a u s a l e x p l a n a t i o n s s u p e r c e d e symptomatic ones when t h e y can be g i v e n , and,when we s u s p e c t we a r e d e a l i n g w i t h symptoms,' we l o o k h a r d f o r a c a u s a l e x p l a n a t i o n . The r e a s o n i s t h a t a c a u s a l e x p l a n a t i o n i s l i k e l y t o p r o v i d e a more homogeneous r e f e r e n c e c l a s s t h a n does a symptomatic e x p l a n a t i o n . C a u s a l p r o x i m i t y i n c r e a s e s homogeneity. I t w i l l be r e c a l l e d t h a t t h e p r o p e r t y C i s s t a t i s t i c a l l y i r r e l e v a n t t o t h e a t t r i b u t e B i n t h e r e f e r e n c e c l a s s A i f and o n l y i f P(A,B) = P ( A . C , B ) . The p r o b a b i l i t y o f a s t o r m on a day when t h e r e i s a sudden dr o p i n a t m o s p h e r i c p r e s s u r e and when my b a r o m e t e r e x e c u t e s a sudden d r o p i s p r e c i s e l y t h e same as t h e p r o b a b i l i t y o f a s t o r m on a day when t h e r e i s a sudden w i d e s p r e a d d r o p i n a t m o s p h e r i c p r e s s u r e . To b o rrow a u s e f u l n o t i o n f r o m R e i c h e n b a c h , we may say t h a t t h e sudden w i d e s p r e a d d r o p i n a t m o s p h e r i c p r e s s u r e s c r e e n s o f f t h e d r o p i n b a r o m e t e r r e a d i n g f rom t h e o c c u r r e n c e o f t h e s t o r m . The c o n v e r s e r e l a t i o n does not h o l d . The p r o b a b i l i t y o f a s t o r m on a day when the reading on my barometer makes a sudden drop i s not equal to the p r o b a b i l i t y of a storm on a day when the reading on my barometer makes a sudden drop and there i s a sudden widespread drop i n the atmospheric pressure. The sudden drop i n barometer reading does not screen o f f the sudden widespread drop i n atmospheric pressure from the occurrence of the storm. More formally, we may say that D screens o f f C from B i n reference c l a s s A i f and only i f P(A.C.D,B) .= P(A.D,B) j- P(A.C,B). For purposes of the foregoing example, l e t A = the c l a s s of days i n the v i c i n i t y of my house, l e t B = the c l a s s of days on which there i s an occurrence of a storm, l e t C = the c l a s s of days on which there i s a sudden drop i n reading on my barometer, and l e t D = the c l a s s of days on which there i s a widespread drop i n atmospheric pressure i n the area i n which my house i s located. By means of t h i s formal d i f i n i t i o n , we see that D screens o f f C from B, but C does not screen o f f D from B. When one property i n terms of which a s t a t i s t i c a l l y relevant p a r t i t i o n i n a reference c l a s s can be e f f e c t e d screens o f f another property i n terms of which another s t a t i s t i c a l l y relevant p a r t i t i o n of that same reference c l a s s can be ef f e c t e d , then the screened-off property must give way to the property which screens i t o f f . This i s the screening-off r u l e . The screened-off property then becomes i r r e l e v a n t and no longer has explanatory value. This consideration shows us how to handle the barometer case, as the unwanted 'symptomatic explanations' can now be blocked by use of the screening-off concept which i s defined i n terms of s t a t i s t i c a l irrelevance alone. This notion of causal relevance emerges as the dominant factor i n Salmon's l a t e s t writings on explanation. He now admits that he no longer believes that the assemblage of relevant f a c t o r s provides a complete e x p l a n a t i o n — o r much of anything i n the way of an explanation. We do, I b e l i e v e , have a bona f i d e explanation of an event i f we have a complete set of s t a t i s t i c a l l y relevant f a c t o r s , the pertinent p r o b a b i l i t y values, and causal explanations of the relevance r e l a t i o n s . Subsumption of a p a r t i c u l a r occurrence under s t a t i s t i c a l r e g u l a r i t i e s - - w h i c h , we r e c a l l , does not imply anything about the construction of deductive or inductive arguments—is a necessary part of any adequate explanation of i t s occurrence, but i t i s not the whole story. The causal explanation . of the r e g u l a r i t y i s also needed.1 As i s evident from the name of the model, causal mechanisms occupy a v i t a l place i n the C-R model of explanation. Salmon adopts, as the basic p r i n c i p l e of explanation, that "every r e l a t i o n of s t a t i s t i c a l relevance must be explained by r e l a t i o n s of causal r e l e v a n c e . " 2 He maintains that i f we have events of two types A and B, whose respective members are not spatio-temporally contiguous, but whose occurrences are c o r r e l a t e d Salmon (1978), p. 699. 2Salmon (1975), p. 124. 116 with one another, the causal explanation of t h i s r e g u l a r i t y may take e i t h e r of two forms. E i t h e r there i s a d i r e c t causal connection from A to B or from B to A, or there i s a common cause C which accounts for the s t a t i s t i c a l dependency.1 An example of the f i r s t form of causal explanation i s t h i s . Suppose, for instance, that the p i c t u r e on my t e l e v i s i o n receiver o c c a s i o n a l l y breaks up i n t o a s o r t of herringbone pattern. At f i r s t , I may think that t h i s i s occurring randomly, but I then discover that there i s a nearby p o l i c e broadcasting s t a t i o n that goes on the a i r p e r i o d i c a l l y . When I f i n d a strong s t a t i s t i c a l c o r r e l a t i o n between the operation of the p o l i c e transmitter and the breakup of the pi c t u r e , I conclude that the p o l i c e broadcast i s part of the explanation of the t e l e v i s i o n malfunction. Roughly speaking, the operation of the p o l i c e transmitter i s the cause (or a part of the cause) of the bad TV p i c t u r e . T y p i c a l of the second form of causal explanation i s the above-mentioned barometer case. The rap i d dropping of the barometer does not explain the subsequent storm. Likewise, the subsequent storm does not explain the behaviour of the barometer. Both are explained by a common cause, namely, the meteorological conditions that cause the storm and are indicated by the barometer. In t h i s case, there i s a s t a t i s t i c a l relevance r e l a t i o n between the barometer reading and the storm, but neither event i s invoked to explain the other. Instead, both are explained by a common cause. ^Salmon ( 1 9 7 8 ) , p. 699. 117 Various examples w i l l i l l u s t r a t e the p r i n c i p l e of the  common cause more c l e a r l y . I f a l l the e l e c t r i c l i g h t s i n an apartment b u i l d i n g go out simultaneously, we explain t h i s coincidence i n terms of a power f a i l u r e , but not by the chance burning out of each of the l i g h t bulbs at the same time. Or i f two term papers are i d e n t i c a l , and neither has been copied from another, we postulate a common cause (e.g. a paper i n a f r a t e r n i t y f i l e ) . F i n a l l y , l e t us examine Salmon's idea of a causal explanation i n terms of causal processes and causal i n t e r a c t i o n s . Salmon takes processes to be more fundamental than t h e i r i n t e r s e c t i o n s , and he holds that the notion of a causal i n t e r a c t i o n i s a product of the concepts of causal processes and t h e i r i n t e r s e c t i o n s . He says: An electron t r a v e l i n g through space i s a process, and so i s a photon; i f they c o l l i d e , that i s an i n t e r s e c t i o n . . A l i g h t pulse t r a v e l i n g from a beacon to a screen i s a process, and a piece of red glass standing i n the path i s another; the l i g h t passing through the g l a s s i s an i n t e r s e c t i o n . Both of these i n t e r s e c t i o n s c o n s t i t u t e i n t e r a c t i o n s . I f two l i g h t beams cross one another, we have an i n t e r s e c t i o n without an i n t e r a c t i o n — e x c e p t i n the extremely u n l i k e l y event of a p a r t i c l e - l i k e c o l l i s i o n between photons. What we want to say, very roughly, i s that when two processes i n t e r s e c t , and both are modified i n such ways that the changes i n one are c o r r e l a t e d with changes i n the o t h e r — i n the manner of an i n t e r a c t i v e fork ["see Figure l ] —we have a causal i n t e r a c t i o n . Furthermore, by employing Reichenbach 1s 'mark' method, 2 Salmon distinguishes causal processes which can transmit 'marks' or modifications from pseudo-processes which cannot. x I b i d . , p. 696. 2 F o r a d e t a i l e d account of Reichenbach's 'mark* method, see, for example, Salmon (1975), pp. 1 2 9 - 1 3 4 . CAUSAL INTERACTION INTERACTIVE FORK PROCESS 1 ^ c INTERSECTION PROCESS 2 F i g u r e 1 (Adapted from Salmon (1978), p. 697.) 119 In essence, Salmon maintains that i f we have events of two types A and B which are s t a t i s t i c a l l y c o r r e l a t e d with one another, the causal explanation of t h i s r e g u l a r i t y may e i t h e r take the form of a d i r e c t causal connection from A to B or B to A, or i t may take the form of a common cause C which accounts for the S t a t i s t i c a l dependency. In e i t h e r case, those events which stand i n the cause-e f f e c t r e l a t i o n to one another are joined by a causal process. The d i s t i n c t events A, B, and C which are thus r e l a t e d constitute i n t e r a c t i o n s — a s defined i n terms of an i n t e r a c t i v e f o r k — a t the appropriate places i n the respective causal process. The i n t e r a c t i o n s produce modifications i n the causal processes, and the causal processes transmit the m o d i f i c a t i o n s . ! Salmon ( 1 9 7 8 ) , p p . 6 9 9 - 7 0 0 . 120 6.2 A C r i t i q u e of the C-R Model of Explanation I intend to argue i n t h i s section that there are serious shortcomings with the C-R model of explanation. On the one hand, the model, or more s p e c i f i c a l l y , the p r i n c i p l e  of the common cause, i s g u i l t y of demanding f o r hidden var i a b l e s which entertain;.... the notion of •determinism' that Salmon himself regards as wholly unnecessary i n the ex p l i c a t i o n of s c i e n t i f i c explanation. 1 On the other hand, i f hidden va r i a b l e s are to be avoided, then the model would not be able to accomodate most quantum phenomena; thus, i t w i l l f a l l short of being an adequate, comprehensive model of s c i e n t i f i c explanation. 6.2.1 van Fraassen's C r i t i c i s m of the P r i n c i p l e of the  Common Cause and Salmon's Reply van Fraassen r a i s e s a serious charge against Salmon's adoption of the p r i n c i p l e of the common cause. He argues: To assume Reichenbach's p r i n c i p l e to be s a t i s f i a b l e , c o n t i n u i t y aside, i s to ru l e out a l l genuinely in d e t e r m i n i s t i c theories. As example, l e t a theory say that C i s i n v a r i a b l y followed by one of the incompatible events A, B, or D, each with p r o b a b i l i t y 1/3. Let us suppose the theory complete, and i t s p r o b a b i l i t i e s i r r e d u c i b l e , with C the complete s p e c i f i c a t i o n of state. Then we w i l l f i n d a c o r r e l a t i o n for which only C could be the common cause, but i t i s not. Assuming that A, B, D are always preceded by C and that they have low but equal p r i o r p r o b a b i l i t i e s , iSalmon's argument against the theory of hidden variables can be found i n hi s (1977a), pp. 155-158. 121 t h e r e i s a s t a t i s t i c a l c o r r e l a t i o n between (A o r D) and ^ ( B o r D) , f o r P Wft) =P ( = 1 / 2 = P (0) . B u t C, t h e o n l y a v a i l a b l e c a n d i d a t e , does riot s c r e e n o f f 0 from ft: P (^/C&/-)=P(^/^)=l/2^P(jzi/C) w h i c h i s 2/3. A l t h o u g h t h i s may sound c o m p l i c a t e d , t h e c o n s t r u c t i o n i s so g e n e r a l t h a t a l m o s t any i r r e d u c i b l y p r o b a b i l i s t i c s i t u a t i o n w i l l g i v e a s i m i l a r example. Thus R e i c h e n b a c h ' s p r i n c i p l e o f  t h e common cause i s i n f a c t a demand f o r h i d d e n v a r i a b l e s [ i t a l i c s mine] Salmon i s c o n v i n c e d by van F r a a s s e n ' s c r i t i c i s m t h a t R e i c h e n b a c h ' s f o r m u l a t i o n o f t h e p r i n c i p l e o f t h e common cause i n terms o f t h e c o n j u n c t i v e f o r k ( F i g u r e 2 ) , i n w h i c h t h e common cause s c r e e n s o f f t h e one e f f e c t f r om t h e o t h e r , i s f a u l t y . B u t he t h i n k s t h a t van F r a a s s e n ' s c o u n t e r e x a m p l e can be h a n d l e d by t h e i n t r o d u c t i o n o f a second k i n d o f c a u s a l i n t e r a c t i o n i n w h i c h t h e r e s u l t i n g e f f e c t s a r e more s t r o n g l y c o r r e l a t e d w i t h one a n o t h e r t h a n i s a l l o w e d i n R e i c h e n b a c h * s c o n j u n c t i v e f o r k s . He c a l l s t h i s k i n d o f c a u s a l i n t e r a c t i o n t h e i n t e r a c t i v e f o r k ( F i g u r e 3) w h i c h i s e x e m p l i f i e d by t h e Compton s c a t t e r i n g o f a photon and an e l e c t r o n . 2 I f an e n e r g e t i c photon c o l l i d e s w i t h an e l e c t r o n i n a Compton s c a t t e r i n g e x p e r i m e n t , t h e r e i s a c e r t a i n p r o b a b i l i t y t h a t a photon w i t h a g i v e n s m a l l e r e n ergy w i l l emerge, and t h e r e i s a c e r t a i n p r o b a b i l i t y t h a t t h e e l e c t r o n w i l l be k i c k e d o u t w i t h a g i v e n k i n e t i c e n e r g y . However, because o f th e law o f c o n s e r v a t i o n o f e n e r g y , t h e r e i s a s t r o n g c o r r e s p o n d e n c e between t h e two e n e r g i e s — t h e i r sum must be c l o s e t o t h e energy o f t h e i n c i d e n t p h o t o n . Thus, t h e -•-van F r a a s s e n (1977), p. 146. 2 T h i s Compton s c a t t e r i n g example i s p r o v i d e d by Salmon i n h i s (1978) , pp. 692-694. 122 LEUKEMIA CONJUNCTIVE FORK P(A&B/C) = P(A/C)xP(B/C) SCREEN ING-OFF VICTIM F i g u r e 2 (Adapted f rom Salmon (1978), p. 695.) 1 S e e Salmon (1978), p. 688, f o r an a c c o u n t o f t h e l e u k e m i a example. -GOMPTON SCATTERING ELECTRON INTERACTIVE FORK P(A&B/C)> P(A/C)xP(B/C) N O N - S C R E E N 1 N G - O F F PHOTON E 2 E = E, + E 2 F i g u r e 3 (Adapted from Salmon (1978), p. 693.) 124 p r o b a b i l i t y o f g e t t i n g a p h o t o n w i t h e n e r g y a n d a n e l e c t r o n w i t h e n e r g y E 2 / w h e r e E ^ + E_ 2 i s a p p r o x i m a t e l y e q u a l t o E ( t h e e n e r g y o f t h e i n c i d e n t p h o t o n ) , i s m u c h g r e a t e r t h a n t h e p r o d u c t o f t h e p r o b a b i l i t i e s o f e a c h e n e r g y o c c u r r i n g s e p a r a t e l y . A s s u m e , f o r e x a m p l e , t h a t t h e r e i s a p r o b a b i l i t y o f 0.1 t h a t a p h o t o n o f e n e r g y E j w i l l e m e r g e i f a p h o t o n o f e n e r g y E i m p i n g e s o n a g i v e n t a r g e t , a n d a s s u m e t h a t t h e r e i s a p r o b a b i l i t y o f 0.1 t h a t a n e l e c t r o n w i t h k i n e t i c e n e r g y Ej? w i l l e m e r g e u n d e r t h e same c i r c u m s t a n c e s (where E , E ^ , a n d E_ 2 a r e r e l a t e d a s t h e l a w o f c o n s e r v a t i o n o f e n e r g y d e m a n d s ) . I n t h i s c a s e , t h e p r o b a b i l i t y o f t h e j o i n t r e s u l t i s n o t 0.01, t h e p r o d u c t o f t h e s e p a r a t e p r o b a b i l i t i e s , b u t 0.1, f o r e a c h r e s u l t w i l l o c c u r i f a n d o n l y i f t h e o t h e r d o e s . T h e same r e l a t i o n s h i p s c o u l d b e i l l u s t r a t e d b y s u c h m a c r o s c o p i c e v e n t s a s c o l l i s i o n s o f b i l l i a r d b a l l s . I n f o r k s o f t h e i n t e r a c t i v e s o r t s u c h a s t h e a b o v e C o m p t o n s c a t t e r i n g o f a p h o t o n a n d a n e l e c t r o n , t h e common c a u s e d o e s n o t s c r e e n - o f f t h e o n e e f f e c t f r o m t h e o t h e r . T h e p r o b a b i l i t y t h a t t h e e l e c t r o n w i l l b e e j e c t e d w i t h k i n e t i c e n e r g y E _ 2 g i v e n a n i n c i d e n t p h o t o n o f e n e r g y E i s n o t e q u a l t o t h e p r o b a b i l i t y t h a t t h e e l e c t r o n w i l l e m e r g e w i t h e n e r g y E_2 g i v e n a n i n c i d e n t p h o t o n o f e n e r g y E a n d a s c a t t e r e d p h o t o n o f e n e r g y E j _ . I n t h e c o n j u n c t i v e f o r k , t h e common c a u s e C a b s o r b s t h e d e p e n d e n c y b e t w e e n t h e e f f e c t s A a n d B , f o r t h e p r o b a b i l i t y o f A a n d B g i v e n C i s e q u a l t o t h e p r o d u c t o f t h e p r o b a b i l i t y o f A g i v e n C a n d t h e p r o b a b i l i t y o f B g i v e n C. I n t h e i n t e r a c t i v e f o r k , t h e common c a u s e C d o e s n o t a b s o r b t h e d e p e n d e n c y b e t w e e n t h e e f f e c t s A a n d B , f o r t h e p r o b a b i l i t y o f A a n d B g i v e n C i s g r e a t e r t h a n t h e p r o d u c t o f t h e t w o s e p a r a t e c o n d i t i o n a l p r o b a b i l i t i e s . 1 " Salmon . (1978) , p . 694 . 125 6.2.2 The Ghost of the Hidden Variables i s S t i l l Haunting Salmon appears to think that having disposed of van Fraassen's counterexample by introducing the i n t e r a c t i v e fork, he has thereby salvaged the p r i n c i p l e of the common  cause from the charge of demanding f o r hidden v a r i a b l e s . Unfortunately, t h i s i s not the case. Consider the following case.-'- We examine, one-at-a-time, a very large number of atoms of type A. Each examination l a s t s for a time i n t e r v a l i : , and t i s s l i g h t l y less than the h a l f - l i f e of substance A. The r e s u l t s show that 40% of the examined cases emit r a d i a t i o n , that i s , P(R,A)=.4. Now we assume that the reference c l a s s as s p e c i f i e d i s o b j e c t i v e l y homogeneous, that i s , there are no f a c t o r s C such that the employment of C can e f f e c t a s t a t i s t i c a l l y relevant p a r t i t i o n of the reference c l a s s A. (Of course, i n t h i s case, the lack of such factors i s not due to a lack of knowledge, f o r we have phy s i c a l grounds f o r supposing that there are no such factors.) Do we have, according to Salmon's C-R model (in which an explanation of an event includes a complete set of s t a t i s t i c a l l y relevant f a c t o r s , the pertinent p r o b a b i l i t y values, and a causal explanation of the relevant r e l a t i o n s 2 ) , x T h i s case and i t s s i g n i f i c a n c e are suggested to me by Professor Edwin Levy. 2Though there i s a sharp conceptual d i s t i n c t i o n between the terms ' s t a t i s t i c a l relevance r e l a t i o n s * and ' s t a t i s t i c a l r e g u l a r i t i e s , ' Salmon himself i s using them as interchangeable terms. a bona f i d e explanation i n a case of the above sort i n which we f i n d that an atom has emitted radiation? The answer i s negative. For Salmon maintains that "the s t a t i s t i c a l relevance r e l a t i o n s are s t a t i s t i c a l r e g u l a r i t i e s , " 1 and " r e g u l a r i t i e s — a n d t h i s c e r t a i n l y includes s t a t i s t i c a l r e g u l a r i t i e s — r e q u i r e causal explanation." 2 That i s to say, " i f the r e g u l a r i t y invoked i s not a causal r e g u l a r i t y , then a causal explanation of that very r e g u l a r i t y must be made part of the explanation of the event." 3 But the s t a t i s t i c a l r e g u l a r i t y i n question, namely "40% of the atoms of type A emit r a d i a t i o n , " being i r r e d u c i b l y s t a t i s t i c a l , seems to defy any causal explanation. Salmon claims that there i s a clear conceptual d i s t i n c t i o n between c a u s a l i t y and determinism; the l a t t e r he takes to be, the doctrine which says that "there are no genuinely homogeneous reference classes except i n the l i m i t i n g cases when a l l A are B or no A are B."^ And he thinks that "by employing a s t a t i s t i c a l conception of causation . . . , i t i s possible to f i t together harmoniously the causal and s t a t i s t i c a l f actors i n explanatory contexts." 5 In h i s opinion there i s no reason to make an a p r i o r i d e c i s i o n as to the nature of further p h y s i c a l theories. Perhaps, i n ^-Salmon (1978), p. 699. 2 I b i d . , p. 688. 3 I b i d . , p. 699. 4Salmon (1977a), p. 155. 5Salmon (1978), p. 688. 127 t h e f u t u r e , improved t h e o r i e s w i l l p r o v i d e a d e t e r m i n i s t i c a c c o u n t o f e v e n t s t h a t c u r r e n t t h e o r y r e g a r d s as c a u s a l l y u n d e t e r m i n e d — b u t p e r h a p s t h e y w i l l n o t . We s h o u l d be p r e p a r e d f o r t h e p o s s i b i l i t y t h a t t h e i n d e t e r m i n i s t i c c h a r a c t e r o f p h y s i c a l t h e o r i e s i s c o r r e c t and t h a t t h e r e a r e e v e n t s w h i c h a r e i n t r i n s i c a l l y i m p r o b a b l e , n o t m e r e l y i m p r o b a b l e i n r e l a t i o n t o o u r p r e s e n t i n c o m p l e t e knowledge. . . . I t seems d e s i r a b l e f o r a t h e o r y o f e x p l a n a t i o n t o ad m i t t h e p o s s i b i l i t y o f e v e n t s t h a t a r e i n t r i n s 1 c a 1 l y  u n d e t e r m i n e d [ i t a l i c s mine] . . . . To deny t h a t any e v e n t s a r e u n d e t e r m i n e d seems t o i n v o l v e an u n w a r r a n t e d a p r i o r i commitment t o d e t e r m i n i s m . 1 E l s e w h e r e , i n t h e c o n t e x t o f h i s l e u k e m i a example, Salmon s a y s : A t each end o f t h e c a u s a l p r o c e s s — i . e . , t h e t r a n s m i s s i o n o f r a d i a t i o n from t h e bomb t o t h e p e r s o n - — t h e r e i s a c a u s a l i n t e r a c t i o n . The r a d i a t i o n i s e m i t t e d as a r e s u l t o f a .nuc l e a r i n t e r a c t i o n when t h e bomb e x p l o d e s , and i t i s a b s o r b e d by c e l l s i n t h e body o f t h e v i c t i m . Each o f t h e s e i n t e r a c t i o n s may w e l l be i r r e d u c i b l y s t a t i s t i c a l and i n d e t e r m i n i s t i c , b u t t h a t i s no r e a s o n t o deny t h a t t h e y a r e c a u s a l . 2 The r e a l p r o b l e m now i s t o u n d e r s t a n d how a c a u s a l e x p l a n a t i o n can be o f f e r e d f o r t h e i r r e d u c i b l y s t a t i s t i c a l r e g u l a r i t y t h a t "40% o f t h e atoms o f t y p e A e m i t r a d i a t i o n . " -C h a r a c t e r i s t i c o f most, i f n o t a l l , quantum phenomena, t h e f o r e g o i n g a t o m - e m i t s - r a d i a t i o n c a s e e x h i b i t s two f e a t u r e s , namely b e i n g i r r e d u c i b l y s t a t i s t i c a l o r g e n u i n e l y i n d e t e r m i n i s t i c and i n v o l v i n g s p a t i o - t e m p o r a l l y d i s c o n t i n u i t i e s , w h i c h seem t o p r e s e n t d i f f i c u l t i e s t o Salmon's C-R model o f e x p l a n a t i o n . I n o t h e r words, i f t h e C-R model p u r p o r t s t o be an ade q u a t e , comprehensive model o f s c i e n t i f i c e x p l a n a t i o n , t h e n Salmon has t o be a b l e t o p r o v i d e " c a u s a l e x p l a n a t i o n s ± S a l m o n (1971), p. 10. 2 S a l m o n (1978), p. 689. 128 (via causal processes and causal i n t e r a c t i o n s ) of i r r e d u c i b l y s t a t i s t i c a l r e g u l a r i t i e s " as well as "causal explanations (via causal processes and causal i n t e r a c t i o n s ) of sp a t i o -temporal d i s c o n t i n u i t i e s . " In d i s t i n g u i s h i n g determinism from c a u s a l i t y , Salmon argues that the f a i l u r e of determinism i s one t h i n g t h e v i o l a t i o n of c a u s a l i t y quite another. As I understand i t , determinism i s the thesis that (loosely speaking) the occurrence of an event has p r o b a b i l i t y zero or one i n the presence of a complete set of s t a t i s t i c a l l y relevant conditions. Indeterminism, by contrast, obtains i f there are complete sets of s t a t i s t i c a l l y relevant conditions ( i . e . homogeneous reference classes) with respect to which the event may e i t h e r happen or not — t h e p r o b a b i l i t y of i t s occurrence has some intermediate value other than zero or one. The breakdown of c a u s a l i t y l i e s i n the fac t that (in the quantum domain) causal influence i s not transmitted with spatio-temporal continuity. . . . Causal influence need not be deter m i n i s t i c to e x h i b i t c o n t i n u i t y ; we are construing causal relevance as a species of s t a t i s t i c a l relevance. Causality, i n t h i s sense, i s e n t i r e l y compatible with indeterminism, but quantum mechanics goes beyond indeterminism i n i t s admission of f a m i l i a r s p a t i o -temporal d i s c o n t i n u i t i e s . ! He also says: One very basic and important p r i n c i p l e concerning causal r e l e v a n c e — i . e . the transmission of m a r k s — i s , nevertheless, that i t seems to be embedded i n continuous processes [ i t a l i c s mine] . Marks (or information) are transmitted continuously i n space and time. . . . The fact that spatio-temporal c o n t i n u i t y breaks down i n quantum mechanics—quantum mechanics seems avoidably to engender causal a n o m a l i e s — i s a source of great d i s t r e s s . 2 He concludes that " i t i s a fac t about t h i s world (at l e a s t as long as we stay out of the quantum realm) that there Salmon (1975) , p.. 133. 2.ibid., pp. 132-133. 1 2 9 a r e many c o n t i n u o u s c a u s a l p r o c e s s e s t h a t do t r a n s m i t marks." 1' I t i s e v i d e n t f r o m t h e above p a s s a g e s t h a t Salmon t h i n k s he can p r o v i d e " c a u s a l e x p l a n a t i o n s ( v i a c a u s a l p r o c e s s e s and c a u s a l i n t e r a c t i o n s ) o f i r r e d u c i b l y s t a t i s t i c a l o r g e n u i n e l y i n d e t e r m i n i s t i c r e g u l a r i t i e s . " But i t c e r t a i n l y a p p e a r s t h a t he has g i v e n up any a t t e m p t t o p r o v i d e " c a u s a l e x p l a n a t i o n s ( v i a c a u s a l p r o c e s s e s and c a u s a l i n t e r a c t i o n s ) o f s p a t i o - t e m p o r a l d i s c o n t i n u i t i e s " b e s t e x e m p l i f i e d by quantum phenomena, f o r t h e t r a n s m i s s i o n o f marks, w h i c h i s t h e c h i e f ( o n l y ? ) c h a r a c t e r i s t i c o f a c a u s a l p r o c e s s , i s a l w a y s "embedded i n c o n t i n u o u s p r o c e s s e s . " K a t z ( i n p e r s o n a l communication) t r i e s t o remove t h e c l a s h between t h e c o n t i n u o u s n a t u r e o f c a u s a l p r o c e s s e s and t h e s p a t i o - t e m p o r a l d i s c o n t i n u i t i e s i n quantum m e c h a n i c s by means o f an a d a p t a t i o n o f R u s s e l l ' s ' a t - a t ' t h e o r y o f m o t i o n and a p r o p e r c h a r a c t e r i z a t i o n o f R e i c h e n b a c h * s 'mark' method. U p h o l d i n g t h a t Salmon need n o t be s a d d l e d w i t h c o n t i n u o u s c a u s a l p r o c e s s e s , he a r g u e s t h a t a p r o c e s s i s q u a l i f i e d a s c a u s a l o n l y i f i t t r a n s m i t s a 'mark.' The p r o p a g a t i o n o f t h i s mark g u a r a n t e e s t h e ' r e l a t e d n e s s ' o f t h i s p r o c e s s , and t h i s ' r e l a t e d n e s s ' i s p r o v i d e d by t h e 'mark' b e i n g ' a t ' t h e end i f i t i s ' a t ' t h e b e g i n n i n g o f t h e p r o c e s s (and ' a t * each p o i n t i n between). Yet none o f t h i s i m p l i e s , he m a i n t a i n s , t h a t c a u s a l p r o c e s s e s must be c o n t i n u o u s . I b i d . , p. 1 4 0 . In my opinion, Katz's attempt at removing the clash between the continuous nature of causal processes and the spatio-temporal d i s c o n t i n u i t i e s i s unsuccessful. His basic idea seems to be t h i s . I f we observe that a c e r t a i n 'mark' i s 'at' the beginning of a process, be i t continuous or discontinuous, and that i t i s also 'at* the end of the same process, then the 'mark' i s regarded as properly propagated throughout the process. Thus, t h i s process i s a causal process because i t transmits a 'mark.* But a serious d i f f i c u l t y a r i s e s . On what grounds are we j u s t i f i e d i n s t i p u l a t i n g that the 'mark,' which i s 'at' the beginning of a spatio-temporally discontinuous process, i s the very same 'mark' that i s 'at' the end of the process? I can f i n d no adequate grounds for such a s t i p u l a t i o n . Yet i f t h i s f a c t cannot be established, then we are hot e n t i t l e d to hold that the spatio-temporally discontinuous process i s causal i n character, f o r we do not know whether the 'mark' i s a c t u a l l y propagated throughout the process or not. Here, we must d i s t i n g u i s h between processes which are inherently spatio-temporally discontinuous, and processes which are merely epistemically or perceptually discontinuous Quantum mechanics provides the best (only?) cases f o r the former. A t y p i c a l case of the l a t t e r looks l i k e t h i s . We observe that an aeroplane leaves the ground and f l i e s across the sky; we then lose sight of i t when i t plunges into a thick cloud; we see i t again a few seconds l a t e r when i t gets out of the cloud. I t i s not possible for us to trace 131 v i s u a l l y a s p a t i o - t e m p o r a l l y c o n t i n u o u s t r a j e c t o r y o f t h e p l a n e from t h e t i m e i t l e a v e s t h e ground t o t h e t i m e when we see i t a g a i n as i t g e t s o u t o f t h e c l o u d . Y e t , we can draw a c c e p t a b l e t r a j e c t o r i e s i f we assume t h a t t h e movements o f t h e p l a n e a r e s p a t i o - t e m p o r a l l y c o n t i n u o u s . When I use t h e t e r m ' s p a t i o - t e m p o r a l l y d i s c o n t i n u o u s , ' I mean t h o s e p r o c e s s e s w h i c h a r e i n h e r e n t l y , n o t m e r e l y e p i s t e m i c a l l y o r p e r c e p t u a l l y , d i s c o n t i n u o u s . As a m a t t e r o f f a c t , a s i m i l a r d i f f i c u l t y can be r a i s e d i n a d i f f e r e n t way. I n a d o p t i n g R u s s e l l ' s ' a t - a t ' t h e o r y o f m o t i o n and R e i c h e n b a c h * s 'mark' method, Salmon m a i n t a i n s t h a t " t h e t r a n s m i s s i o n o f a mark from p o i n t A i n a c a u s a l p r o c e s s t o p o i n t B i n t h e same [ i t a l i c s mine] p r o c e s s i_s t h e f a c t t h a t i t app e a r s a t each p o i n t between A and B w i t h o u t f u r t h e r i n t e r a c t i o n s . " x But t h e p r o b l e m i s what p r o v i d e s f o r t h e p r o c e s s a t B b e i n g t h e same p r o c e s s t h a t o c c u r s a t A; c e r t a i n l y , t h e p r o p a g a t i o n o f a 'mark* won't do, even i f t h e p r o c e s s i s c o n t i n u o u s , b u t p a r t i c u l a r l y when t h e p r o c e s s i s s p a t i o - t e m p o r a l l y d i s c o n t i n u o u s . A c t u a l l y , i t i s v e r y d o u b t f u l t h a t , i n o u r a t o m - e m i t s -r a d i a t i o n c a s e , t h e p r o c e s s by w h i c h a s u b - a t o m i c p a r t i c l e g e t s f r o m t h e atom t o a s c i n t i l l a t i o n s c r e e n can be d e s c r i b e d by an ' a t - a t ' c a u s a l p r o c e s s . Any such a t t e m p t w o u l d seem t o i n v o k e h i d d e n v a r i a b l e s , f o r how e l s e c o u l d we g e t a t r a j e c t o r y i n t h i s c a s e ? Salmon ( 1 9 7 7 c ) , p. 221. 132 Now, even supposing that there can be causal processes containing spatio-temporal d i s c o n t i n u i t i e s , i s Salmon able to provide a causal account of why a c e r t a i n atom of type A emitted radiation? A causal account might go i n the following way. What happens i s that we observe a f l a s h on a s c i n t i l l a t i o n s c r e e n — t h i s i s an i n t e r a c t i v e event. And i f we ask why t h i s event came about, the answer i s that a previous event has occurred, that i s , the atom has (spontaneously) emitted r a d i a t i o n . These two i n t e r a c t i v e events or i n t e r a c t i o n s are supposedly connected by a causal process. But suppose we ask about the spontaneous emission of r a d i a t i o n from the atom, "Why did the atom emit ra d i a t i o n ? " Can there be a causal answer to t h i s question? Presumably the answer i s 'Yes.' But since a causal i n t e r a c t i o n , as suggested by Salmon, i s the ' i n t e r s e c t i o n * of (at least) two causal processes, what would the causal processes i n t h i s case? As the emission of r a d i a t i o n from the atom i s spontaneous, I guess the only causal processes that we can c i t e would be those which account f o r the atom being 'observed' by us. For example, one causal process might be my carrying a box with type A atoms i n s i d e and the other causal process i s my brother running q u i c k l y towards me; the i n t e r s e c t i o n i s the two people c o l l i d i n g and the box pops open, thereby rel e a s i n g the atoms which can now be 'observed' by us. Yet, t h i s i s such an attenuated sense of causal process that i t seems pickwickian to regard i t as a causal answer to the question, "Why did the atom emit 133 r a d i a t i o n ? " . Furthermore, i t appears that Salmon could not provide a causal explanation of why the p a r t i c u l a r atom under consideration emitted r a d i a t i o n whereas another atom i n the same box d i d not unless he invokes hidden v a r i a b l e s . The same conclusion can be drawn concerning the Compton sc a t t e r i n g case. Let us grant that Salmon i s c o r r e c t i n saying that the c o r r e l a t i o n between and E 2 i s explained by an i n t e r a c t i v e fork (see Figure 3 above). Now assume that there are only three possible energy values obtainable i n the upper beam, namely E^ or E]_' or E^", and a l s o three possible energy values i n the lower beam, namely E 2 or E 2 ' or E 2 " . Assume further that each of these values occurs one-third of the time, and they are c o r r e l a t e d as the law of conservation of energy demands (Figure 4 ) . Now i f we take those cases i n which E^ occur and ask the question, "Why did energy value E-^_ show up?", i t seems that no causal explanation of t h i s f a c t can be given unless we invoke hidden v a r i a b l e s . By introducing the notion of an i n t e r a c t i v e fork, Salmon can indeed explain the c o r r e l a t i o n between E^ and E 2 by invoking the common cause E. However, the foregoing question asks, not about the c o r r e l a t i o n between E^ and E 2 , but why did E-^  show up, and there does not seem to be a causal explanation of the l a t t e r f a c t , even i n terms of the i n t e r a c t i v e fork, i f we do not invoke hidden v a r i a b l e s . The foregoing discussion has two consequences, neither 134 COMPTON SCATTERING ELECTRON E = E 1 + E 2 or E^+ E 2 ' o r E-,"+ E 2" F i g u r e 4 135 of which i s welcome to Salmon's C-R model of explanation. On the one hand, quantum phenomena (best exemplified by the atom-emits-radiation case), being both spatio-temporally discontinuous and i r r e d u c i b l y s t a t i s t i c a l , do seem to defy causal explanations i n Salmon's sense unless hidden v a r i a b l e s are invoked. But a theory of hidden v a r i a b l e s always involves some form of 'determinism,' and t h i s destroys Salmon's conviction that "regardless of whether indeterminism i s true, we need an e x p l i c a t i o n of s c i e n t i f i c explanation which i s neutral regarding that i s s u e . O n the other hand, i f hidden variables are t o be avoided, then most quantum phenomena would have to be relegated to the domain of ine x p l i c a b l e phenomena i f the C-R model of explanation i s adopted. This w i l l r e s u l t i n a large lacuna i n science that the C-R model cannot f i l l ; thus, i t f a l l s short of being an adequate, comprehensive model of s c i e n t i f i c explanation. Salmon (1977a), p. 158. 136 6.3 A M o d i f i e d I-S Model o f E x p l a n a t i o n In t h i s s e c t i o n , I s h a l l argue t h a t a m o d i f i e d I-S model of e x p l a n a t i o n a v o i d s the d i f f i c u l t i e s t h a t Salmon r a i s e s a g a i n s t i t . As a r e s u l t , i t w i l l be as adeguate a model o f e x p l a n a t i o n as Salmon's C-R model. 6 . 3 . 1 E x p l a n a t i o n s as Arguments As Salmon has c o n s i s t e n t l y p o i n t e d o u t , b o t h the D-N and the I-S model of e x p l a n a t i o n are c h a r a c t e r i z e d by Hempel as "an argument [ i t a l i c s mine] t o the e f f e c t t h a t t h e phenomenon t o be e x p l a i n e d , the explanandum phenomenon, was t o be expected i n v i r t u e o f c e r t a i n e x p l a n a t o r y f a c t s . H e m p e l h o l d s t h i s because he t h i n k s t h a t an e x p l a n a t i o n s h o u l d answer the q u e s t i o n "'Why d i d the explanandum-phenomenon o c c u r ? 1 by showing t h a t , . . . g i v e n the p a r t i c u l a r c i r c u m s t a n c e s and the laws i n q u e s t i o n , t h e o c c u r r e n c e of the phenomenon was to be expected; and i t i s i n t h i s sense t h a t the e x p l a n a t i o n enables us t o understand why the phenomenon o c c u r r e d . " T h i s v e r y same i d e a i s e x p r e s s e d i n a d i f f e r e n t c o n t e x t as the g e n e r a l c o n d i t i o n of adequacy f o r any r a t i o n a l l y  a c c e p t a b l e e x p l a n a t i o n of a p a r t i c u l a r e vent. That c o n d i t i o n i s the f o l l o w i n g : Any r a t i o n a l l y a c c e p t a b l e 336. (1965), p. 337. Hempel (19 65c) , p. Hempel & Oppenheim answer t o t h e q u e s t i o n 'Why d i d e v e n t X o c c u r ? ' must o f f e r i n f o r m a t i o n w h i c h shows t h a t X was t o be e x p e c t e d — i f n o t d e f i n i t e l y , as i n t h e c a s e o f D-N e x p l a n a t i o n , t h e n a t l e a s t w i t h r e a s o n a b l e p r o b a b i l i t y . Thus, t h e e x p l a n a t o r y i n f o r m a t i o n must p r o v i d e good grounds f o r b e l i e v i n g t h a t X d i d i n f a c t o c c u r ; o t h e r w i s e , t h a t i n f o r m a t i o n would g i v e us no adequate r e a s o n f o r s a y i n g : "That e x p l a i n s i t — t h a t does show t h a t X o c c u r r e d . 1 1 1 I s h a l l c a l l t h e r e q u i r e m e n t t h a t t h e explanandum e v e n t 'was t o be e x p e c t e d ' i n v i r t u e o f t h e e x p l a n a n s t h e ' e x p e c t a n c y r e q u i r e m e n t . ' 6 . 3 . 2 Why i s H i g h P r o b a b i l i t y Needed i n I-S E x p l a n a t i o n ? I n a D-N e x p l a n a t i o n i n w h i c h t h e explanandum i s d e d u c i b l e from t h e e x p l a n a n s , t h e ' e x p e c t a n c y r e q u i r e m e n t ' i s a u t o m a t i c a l l y s a t i s f i e d s i n c e t h e e x p l a n a n s l o g i c a l l y n e c e s s i t a t e s t h e explanandum. T h i s c a n n o t be s a i d , however, o f an I-S e x p l a n a t i o n i n w h i c h th e explanandum does n o t f o l l o w l o g i c a l l y from the e x p l a n a n s . But I-S e x p l a n a t i o n s , • Hempel m a i n t a i n s , a r e as g e n u i n e e x p l a n a t i o n s as D-N o n es. He a r g u e s : I t i s a l s o sometimes t h o u g h t t h a t b e c a u s e p r o b a b i l i s t i c arguments a r e n o t l o g i c a l l y c o n c l u s i v e t h e y c annot s e r v e t o e x p l a i n , f o r even i f t h e e x p l a n a n s i s t r u e , i t i s s t i l l p o s s i b l e t h a t t h e explanandum phenomenon m i g h t n o t have come about. . . . B u t t h i s o b j e c t i o n t o t h e i d e a o f p r o b a b i l i s t i c e x p l a n a t i o n r e s t s on a t o o r e s t r i c t i v e c o n c e p t i o n o f s c i e n t i f i c e x p l a n a t i o n f o r many i m p o r t a n t e x p l a n a t o r y a c c o u n t s o f f e r e d by e m p i r i c a l s c i e n c e make q u i t e e x p l i c i t use o f s t a t i s t i c a l l a ws w h i c h , i n c o n j u n c t i o n w i t h t h e r e s t o f t h e e x p l a n a t o r y i n f o r m a t i o n adduced, make, t h e explanandum no more t h a n h i g h l y p r o b a b l e . 2 xHempel (1 9 6 5 c ) , pp. 367-368. 2 I b i d . , p. 391. 138 A c c o r d i n g t o Hempel, i f t h e e x p l a n a t i o n i s i n d u c t i v e -s t a t i s t i c a l , i t " e x p l a i n s a g i v e n phenomenon by s h o w i n g t h a t , i n v i e w o f c e r t a i n p a r t i c u l a r f a c t s and c e r t a i n s t a t i s t i c a l l a w s , i t s o c c u r r e n c e was t o be e x p e c t e d w i t h h i g h l o g i c a l , o r i n d u c t i v e , p r o b a b i l i t y . 1 , 1 I n t h i s c a s e , t h e ' e x p e c t a n c y r e q u i r e m e n t ' i s s a t i s f i e d as t h e e x p l a n a n s c o n f e r s h i g h p r o b a b i l i t y upon t h e explanandum; t h e e x p l a n a n s makes t h e explanandum e v e n t h i g h l y p r o b a b l e . I n Salmon's w o r d s , "Hempel's r e q u i r e m e n t t h a t t h e i n d u c t i v e p r o b a b i l i t y o f t h e c o n c l u s i o n r e l a t i v e t o t h e p r e m i s e s .be h i g h i s b e s t u n d e r s t o o d . . . as a consequence o f t h e f a c t t h a t any s u i t a b l e i n d u c t i v e r u l e o f a c c e p t a n c e would demand, among o t h e r t h i n g s , a h i g h p r o b a b i l i t y f o r t h e c o n c l u s i o n r e l a t i v e 2 t o t h e p r e m i s e s . " • 6 . 3 . 3 Salmon's Co u n t e r e x a m p l e s and T h e i r D i s s o l u t i o n A g a i n s t t h e Hempelian a c c o u n t s o f e x p l a n a t i o n , Salmon o f f e r s t h e f o l l o w i n g c o u n t e r e x a m p l e s : (1) John Jones was a l m o s t c e r t a i n t o r e c o v e r f r o m h i s c o l d w i t h i n a week, because he t o o k v i t a m i n C, and a l m o s t a l l c o l d s c l e a r up w i t h i n a week a f t e r a d m i n i s t r a t i o n o f v i t a m i n C. T h i s example c o r r e s p o n d s e x a c t l y w i t h Hempel's I-S schema o f e x p l a n a t i o n . But t h e d i f f i c u l t y w i t h (1), as Salmon s e e s xHempel (1962b), p. 14. 2 S a l m o n (1977b), p. 183. 139 i t , i s that colds tend to c l e a r up within a week regardless of the f a c t that the patient has taken vitamin C or n o t . 1 Analogously, Salmon s t i p u l a t e s , counterexamples to the D-N schema can also be constructed: (2) This sample of table s a l t dissolves i n water, f o r i t has had a d i s s o l v i n g s p e l l cast upon i t , and a l l samples of table s a l t that have had d i s s o l v i n g s p e l l s cast upon them disso l v e i n water. (3) John Jones avoided becoming pregnant during the past year, for he has taken h i s wife's b i r t h c o n t r o l p i l l s r e g u l a r l y , and every man who r e g u l a r l y takes b i r t h c o n t r o l p i l l s avoids pregnancy. These two examples, which correspond exactly with the D-N schema, share a s i m i l a r d i f f i c u l t y with (1). For s a l t d i s s o l v e s , s p e l l or no s p e l l , so we do not need to explain the d i s s o l v i n g of t h i s sample i n terms of a hex; and men do not become pregnant, p i l l s or no p i l l s , so the consumption of o r a l contraceptives i s not required to explain the phenomenon i n John Jones' case (though i t may have considerable explanatory force with regard to h i s wife's pregnancy). In Salmon's opinion, the obvious trouble with these This i s s t i l l a hotly debated medical issue at the moment. But f o r the sake of the argument, we w i l l assume that colds do tend to c l e a r up within a week regardless of whether the patient has taken vitamin C or not. 140 h o r r i b l e examples i s t h a t . . . t h e " e x p l a n a t o r y " argument i s n o t needed t o make us see t h a t t h e explanandum e v e n t was t o be e x p e c t e d . There a r e o t h e r , more s a t i s f a c t o r y , grounds f o r t h i s e x p e c t a t i o n . The " e x p l a n a t o r y f a c t s " adduced a r e i r r e l e v a n t [ i t a l i c s minej t o t h e explanandum e v e n t d e s p i t e t h e f a c t t h a t t h e explanandum f o l l o w s ( d e d u c t i v e l y o r i n d u c t i v e l y ) from t h e e x p l a n a n s . T a b l e s a l t d i s s o l v e s i n w a t e r r e g a r d l e s s o f h e x i n g , a l m o s t a l l c o l d s c l e a r up w i t h i n a week r e g a r d l e s s o f t r e a t m e n t , m ales do n o t g e t p r e g n a n t r e g a r d l e s s o f p i l l s . . . . Each o f t h e s e explanandum e v e n t s has a h i g h p r i o r p r o b a b i l i t y i n d e p e n d e n t o f t h e e x p l a n a t o r y f a c t s , and t h e p r o b a b i l i t y o f t h e explanandum e v e n t r e l a t i v e t o t h e e x p l a n a t o r y f a c t s i s t h e same as t h i s p r i o r p r o b a b i l i t y . I n t h i s sense t h e e x p l a n a t o r y f a c t s do n o t h i n g t o enhance t h e p r o b a b i l i t y o f t h e explanandum e v e n t o r t o make us more c e r t a i n o f i t s o c c u r r e n c e t h a n we w o u l d o t h e r w i s e have been. T h i s i s n o t because we know t h a t t h e f a c t t o be e x p l a i n e d has o c c u r r e d ; i t i s b e c a u s e we had o t h e r grounds f o r e x p e c t i n g i t t o o c c u r , even i f we  had h o t a l r e a d y w i t n e s s e d i t . x The u l t i m a t e a im o f Salmon's c o u n t e r e x a m p l e s , I g a t h e r , i s t o s u b s t a n t i a t e h i s c o n t e n t i o n t h a t i t i s i n c o r r e c t t o c h a r a c t e r i z e e x p l a n a t o r y a c c o u n t s as arguments showing t h a t t h e explanandum e v e n t was t o be e x p e c t e d . F o r , a s i s e v i d e n t from t h e s e examples, t h e r e a r e ' o t h e r more s a t i s f a c t o r y g r o u n d s , ' b u t n o t t h e ' i r r e l e v a n t ' e x p l a n a t o r y f a c t s adduced, w h i c h show t h a t t h e explanandum e v e n t was t o be e x p e c t e d . But what e x a c t l y a r e t h e s e ' o t h e r more s a t i s f a c t o r y g r o u n d s ' and i n what sense a r e t h e e x p l a n a t o r y f a c t s adduced ' i r r e l e v a n t ' ? L e t us examine t h i s i s s u e i n t h e c o n t e x t o f c o u n t e r e x a m p l e ( 3 ) . F o l l o w i n g Salmon's argument, the-explanandum event "John Jones i s n o t p r e g n a n t " i s n o t Salmon (1971), p. 36. 141 e x p l a i n e d b y t h e e x p l a n a t o r y f a c t s (a) J o h n J o n e s i s a man who t a k e s b i r t h c o n t r o l p i l l s a n d (b) a l l men who t a k e b i r t h c o n t r o l p i l l s r e g u l a r l y d o n o t g e t p r e g n a n t ; f o r t h e r e a r e ' o t h e r m o r e s a t i s f a c t o r y g r o u n d s , ' n a m e l y ( a ' ) J o h n J o n e s i s a man a n d ( b ' ) a l l men d o n o t g e t p r e g n a n t r e g a r d l e s s o f p i l l s , w h i c h show t h a t t h e e x p l a n a n d u m e v e n t was t o b e e x p e c t e d . O n l y ( a ' ) a n d ( b ' ) , b u t n o t (a) a n d ( b ) , c o n s t i t u t e g e n u i n e e x p l a n a t o r y f a c t s f o r t h e e x p l a n a n d u m e v e n t b e c a u s e t h e l a t t e r two f a c t s a r e ' i r r e l e v a n t ' t o t h e e x p l a n a n d u m e v e n t , d e s p i t e t h e f a c t t h a t t h e e v e n t f o l l o w s d e d u c t i v e l y f r o m t h e m . I n o t h e r w o r d s , n o t e v e r y a r g u m e n t , d e d u c t i v e o r i n d u c t i v e , w h i c h s a t i s f i e s H e m p e l ' s c o n d i t i o n s o f a d e q u a c y , w o u l d a u t o m a t i c a l l y c o u n t a s a g e n u i n e e x p l a n a t i o n . I c a n n o t a g r e e , h o w e v e r , w i t h S a l m o n ' s s t i p u l a t i o n s . R a t h e r , I w o u l d m a i n t a i n t h a t t h e e x p l a n a t i o n o f " J o h n J o n e s -i s n o t p r e g n a n t " b y means o f (a) a n d (b) i s a s g e n u i n e a n e x p l a n a t i o n a s t h e o n e c o m p r i s i n g ( a ' ) a n d ( b * ) , t h o u g h t h e l a t t e r e x p l a n a t i o n i s a d m i t t e d l y a ' b e t t e r ' e x p l a n a t i o n t h a n t h e f o r m e r o n e b e c a u s e i t i s ' g r e a t e r i n g e n e r a l i t y . ' ^ T h e r e i s n o t h i n g o b j e c t i o n a b l e a b o u t S a l m o n ' s c l a i m t h a t t h e e x p l a n a t o r y f a c t s a d d u c e d , v i z . (a) a n d ( b ) , a r e ' i r r e l e v a n t ' t o t h e e x p l a n a n d u m e v e n t b e c a u s e " t h e e x p l a n a n d u m e v e n t h a s a h i g h p r i o r p r o b a b i l i t y i n d e p e n d e n t o f t h e e x p l a n a t o r y f a c t s , a n d t h e p r o b a b i l i t y o f t h e e x p l a n a n d u m e v e n t r e l a t i v e See s e c . 5.4. 142 to the explanatory f a c t s i s the same as t h i s p r i o r p r o b a b i l i t y , 1 , 1 except for the fac t that he has t a c i t l y  assumed (and on excellant grounds too) that (b') i s always known p r i o r to (b). This i s true, as a matter of f a c t , i n the case of (3) as we know that men do not get pregnant long before we know that taking b i r t h c o n t r o l p i l l s r e g u l a r l y w i l l prevent pregnancy. This p r i o r i t y , however, i s not l o g i c a l but only f a c t u a l . I t i s not only t h e o r e t i c a l l y conceivable that we might have known the less general f a c t (b) p r i o r to the more general f a c t (b") and consequently use (b) i n our i n i t i a l explanation as.(b*) i s not yet a v a i l a b l e , but that we often progress from a les s general f a c t to a more general one. For example, children normally learn f i r s t the p a r t i c u l a r f a c t that adding one apple or cup to another w i l l give two apples or cups before they acquire the general f a c t that 1+1=2. And i f (b) were known p r i o r to ( b 1 ) , the explanandum event that "John Jones i s not pregnant" would not have a known high p r i o r p r o b a b i l i t y independent of the explanatory f a c t s (a) and (b) since the very 'pri o r ' p r o b a b i l i t y i s dependent upon (a) and (b). Consequently, the explanatory fa c t s (a) and (b) would indeed be relevant to the explanandum event. Surely since, as a matter of f a c t , we know (b') long before we know (b), we are apt to ref e r to 'the' explanation Salmon (1971), p. 36. 143 of why "John Jones does not get pregnant" as c o m p r i s i n g (a') and ( b ' ) , but not (a) and (b). T h i s does not go t o show, however, t h a t o n l y the former, but not the l a t t e r , i s a genuine e x p l a n a t i o n . I t o n l y r e v e a l s t h a t the former i s • g r e a t e r i n g e n e r a l i t y ' than the l a t t e r and a f f o r d s t h e r e f o r e a ' b e t t e r ' e x p l a n a t i o n of the explanandum e v e n t . In view o f the f o r e g o i n g a n a l y s i s , i t s h o u l d be s a f e t o conclude t h a t counterexample (3) has been s t r i p p e d o f i t s i n i t i a l p e r s u a s i v e n e s s . In l i k e manner, counterexamples (1) and (2) can a l s o be d i s m i s s e d . I t s h o u l d be p o i n t e d out here t h a t Hempel h i m s e l f i s v e r y much aware of the danger of i r r e l e v a n t p remises c r e e p i n g ,into the explanans of a D-N e x p l a n a t i o n which t a k e s the form of a d e d u c t i v e argument. For i n a d e d u c t i v e argument, the c o n c l u s i o n 'was t o be expected' i f i t i s l o g i c a l l y i m p l i e d by the premises, but any a d d i t i o n a l o r redundant premises would not d e s t r o y the v a l i d i t y o f the argument i t s e l f . As Salmon promptly p o i n t s out, i f one were t o o f f e r the argument, A l l men are m o r t a l . S o c r a t e s i s a man. Xahtippe i s a woman. So c r a t e s i s m o r t a l . i t would seem s t r a n g e , and perhaps m i l d l y amusing, b u t i t s l o g i c a l s t a t u s would not be i m p a i r e d by the p r e s e n c e of the t h i r d p r e m i s e . 1 Yet, Salmon m a i n t a i n s , t h i s d e d u c t i v e l y v a l i d argument does Salmon (1977a), p. 150. 144 not q u a l i f y as a D-N explanation because the t h i r d premise, which may be retained i n the argument despite the f a c t that i t i s not needed for the de r i v a t i o n of the conclusion, cannot be kept as part of the explanans i n the explanation as i t serves no explanatory function at a l l . In my opinion, Hempel's condition (R2), which states that "the explanans must contain general laws, and these must a c t u a l l y be required [ i t a l i c s mine] for the d e r i v a t i o n of the explanandum," 1 i s proposed i n order to keep i r r e l e v a n t premises away from the explanans. That i s to say, i t i s not allowed to add another law, say " A l l men are mammals,' to the above argument because t h i s law would riot a c t u a l l y be required f o r the d e r i v a t i o n of the conclusion. Although i r r e l e v a n t laws are kept away from the explanans by means of (R2), i r r e l e v a n t antecedent conditions such as the t h i r d . premise i n the foregoing argument are s t i l l not. Thus, I suggest that (R2) be expanded to include antecedent conditions as well as general laws, such that both must a c t u a l l y be required f o r the d e r i v a t i o n of the explanandum. In so doing, no i r r e l e v a n t or redundant premises w i l l be able to creep into the explanans any more. However, we must bear i n mind that condition (R2) or i t s expanded version does not rule out Salmon's counterexamples which contain no i r r e l e v a n t premises. These counterexamples are dissolved i n a d i f f e r e n t manner as I have Hempel & Oppenheim ( 1 9 6 5 ) , p. 248. argued e a r l i e r i n t h i s section. But one thing i s c l e a r ; though, f o r Hempel, a l l D-N explanations are deductive arguments, not every deductive argument i s a D-N explanation for the l a t t e r has to s a t i s f y other l o g i c a l and em p i r i c a l conditions, notably ( R 2 ) . 6.3.4 I-S Explanation without Inductive 'Acceptance Rule's' For c l a r i t y , l e t us follow Salmon's advice i n reserving the term 'rule of detachment' f o r modus ponens and possibly other r u l e s of deductive l o g i c , and l e t us r e s t r i c t the term 'rule of acceptance' for inductive contexts. Hempel notes c o r r e c t l y that as opposed to a deductive argument whose conclusion cannot f a i l to be true i f the premises are true, the 'premises' i n an inductive inference lend only p a r t i a l support to the 'conclusion,' and t r u t h i s not therefore automatically transferred from the former to the l a t t e r . Hence, i n the inductive case, even i f the premises a l l belong to the c l a s s of statements previously accepted or possessed, the conclusion cannot be added to the c l a s s ; i t can only be q u a l i f i e d by a number representing i t s p r o b a b i l i t y r e l a t i v e to the premises. In reference to inductive "inferences" or "arguments," therefore, one can speak of a "conclusion" only cum grano s a l i s : the conclusion cannot be detached from the premises and asserted on i t s own when the premises are t r u e . 1 Hempel argues conclusively against the a l l too common error of incorporating some sort of p r o b a b i l i t y q u a l i f i e r 1Hempel ( 1 9 6 5 a ) , p. 61. 146 i n t o t h e c o n c l u s i o n / so t h a t i t c h a r a c t e r i z e s t h e c o n c l u s i o n a l o n e , r a t h e r t h a n t h e r e l a t i o n between p r e m i s e s and c o n c l u s i o n . 1 I n p a r t i c u l a r , he r e j e c t s t h e n o t i o n t h a t i n d u c t i v e l o g i c p o s s e s s e s any k i n d o f r u l e t h a t a l l o w s one t o ' a c c e p t ' a c o n c l u s i o n o f t h e form ' p r o b a b l y P,' and t o a s s e r t i t u n q u a l i f i e d l y , i f one a c c e p t s t h e p r e m i s e s o f t h e argument and i f t h e y embody a l l a v a i l a b l e r e l e v a n t e v i d e n c e . F o r i f 'P' i s a s t a t e m e n t , t h e n t h e e x p r e s s i o n s ' c e r t a i n l y P' and ' p r o b a b l y P' . . . a r e n o t s t a t e m e n t s . I f we ask how one would go about t r y i n g t o a s c e r t a i n w h e t h e r t h e y were t r u e , we r e a l i z e t h a t we a r e e n t i r e l y a t a l o s s u n l e s s and u n t i l a r e f e r e n c e s e t o f s t a t e m e n t s o r a s s u m p t i o n s has been s p e c i f i e d r e l a t i v e t o w h i c h P may t h e n be found t o be c e r t a i n , o r t o be h i g h l y p r o b a b l e , o r n e i t h e r . The e x p r e s s i o n s i n q u e s t i o n s , t h e n , a r e e s s e n t i a l l y i n c o m p l e t e ; t h e y a r e e l l i p t i c f o r m u l a t i o n s o f r e l a t i o n a l s t a t e m e n t s ; n e i t h e r o f them c a n be t h e c o n c l u s i o n o f an i n f e r e n c e . . . . .. .[T] h e r e i s «no a n a l o g u e i n i n d u c t i v e l o g i c t o modus poriens, o r t h e " r u l e o f detachment," o f d e d u c t i v e l o g i c , w h i c h , g i v e n t h e i n f o r m a t i o n t h a t !D', and a l s o ' i f D t h e n P', a r e t r u e s t a t e m e n t s , a u t h o r i z e s us t o d e t a c h t h e c o n s e q u e n t 'P' i n t h e c o n d i t i o n a l p r e m i s e and t o a s s e r t i t as a s e l f -c o n t a i n e d s t a t e m e n t w h i c h must t h e n be t r u e a s w e l l . Though Hempel e x p l i c i t l y d e n i e s t h a t t h e r e i s any an a l o g u e i n i n d u c t i v e l o g i c t o t h e ' r u l e o f detachment' i n d e d u c t i v e l o g i c , he, c o n t r a t o e x p e c t a t i o n , commits h i m s e l f t o an a n a l y s i s o f t h e p o s s i b i l i t y o f an i n d u c t i v e a c c e p t a n c e r u l e t h a t would a l l o w one, under s u i t a b l e c i r c u m s t a n c e s , t o a c c e p t a p r o b a b i l i s t i c a l l y u n q u a l i f i e d c o n c l u s i o n o f t h e x S e e , f o r example, Hempel (1965a), pp. 53-63 and Hempel ( 1 9 6 5 c ) , pp. 381-386. 2Hempel (1 9 6 5 c ) , n. 5, p. 384. 147 form 'P. 1 And any such i n d u c t i v e a c c e p t a n c e r u l e w o u l d r e q u i r e , i t seems, a h i g h p r o b a b i l i t y o f t h e c o n c l u s i o n r e l a t i v e t o t h e p r e m i s e s . I n t r o d u c i n g t h e c o n c e p t o f ' e p i s t e m i c u t i l i t y , ' Hempel a t t e m p t s t o f o r m u l a t e s u c h a r u l e i n terms o f t h e r e l a t i v e u t i l i t i e s o f a c c e p t i n g o r n o t a c c e p t i n g v a r i o u s a v a i l a b l e h y p o t h e s e s . He a s s e s s e s one such t e n t a t i v e r u l e , a l o n g w i t h an a s s o c i a t e d measure o f e p i s t e m i c u t i l i t y , and c o n c l u d e s t h a t i t i s u n s a t i s f a c t o r y . However, as p o i n t e d o u t by Salmon, he d i d n o t c o n c l u d e . . . t h a t no r u l e o f a c c e p t a n c e i s p o s s i b l e i n i n d u c t i v e l o g i c ; i n s t e a d , he was i n c l i n e d t o l a y t h e blame, n o t upon t h e i d e a o f an i n d u c t i v e a c c e p t a n c e r u l e as s u c h , b u t r a t h e r upon an i n a d e q u a t e e x p l i c a t i o n o f " e p i s t e m i c u t i l i t y . " He w r o t e , " T h i s must n o t be t a k e n t o p r o v e , however, t h a t C a r n a p ' s r u l e f o r r a t i o n a l c h o i c e s i m p l y c a n n o t y i e l d a r e a s o n a b l e a c c e p t a n c e r u l e f o r s c i e n t i f i c h y p o t h e s e s : q u i t e l i k e l y our c r u d e d e f i n i t i o n o f e p i s t e m i c u t i l i t y i s a t f a u l t . " 2 Now a s e r i o u s a m b i g u i t y emerges. As Salmon h a s o b s e r v e d , when Hempel s t a t e s t h a t I-S e x p l a n a t i o n s a r e i n d u c t i v e arguments, t h e r e i s "the q u e s t i o n o f w h e t h e r s u c h arguments a r e t o be u n d e r s t o o d as (1) arguments o f t h e t r a d i t i o n a l s o r t ; c o n t a i n i n g p r e m i s e s and c o n c l u s i o n s , g o v e r n e d by some s o r t o f i n d u c t i v e ' a c c e p t a n c e r u l e , ' o r (2) something more c l o s e l y a k i n t o Carnap's d e g r e e o f c o n f i r m a t i o n s t a t e m e n t s w h i c h o c c u r i n an i n d u c t i v e l o g i c w h i c h e n t i r e l y eschews i n d u c t i v e ' a c c e p t a n c e r u l e s . * " 4 x S e e Hempel (1965a), pp. 73-79. 2Hempel (1965a), p. 77. 3Salmon (1977b), pp.. 181-182. 4 I b i d . , p . 1 8 0 . 148 Hempel's writings seem to be equivocal on t h i s issue. On the one hand,: h i s analysis of the p o s s i b i l i t y of an inductive acceptance rule that would allow one to accept a p r o b a b i l i s t i c a l l y u n qualified conclusion of the form 'P* points to the understanding of 'arguments' i n the sense of (1). Yet, on the other hand, his constantly pronounced a f f i l i a t i o n with Carnap's conception of inductive l o g i c seems to place the notion of 'arguments' i n the sense of (2). This ambiguity of reference i s conducive to a number of c r i t i c i s m s r a i sed against the apparent inconsistency of endorsing both (1) and (2) at the same time. Salmon, f o r one, argues that when we look at Carnap's inductive l o g i c , we discover a shocking f a c t : i n that system of inductive l o g i c (the one to which Hempel e x p l i c i t l y r e f e r s i n connection with the concept of inductive p r o b a b i l i t y ) , there i s no such  thing as inductive inference i n the sense [ i . e . sense (1)] required for Hempel's account of i n d u c t i v e -s t a t i s t i c a l explanation! In Carnap's inductive l o g i c there are no inductive arguments c o n s i s t i n g of premises and conclusion, which allow you to a f f i r m the conclusion (with some degree of p r o b a b i l i t y ) i f you are prepared to assert the premises. x Furthermore, Salmon maintains, i t was an exceedingly profound i n s i g h t on Carnap's part to r e a l i z e that inductive l o g i c can, to a large extent anyway, dispense e n t i r e l y with r u l e s of acceptance and inductive inferences i n the ordinary sense. Instead, inductive l o g i c attaches numbers to hypotheses, and these numbers are used to make p r a c t i c a l d e cisions. In some circumstances such numbers, the degree of confirmation, may serve as f a i r b e t t i n g quotients to determine the odds for a f a i r bet on a given hypothesis. There i s no rule that t e l l s one when to accept an hypothesis or when to r e j e c t i t ; instead, there i s a rule of p r a c t i c a l behaviour that prescribes Salmon (1971), pp. 8-9. 149 that we so act as to maximize our expectation of u t i l i t y . Hence, inductive l o g i c i s simply not concerned with inductive arguments (regarded as e n t i t i e s composed of premises and c o n c l u s i o n s ) . 1 Not t r y i n g to be p r e f e r e n t i a l towards Hempel, I w i l l give him the b e n e f i t of the doubt that i t i s sense (2) i n which his conception of inductive arguments i s to be understood for two reasons. 2 F i r s t , he would be c o n t r a d i c t i n g his e a r l i e r s t i p u l a t i o n that "there i s no analogue i n inductive l o g i c to modus pohens, or the 'rule of detachment,' of deductive l o g i c " 3 i f inductive arguments are to be understood i n sense (1). Secondly, his knowledge of Carnap's writings would make i t rather u n l i k e l y that i t i s sense (1) which he i s using because t h i s sense of inductive arguments i s obviously subject to the consequences of Carnap's famous l o t t e r y paradox. Carnap observes that i f inductive l o g i c contains r u l e s • of acceptance which enable us to draw conclusions from premises—much as i n deductive l o g i c — t h e n there i s presumably some number r which constitutes a lower bound f o r acceptance. Accordingly, any hypothesis h whose p r o b a b i l i t y on the t o t a l evidence i s greater than or equal to r can be accepted on the basis of the evidence. (Of course, h might subsequently i l b i d . , p. 77. 2There i s no longer any doubt that Hempel's conception of inductive arguments i s to be understood i n the sense of ( 2 ) , for he has once more e x p l i c i t l y stated that he nowhere assumes 'inductive acceptance r u l e s . ' See (3) i n sec. 6.5. 3Hempel (1965a),.n. 5, p. 384. have t o be r e j e c t e d on the b a s i s of f u r t h e r e v i d e n c e . ) The problem i s to s e l e c t an a p p r o p r i a t e v a l u e f o r r . I t seems t h a t no v a l u e i s s a t i s f a c t o r y , f o r no m a t t e r how l a r g e r i s , p r o v i d e d i t i s l e s s than one, we can c o n s t r u c t a f a i r l o t t e r y w i t h a s u f f i c i e n t number of t i c k e t s t o be a b l e t o say f o r each t i c k e t t h a t i t w i l l not win, because the p r o b a b i l i t y o f i t s not winning i s g r e a t e r than r . From t h i s we can conclude t h a t no t i c k e t w i l l win which c o n t r a d i c t s t h e s t i p u l a t i o n t h a t t h i s i s a f a i r l o t t e r y , f o r no l o t t e r y can be c o n s i d e r e d f a i r i f t h e r e i s no w i n n i n g t i c k e t . -Indeed, Hempel i s l a r g e l y r e s p o n s i b l e f o r c r e a t i n g the i m p r e s s i o n t h a t h i s n o t i o n of i n d u c t i v e arguments i s t o be understood i n terms of sense (1) by i n s i s t i n g upon h i g h p r o b a b i l i t y as a desideratum f o r I-S e x p l a n a t i o n s , because i t appears c o u n t e r i n t u i t i v e t o accept an i n d u c t i v e c o n c l u s i o n which i s not h i g h l y p r o b a b l e r e l a t i v e t o i t s p r e m i s e s . However, i n d u c t i v e acceptance r u l e s and t h e subsequent h i g h p r o b a b i l i t y requirement, which are both l e g a c i e s o f the D-N model, have no p l a c e i n I-S e x p l a n a t i o n s . F o r Hempel h i m s e l f r e c o g n i z e s t h a t I-S e x p l a n a t i o n s s h o u l d n o t be p a r a s i t i c upon t h e i r D-N c o u n t e r p a r t s as they c o n s t i t u t e an i m p o r t a n t type of e x p l a n a t i o n i n t h e i r own r i g h t . He s a y s : I t seems i n a d v i s a b l e t o c o n s t r u c t an extended concept of e x p l a n a t o r y completeness i n such a way as t o q u a l i f y a l l s t a t i s t i c a l e x p l a n a t i o n s as i n c o m p l e t e . F o r t h i s q u a l i f i c a t i o n c a r r i e s w i t h i t c o n n o t a t i o n s of a d e f i c i e n c y , and s u r e l y , we cannot r e g a r d s t a t i s t i c a l e x p l a n a t i o n s simply as u n s u c c e s s f u l D-N e x p l a n a t i o n s : they c o n s t i t u t e an important type o f e x p l a n t i o n i n t h e i r own r i g h t . 1 J-Hempel (1965c), p.417. 151 If we were to renounce a l l inductive acceptance r u l e s , as Salmon r i g h t l y observes, there w i l l be no reason f o r regarding high p r o b a b i l i t y as a desideratum for i n d u c t i v e - s t a t i s t i c a l explanations. If no question of acceptance a r i s e s , i t seems . . . that high p r o b a b i l i t i e s are no better or more des i r a b l e than medium or low p r o b a b i l i t i e s . I t i s not the s i z e of the p r o b a b i l i t y that i s important, but only the accurate knowledge of i t regardless of s i z e . Since inductive l o g i c would no longer have any place f o r acceptance of hypotheses, i t does not seem e s s e n t i a l that an explanation provide evidence r e l a t i v e to which "the event was to be expected."1 We might s t i l l have l i n g e r i n g doubts as to whether Hempel's conception of inductive arguments i s to be understood i n the sense of (2) instead of (1) i f he would not l e t go of the high p r o b a b i l i t y requirement. But the f a c t i s that he has now retracted t h i s requirement from h i s 2 • • I-S model. Thus, we can re s t assured that he xs usxng inductive arguments i n the sense of (2) which does not r e l y on any inductive acceptance rules at a l l . 6 . 3 . 5 A Modified I-S Model of Explanation and the  Explanation of Low P r o b a b i l i t y Events In the absence of both inductive acceptance r u l e s and the high p r o b a b i l i t y requirement, i t i s obvious that the 'expectancy requirement,' which i s always f u l f i l l e d i n a D-N explanation, cannot be s a t i s f i e d i n an I-S explanation. iSalmon (1977b), p. 184. 2Hempel is.now convinced by Salmon and J e f f r e y that a s t a t i s t i c a l explanation need not show that the explanandum event was to be expected with high p r o b a b i l i t y . See (1) i n sec.6.5. 152 I f t h i s i s the case, then i n what manner can we claim to have characterized I-S explanations? Adopting Carnap's notion of f a i r betting quotients, I would claim that to say that an explanandum event i s explained by i t s explanans i s to maintain that the former i s "to be expected to a c e r t a i n . degree" r e l a t i v e to the l a t t e r . 1 Put d i f f e r e n t l y , we know p r e c i s e l y what degree of expectation i s r a t i o n a l and can then proceed to act r a t i o n a l l y on the basis of t h i s knowledge. In a s i m i l a r vein, Sklar says: In f i n d i n g s t a t i s t i c a l explanations we are discovering p r o b a b i l i t i e s r e l a t i v e to reference c l a s s e s i n t o which the object i n question f a l l s . These p r o b a b i l i t i e s are guides to r a t i o n a l action (and some would add, b e l i e f ) i n the face of uncertainty and r i s k . 2 The most important feature of t h i s modified I-S model of explanation i s that low p r o b a b i l i t y events (such as the paresis case) as well as high p r o b a b i l i t y events can be explained within t h i s model, for a high b e t t i n g quotient i s -not n e c e s s a r i l y better than a low one. As a r e s u l t , Hempel would no longer be subject to Salmon's complaint that i t i s "an e c c e n t r i c prejudice which leads us to d i s c r i m i n a t e against the minority [ i . e . low p r o b a b i l i t y events], •a condemning i t s members to the realm of the inexplicable."°. x 0 r as Hempel would say now, the explanandum event "was to be expected with a c e r t a i n nomological p r o b a b i l i t y . " See (2) i n sec . 6 . 5 2 S k l a r ( 1 9 7 3 ) , p. 199. 3Salmon ( 1 9 7 7 a ) , p. 152. 153 6 . 4 T h e M o d i f i e d H e m p e l T h e H e m p e l whom I h a v e g r a d u a l l y m o l d e d t h r o u g h o u t t h e p r e s e n t c h a p t e r i s n o t e x a c t l y t h e same p e r s o n who i s t r a d i t i o n a l l y u n d e r s t o o d b y p h i l o s o p h e r s . S o o n e c o u l d i n d e e d a r g u e t h a t i t i s n o t H e m p e l b u t a n i m a g i n a r y f i g u r e whom I am d e f e n d i n g . A s f a r a s t h e n a t u r e o f s c i e n t i f i c e x p l a n a t i o n i s c o n c e r n e d , h o w e v e r , I d o n o t t h i n k i t m a t t e r s w h e t h e r i t i s r e a l l y H e m p e l o r a n y o n e e l s e f o r t h a t m a t t e r whom I am d e f e n d i n g . W h a t c o n c e r n s me i s w h e t h e r t h e m o d e l ( o r m o d e l s ) o f s c i e n t i f i c e x p l a n a t i o n w h i c h I h a v e p r o p o s e d i s a d e q u a t e o r n o t . Now l e t u s t r y t o f o r m u l a t e a c o m p l e t e , m o d i f i e d H e m p e l i a n v i e w o f s c i e n t i f i c e x p l a n a t i o n . O u r p r e -e x p l i c a t i o n i n t u i t i o n o f a s c i e n t i f i c e x p l a n a t i o n i s t h a t i t a l w a y s a n s w e r s t h e q u e s t i o n "Why d i d a n e v e n t o c c u r ? " b y s h o w i n g t h a t t h e e v e n t 'was t o be e x p e c t e d . ' T h i s ' e x p e c t a n c y r e q u i r e m e n t ' i s a l w a y s s a t i s f i e d i n t h e c a s e o f a D - N e x p l a n a t i o n t h a t t a k e s t h e f o r m o f a d e d u c t i v e a r g u m e n t i n w h i c h t h e e x p l a n a n d u m e v e n t 'was t o b e e x p e c t e d ' a s i t f o l l o w s l o g i c a l l y f r o m t h e e x p l a n a n s . I n o t h e r w o r d s , a l l D - N e x p l a n a t i o n s are a r g u m e n t s ; y e t t h e y a r e e x p l a n a t i o n s , n o t because they are a r g u m e n t s , b u t b e c a u s e t h e y a l w a y s s a t i s f y t h e 'expectancy r e q u i r e m e n t ' w h i c h j u s t h a p p e n s t o b e always f u l f i l l e d i n d e d u c t i v e a r g u m e n t s . T h e ' e x p e c t a n c y requirement,' not 'argument,' i s the key concept i n D-N e x p l a n a t i o n s . But when i t comes to I-S e x p l a n a t i o n s , the 'expectancy requirement' can no l o n g e r be s a t i s f i e d . Rather than d r o p p i n g i t c o m p l e t e l y , we modify i t by adding the q u a l i f i c a t i o n 'to a c e r t a i n degree' t o i t . The m o d i f i e d 'expectancy requirement' w i l l now read 'was t o be expected t o a c e r t a i n degree' which i s e x a c t l y Carnap's n o t i o n o f f a i r b e t t i n g q u o t i e n t s . Now t o e x p l a i n an event i s t o p r o v i d e the b e s t p o s s i b l e grounds we c o u l d have had f o r making p r e d i c t i o n s c o n c e r n i n g i t or t o show to what degree i t was t o be expected. S i n c e t h i s concept of e x p l a n a t i o n has n o t h i n g t o do w i t h i n d u c t i v e arguments (regarded as e n t i t i e s c o n s i s t i n g o f premises and c o n c l u s i o n s ) , Hempel should never have used the term ' i n d u c t i v e arguments' i n h i s e x p o s i t i o n o f the s t r u c t u r e o f I-S e x p l a n a t i o n s . But, as I have argued a t l e n g t h , Hempel seems t o have the c o r r e c t i d e a a l l a l o n g . Although he has found i t v e r y d i f f i c u l t to f r e e h i m s e l f c o m p l e t e l y from the l e g a c y o f the D-N model which r e s u l t s i n a demand f o r h i g h p r o b a b i l i t y i n I-S e x p l a n a t i o n s , he f i n a l l y r e a l i z e s t h a t t h i s h i g h p r o b a b i l i t y requirement i s not n e c e s s a r y and has s u b s e q u e n t l y dropped i t . With t h i s requirement out o f the way, t h e r e does not seem to be any more c o n s i d e r a t i o n s l e f t t o h o l d him back from d i s p e n s i n g with the n o t i o n of i n d u c t i v e arguments (regarded as e n t i t i e s c o n s i s t i n g of premises and c o n c l u s i o n s ) 155 i n h i s I-S m o d e l o f e x p l a n a t i o n . 1 l l n f a c t , Hempel now c l a i m s t h a t h i s n o t i o n o f a ' s t a t i s t i c a l a r g u m e n t ' i s i n f u n d a m e n t a l a c c o r d w i t h S a l m o n ' s n o t i o n o f r a t i o n a l e x p e c t a t i o n v a l u e s w h i c h i s s i m i l a r t o C a r n a p ' s n o t i o n o f f a i r b e t t i n g q u o t i e n t s . See (3) i n s e c . 6.5, 156 6-5 Pos t s c r i p t I t i s important to point out here that my discussions on inductive acceptance r u l e s and the high p r o b a b i l i t y requirement, and p a r t i c u l a r l y my proposed modification of Hempel's o r i g i n a l I-S model of explanation, were written i n the absence of the knowledge that Hempel has also a r r i v e d at e s s e n t i a l l y s i m i l a r conclusions i n h i s " P o s t s c r i p t 1976." But instead of rewriting a l l the relevant passages, I s h a l l present h i s l a t e s t ideas that are pertinent to the present discussion i n t h i s p o s t s c r i p t . Furthermore, I s h a l l make reference to t h i s p o s t s c r i p t at places i n the text where Hempel's ideas and mine seem to coincide. In h i s "Postscript 1976," Hempel discusses i n d e t a i l the recent ideas on the problem of s t a t i s t i c a l explanation developed by J e f f r e y , Salmon, and Stegmuller. The following are some of the points that he makes which I think are relevant to the present discussion: (1) Hempel now thinks that J e f f r e y and ( e a r l i e r ) Salmon's fundamental ideas on s t a t i s t i c a l explanation are r i g h t , that i s , the understanding that s t a t i s t i c a l laws give us regarding p a r t i c u l a r events i s not the deeper or more complete the greater i s the p r o b a b i l i t y which the laws a t t r i b u t e to the e f f e c t . The explanation c o n s i s t s of the chara c t e r i z a t i o n of the stochastic process that produced 157 the event and of what the s t a t i s t i c a l law(s) s p e c i f i e s fo r the p r o b a b i l i t y of the various r e s u l t s of t h i s process. These statements explain then the occurrence of an improbable event no better than the occurrence of a very probable r e s u l t . Thus, Hempel now gives up h i s demand that a s t a t i s t i c a l explanation must show that the explanandum event was to be expected with high p r o b a b i l i t y . (2) Hempel thinks that a s t a t i s t i c a l explanation y i e l d s the most complete nomological i n s i g h t i n t o the occurrence of the explanandum events i n question that i s o b j e c t i v e l y possible. In contrast to a deductive-nomological explanation, a s t a t i s t i c a l explanation does not show that the explanandum event was to be expected with nomological necessity (that i s , the explanandum statement i s l o g i c a l l y deducible from the d e s c r i p t i o n of the preceding events given i n the explanans and the laws c i t e d i n the explanans). But perhaps, Hempel now says, a s t a t i s t i c a l explanation does show that the explanandum event was to be expected with a c e r t a i n nomological  p r o b a b i l i t y . (3) In Hempel's opinion, Salmon's r e j e c t i o n of h i s representation of s t a t i s t i c a l explanations i n argument form i s apparently c h i e f l y because Salmon thinks that there i s no r u l e of detachment of the kind c h a r a c t e r i s t i c of deductive arguments for s t a t i s t i c a l inferences. But Hempel rein s t a t e s t h a t he nowhere assumes such r u l e s ; he 158 reminds us that he has e x p l i c i t l y stated that there i s no analogous separation r u l e i n inductive inferences which allows a 'conclusion' to be detached from the 'premises.' As a matter of f a c t , Hempel t e l l s us that he employs.the word 'argument' rather i n a broader sense which i s i l l u s t r a t e d by means of schema (3o): x p(G,F)=r Fb ========= [r] Gb Here, ' r ' i s the expectation value which the explanans confers upon the explanandum. Thus, Hempel claims that h i s notion of a ' s t a t i s t i c a l argument' i s i n fundamental accord with Salmon's notion of r a t i o n a l expectation values which i s s i m i l a r to Carnap's notion of f a i r b e t ting quotients. 1 H e m p e l ( 1 9 6 5 c ) , p . 3 9 9 . 159 6.6 Conclusion Salmon's e a r l i e r S-R model of explanation i s indeed an improvement upon Hempel's o r i g i n a l I-S model since low p r o b a b i l i t y events, including those of the quantum nature, are amenable to explanation within the S-R model. But these low p r o b a b i l i t y events cannot be explained within the o r i g i n a l I-S model because they f a i l to s a t i s f y the high p r o b a b i l i t y requirement. • Yet Salmon's l a t e s t C-R model, as I have argued, i s unable to accomodate i r r e d u c i b l y s t a t i s t i c a l quantum phenomena unless i t endorses some version of the theory of hidden v a r i a b l e s . Such an endorsement, however, seems out of the question since Salmon himself openly r e j e c t s any theory of hidden v a r i a b l e s . But i f that i s the case, then the modified I-S model w i l l have a wider scope of a p p l i c a t i o n than the C-R model, f o r a l l kinds of low p r o b a b i l i t y events, whether they are i r r e d u c i b l y s t a t i s t i c a l or not, are amenable to explanation within the modified I-S model. I n c i d e n t a l l y , t h i s i s exactly what the e a r l i e r S-R model i s intended to do. Furthermore, Hempel does not share Salmon's idea that i n the explanation of an event, " i f the r e g u l a r i t y invoked i s not a causal r e g u l a r i t y , then a causal explanation of that very r e g u l a r i t y must be made part of the explanation 160 of the event." On the c o n t r a r y , Hempel ma i n t a i n s , i n Salmon's words, t h a t the mere subsumption . . . of an event under a l a w f u l r e g u l a r i t y c o n s t i t u t e s a complete e x p l a n a t i o n . One can . . . go on to ask f o r an e x p l a n a t i o n of any law used to e x p l a i n a g i v e n event, but t h a t i s a d i f f e r e n t explanation.2 In other words, f o r Hempel, as long as an event i s subsumed under a l a w f u l r e g u l a r i t y , whether i t i s a c a u s a l , symptomatic, or simple e m p i r i c a l r e g u l a r i t y , a complete e x p l a n a t i o n had been g i v e n . Of course, as I have argued b e f o r e , 3 s i n c e a c a u s a l e x p l a n a t i o n i s ' b e t t e r ' than e i t h e r a symptomatic or an e m p i r i c a l one as i t i s 'greater i n g e n e r a l i t y , ' we can always, and sometimes should, t r y to subsume a symptomatic or an e m p i r i c a l r e g u l a r i t y under a c a u s a l law i n order to o b t a i n a ' b e t t e r ' e x p l a n a t i o n . But t h a t i s a d i f f e r e n t e x p l a n a t i o n . The l a t e r Salmon, however, cannot condone t h i s f o r he i s committed to the view t h a t an e x p l a n a t i o n i s a genuine e x p l a n a t i o n o n l y i f i t i s c a u s a l i n nature. Yet, as I have attempted to show e a r l i e r , 4 to h o l d t h a t an e x p l a n a t i o n i s not genuine un l e s s i t i s c a u s a l i s to confuse the 'why' with the 'how' aspect of e x p l a n a t i o n . Besides, e x p l a n a t i o n s which invoke laws o f c o e x i s t e n c e such as the pendulum law, Boyle's law, Ohm's law, e t c . , i n which a 1Salmon (1978), p. 699 2 I b i d . 3 See sec. 5.4. ^ A l s o see sec. 5.4. 161 p a r t i c u l a r f a c t i s e x p l a i n e d , not by c a u s a l antecedents, but by r e f e r e n c e to another contemporaneous f a c t , w i l l thus have to be regarded as non-explanatory i f the c a u s a l model of e x p l a n a t i o n i s adopted. T h i s might be too b i g a p r i c e to pay, p a r t i c u l a r l y s i n c e s c i e n t i s t s do t r e a t these non-c a u s a l e x p l a n a t i o n s as genuine e x p l a n a t i o n s . In sum, s i n c e i t has been shown t h a t the m o d i f i e d I-S model of e x p l a n a t i o n has a wider scope of a p p l i c a t i o n than the C-R model, I conclude t h a t the former i s a more adequate, comprehensive model of s c i e n t i f i c e x p l a n a t i o n than the l a t t e r . CHAPTER S E V E N IS EVOLUTIONARY THEORY C O N S I S T E N T WITH H E M P E L ' S C O V E R I N G - L A W MODELS? 7 . 1 I n t r o d u c t i o n E v o l u t i o n a r y t h e o r y h a s o f t e n b e e n c i t e d as a c o u n t e r e x a m p l e t o t h e s y m m e t r y t h e s i s . B u t s u c h i s n o t t h e c a s e as I h a v e a r g u e d i n S e c t i o n 4 . 1 . I n my o p i n i o n , e v o l u t i o n a r y t h e o r y a p p e a r s t o be a c o u n t e r e x a m p l e t o t h e s y m m e t r y t h e s i s b e c a u s e c e r t a i n a s p e c t s o f t h e t h e o r y h a v e o f t e n b e e n m i s i n t e r p r e t e d a n d m i s u n d e r s t o o d , t h e m o s t p r o m i n e n t b e i n g t h e p u r p o r t e d u n f a l s i f i a b i l i t y o f t h e t h e o r y . T h i s c h a p t e r a i m s a t r e m o v i n g t h e s e m i s i n t e r p r e t a t i o n s a n d m i s u n d e r s t a n d i n g s . Y e t , w i t h i n t h e s c o p e o f t h e p r e s e n t t h e s i s , o n l y t h o s e a s p e c t s o f t h e o r y w h i c h h a v e d i r e c t b e a r i n g o n t h e s y m m e t r y t h e s i s w i l l b e d i s c u s s e d . 163 7.2 I s E v o l u t i o n a r y Theory F a l s i f i a b l e ? Some p h i l o s o p h e r s and b i o l o g i s t s have charged t h a t e v o l u t i o n a r y t h e o r y , even i n i t s n e o - D a r w i n i a n form, does not f i t i n t o t h e c o v e r i n g - l a w model o f e x p l a n a t i o n because i t i s u n f a I s i f l a b l e . T h e i r c r i t i c i s m s r e v o l v e around t h r e e i m p o r t a n t a s p e c t s o f t h e t h e o r y , namely i t s c o n c e p t o f ' f i t n e s s , 1 i t s p r e d i c t a b i l i t y , and i t s e x p e r i m e n t a l r e p r o d u c i b i l i t y . L e t us c o n s i d e r each a s p e c t i n t u r n . 7.2.1 C i r c u l a r i t y o f t h e D e f i n i t i o n o f ' F i t n e s s ' The common d i s s a t i s f a c t i o n w i t h t h e p r i n c i p l e o f n a t u r a l s e l e c t i o n , i . e . ' s u r v i v a l o f t h e f i t t e s t , ' as b e i n g a mere t a u t o l o g y and e m p i r i c a l l y vacuous, i s sh a r e d by many p h i l o s o p h e r s and b i o l o g i s t s . Manser, f o r example, a s s e r t s t h a t " t h e r e can be no independent c r i t e r i o n o f f i t n e s s o r a d a p t a b i l i t y ; s u r v i v a l and a d a p t a b i l i t y o r f i t n e s s a r e n e c e s s a r i l y connected."-'- Smart t o o i n s i s t s t h a t u n l e s s some c h a r a c t e r i z a t i o n o f t h e ' f i t t e s t , ' a p a r t from i t s s u r v i v a l v a l u e , i s p r o v i d e d , t h e r e i s no way t o c o n f i r m o r f a l s i f y t he p r o p o s i t i o n t h a t t h e f i t t e s t s u r v i v e . 2 U n l i k e Manser and iManser (1965), p. 26. 2 S m a r t (1959), p. 266. 164 Smart who have made no apparent attempt t o p r o v i d e a n o n c i r c u l a r d e f i n i t i o n of ' f i t n e s s ' without r e f e r e n c e s u r v i v a l , S c r i v e n a t l e a s t t r i e s t o do so. But he i s to the same c o n c l u s i o n t h a t the d e f i n i t i o n o f "the f i t t e s t " i s d i f f i c u l t even when made r e l a t i v e t o a p a r t i c u l a r environment. I t i s f a i r l y obvious t h a t no c h a r a c t e r i s t i c s can be i d e n t i f i e d as c o n t r i b u t i n g to " f i t n e s s " i n a l l environments. Thus, s t r e n g t h may i n c r e a s e the chance of s u r v i v a l , and i n t e l l i g e n c e may be a n t i a d a p t i v e i n a n t i - i n t e l l e c t u a l s o c i e t i e s . Furthermore, maximum s p e c i a l i z a t i o n f o r a p a r t i c u l a r environment i s i n ge n e r a l i n c o m p a t i b l e , m o r p h o l o g i c a l l y and g e n e t i c a l l y , with maximum f l e x i b i l i t y t o wit h s t a n d sudden environmental changes. We are i n c l i n e d t o say t h a t the organisms adopting the former l i n e of development tend t o be " f i t t e r " unt11 the change o c c u r s , and the l a t t e r f i t t e r When i t occurs.1 That i s t o say, d e f i n i t i o n of 'the f i t t e s t ' i s d i f f i c u l t even when made r e l a t i v e t o a p a r t i c u l a r environment, f o r , i n the s h o r t run and i n a r e l a t i v e l y s t a b l e environment, the ' f i t t e s t ' which s u r v i v e w i l l be those t h a t are maximally s p e c i a l i z e d t o l i v e i n such an environment. But maximum s p e c i a l i z a t i o n becomes a stumbling b l o c k to s u r v i v a l when the environment changes. Now chances are t h a t organisms which are ' f i t t e s t ' i n the o l d environment w i l l prove l e s s so wh i l e the most s u c c e s s f u l organisms i n t h i s new environment w i l l probably be descendants of those which were maximally f l e x i b l e i n the o l d environment. Thus, i t seems t h a t n e i t h e r s p e c i a l i z a t i o n nor f l e x i b i l i t y alone can supply the s o l e m e t r e - s t i c k i n measuring the ' f i t n e s s ' o f organisms. t o f o r c e d S c r i v e n (1959a), p. 478. 165 Is t h ere any c o n c e i v a b l e way of d e f i n i n g ' f i t n e s s 1 without r e f e r e n c e to s u r v i v a l at a l l ? I t h i n k there i s . In f a c t , I would m a i n t a i n t h a t S c r i v e n has a l r e a d y p r o v i d e d us w i t h a very promising candidate f o r such a d e f i n i t i o n , namely the n o t i o n o f g e n e t i c f l e x i b i l i t y or v a r i a b i l i t y . But he cannot see i t s importance, due mainly t o h i s mistaken conception t h a t maximum s p e c i a l i z a t i o n i s g e n e r a l l y i n c o m p a t i b l e with maximum f l e x i b i l i t y . In other words, he can conceive of no way of d e f i n i n g the ' f i t t e s t ' i n terms of maximum f l e x i b i l i t y and s t i l l be able t o say t h a t i n a c e r t a i n unchanging environment, the ' f i t t e s t ' are those t h a t are maximally s p e c i a l i z e d . In what f o l l o w s , I s h a l l argue t h a t the d e f i n i t i o n of ' f i t n e s s ' w i l l not be c i r c u l a r i f i t i s d e f i n e d i n terms of the 'degree o f g e n e t i c v a r i a b i l i t y ' ; t h a t i s t o say, the more g e n e t i c a l l y v a r i a b l e the organisms are, the ' f i t t e r ' they w i l l be. Not s u r p r i s i n g l y , the h y p o t h e s i s t h a t the ' f i t t e r ' organisms are those which are g e n e t i c a l l y more v a r i a b l e i s b e a u t i f u l l y v e r i f i e d i n changing environments s i n c e i t i s a b i o l o g i c a l f a c t t h a t the g e n e t i c a l l y more v a r i a b l e organisms are b e t t e r capable o f adapting t o new environments and s u r v i v e , thus making them the ' f i t t e r ' organisms. But i t needs t o be argued how t h i s h y p o t h e s i s i s a l s o t r u e of organisms i n s t a b l e environments where the ' f i t t e r ' organisms appear t o be those which are maximally s p e c i a l i z e d . Two very e s s e n t i a l f e a t u r e s of the theory of e v o l u t i o n through n a t u r a l s e l e c t i o n must be made a b s o l u t e l y c l e a r a t 166 the o u t s e t . To begin w i t h , the theory presupposes, as a matter of s c i e n t i f i c f a c t , t h a t the environment i n v a r i a b l y changes, whether g r a d u a l l y and m i n i m a l l y or d r a s t i c a l l y on a l a r g e s c a l e . Of course, I am not denying t h a t there are p e r i o d s of time i n which the environment remains s t a b l e . What I am c l a i m i n g i s merely t h a t such a s t a b l e s t a t e i s never permanent, and changes are bound to occur sooner or l a t e r . Thus, environmental changes are indeed an i n t e g r a l p a r t of Darwin's theory of e v o l u t i o n through n a t u r a l s e l e c t i o n . Next, we must r e c o g n i z e t h a t e v o l u t i o n a r y theory i s meant to apply o n l y t o l a r g e p o p u l a t i o n s , but not s m a l l ones. For i n s m a l l p o p u l a t i o n s (Sewall Wright has suggested t h a t 'small' amounts to 1,000 i n d i v i d u a l s ) , random f l u c t u a t i o n s i n gene f r e q u e n c i e s o f t e n o v e r r i d e the e f f e c t s of n a t u r a l s e l e c t i o n . T h i s phenomenon i s known as g e n e t i c d r i f t . S i n ce n a t u r a l , i n t e r b r e e d i n g p o p u l a t i o n s are f r e q u e n t l y s m a l l and are thus s u b j e c t to s t r o n g chance assortment, e v o l u t i o n a r y changes i n these p o p u l a t i o n s can be e x p l a i n e d without recourse to the theory of n a t u r a l s e l e c t i o n . T h i s i s p a r t i c u l a r l y t r u e of founder p o p u l a t i o n s i n the c o l o n i z a t i o n of new lands such as new oceanic i s l a n d s . 1 I n c i d e n t a l l y , the very f a c t t h a t the ' f i t t e s t ' ( i . e . i n d i v i d u a l s w i t h the most g e n e t i c v a r i a b i l i t y ) do not always s u r v i v e i n s m a l l p o p u l a t i o n s c o n s t i t u t e s a s o l i d ground f o r x F o r a d e t a i l e d d i s c u s s i o n of the founder p r i n c i p l e , see Mayr (1963), pp. 211-212 & pp. 529-535. 167 c l a i m i n g t h a t the p r i n c i p l e of the ' s u r v i v a l of the f i t t e s t ' i s not t a u t o l o g i c a l , though i t s a p p l i c a b i l i t y i s c o n f i n e d t o l a r g e p o p u l a t i o n s . One f u r t h e r p o i n t . The f a c t t h a t e v o l u t i o n a r y theory i s not a p p l i c a b l e t o s m a l l p o p u l a t i o n s should not bother us s i n c e i t i s an i n h e r e n t l y s t a t i s t i c a l theory (as w i l l be d i s c u s s e d l a t e r i n S e c t i o n 7.2.2.2) which makes no b o l d c l a i m t h a t every e v o l u t i o n a r y change i s i n p r i n c i p l e e x p l i c a b l e by means of n a t u r a l s e l e c t i o n . Once the above c l a r i f i c a t i o n s are made, I can now proceed w i t h my proposed u n c i r c u l a r d e f i n i t i o n of ' f i t n e s s ' by f i r s t making i n t e l l i g i b l e the concept of maximum g e n e t i c v a r i a b i l i t y and i t s e v o l u t i o n a r y s i g n i f i c a n c e . N e i t h e r the gene p o o l of any outbreeding p o p u l a t i o n , t h a t i s , the sum t o t a l of the d i f f e r e n t a l l e l e s , nor the d i s t r i b u t i o n of a l l e l e s , i s s t a t i c . ( A l l e l e s are the a l t e r n a t e forms of a gene occupying the same l o c u s ; a l o c u s i s the r e l a t i v e p o s i t i o n of a gene i n a chromosome.) The g e n e t i c makeup of a gene p o o l c o n t i n u a l l y changes, i n a minimal manner, through the e f f e c t s of mutations and g e n e t i c recombinations. There are two kinds of mutations, namely p o i n t or gene mutations and chromosomal mutations. Both kinds of mutations take p l a c e most o f t e n when genes are r e p l i c a t i n g t o form co p i e s of themselves, the new gene c o n t a i n s a ' m i s p r i n t ' of the g e n e t i c i n f o r m a t i o n coded i n the o r i g i n a l gene. Mutations occur so r a r e l y t h a t t h e i r c o n t r i b u t i o n to the g e n e t i c v a r i a b i l i t y of a p o p u l a t i o n i n a s i n g l e g e n e r a t i o n i s n e g l i g i b l e . G enetic recombination, on the other hand, i s 168 the constant r e s h u f f l i n g of the thousands of v a r i a n t a l l e l e s found i n the p o p u l a t i o n of a l l s e x u a l l y r e p r o d u c i n g , c r o s s -f e r t i l i z i n g organisms, brought about by the c y c l e of m e i o s i s -f e r t i l i z a t i o n . The gene p o o l i s f u r t h e r e n r i c h e d by a phenomenon known as heterozygote s u p e r i o r i t y . I f a p o p u l a t i o n e x i s t s i n a r e l a t i v e l y homogeneous h a b i t a t so t h a t a l l of i t s i n d i v i d u a l s are competing d i r e c t l y w i t h each o t h e r , two o p p o s i t e a l l e l e s which c o n t r o l v e r y d i f f e r e n t phenotypes ( i . e . the sum of c h a r a c t e r i s t i c s manifested by an organism as c o n t r a s t e d w i t h genotype which i s i t s g e n e t i c makeup) are not l i k e l y t o be r e t a i n e d i n the p o p u l a t i o n f o r an i n d e f i n i t e l e n g t h of time. One phenotype w i l l u s u a l l y have a s e l e c t i v e advantage over the o t h e r , and the a l l e l e c o n t r o l l i n g i t w i l l t h e r e f o r e i n c r e a s e c o n t i n u o u s l y i n frequency. I f the heterozygote ( i . e . an organism which has two d i f f e r e n t a l l e l e s i n the two c o r r e s p o n d i n g l o c i o f a p a i r of chromosomes) has a lower a d a p t i v e v a l u e than the homozygote ( i . e . an organism which has i d e n t i c a l a l l e l e s i n the two c o r r e s p o n d i n g l o c i of a p a i r of chromosomes) f o r the s u p e r i o r a l l e l e , the i n f e r i o r a l l e l e w i l l be d r i v e n out completely. I f the heterozygote i s equal i n a d a p t i v e v a l u e to the s u p e r i o r homozygote, a s i t u a t i o n which u s u a l l y e x i s t s i n the case of dominant and r e c e s s i v e a l l e l e s a t a l o c u s , the r e c e s s i v e a l l e l e w i l l remain i n the p o p u l a t i o n a t a low frequency. I f , however, the heterozygote i s s u p e r i o r to e i t h e r homozygote, the i n f e r i o r a l l e l e w i l l be r e t a i n e d a t 169 a f a i r l y h i g h frequency i n the p o p u l a t i o n (on the b a s i s of the Hardy-Weinberg fo r m u l a ) , even i f genotypes homozygous f o r i t are l e t h a l . S ince the i n f e r i o r a l l e l e s are r e t a i n e d i n the p o p u l a t i o n because t h e i r e f f e c t s i n the heterozygote are more than counterbalanced by those of the s u p e r i o r a l l e l e s , t h i s enrichment of the gene p o o l by heterozygote s u p e r i o r i t y i s c a l l e d the balanced l o a d of g e n e t i c v a r i a b i l i t y . E v o l u t i o n c o n s i s t s c h i e f l y of a d a p t i v e changes i n p o p u l a t i o n s i n response to t h e i r environment. These changes, of n e c e s s i t y , r e q u i r e a l t e r a t i o n s of both the q u a l i t y and q u a n t i t y of g e n e t i c v a r i a t i o n . The amount of v a r i a t i o n may be e i t h e r decreased or i n c r e a s e d . Since continuous e v o l u t i o n i s not l i k e l y t o come by i f r e d u c t i o n s of these v a r i a t i o n s are not g r e a t e r than the c o r r e s p o n d i n g i n c r e a s e s , the e v o l u t i o n a r y s i g n i f i c a n c e of maximum v a r i a b i l i t y i s obvious. I f ' f i t n e s s ' i s to be measured i n terms of the degree of g e n e t i c v a r i a b i l i t y , i n what manner then can we s o l v e S c r i v e n ' s problem over how to account f o r ' f i t n e s s ' of p o p u l a t i o n s of organisms i n a s t a b l e environment by means of maximum s p e c i a l i z a t i o n ? The s o l u t i o n runs l i k e t h i s . Since n a t u r a l s e l e c t i o n b r i n g s about e v o l u t i o n as a r e s u l t of i n t e r a c t i o n s between p o p u l a t i o n s and t h e i r environment, i t s a c t i o n does not always cause e v o l u t i o n to take p l a c e , p a r t i c u l a r l y when there i s no c o n s t a n t i n j e c t i o n of new i n h e r i t a b l e v a r i a t i o n s i n t o the p o p u l a t i o n . As long as the p o p u l a t i o n i s i n t e r a c t i n g w i t h a s t a b l e environment, the 170 i n d i v i d u a l s e l i m i n a t e d by s e l e c t i o n are those t h a t d e v i a t e most from the mode or norm of the p o p u l a t i o n , and those n e a r e s t to the norm w i l l produce the most o f f s p r i n g . Under these unchanging c o n d i t i o n s , the p o p u l a t i o n remains i n dynamic e q u i l i b r i u m and does not change because i t has g r a d u a l l y a c q u i r e d t h e - l e a s t a d a p t a t i o n , v i a maximum s p e c i a l i z a t i o n , w i t h regard to t h a t environment of which i t s gene p o o l i s capable. T h i s phenomenon i s known as ' s t a b i l i z i n g  s e l e c t i o n which i s the a c t i o n of n a t u r a l s e l e c t i o n i n keeping a p o p u l a t i o n c o n s t a n t . In sum, I m a i ntain t h a t the more g e n e t i c a l l y v a r i a b l e the organisms are, the ' f i t t e r ' they w i l l be, f o r they have a b e t t e r chance of s u r v i v a l as they are p o t e n t i a l l y more  capable, as a r e s u l t of t h e i r b e t t e r g e n e t i c v a r i a b i l i t y , of adapting to new and changing environments. And s i n c e the term ' f i t n e s s ' i s here d e f i n e d i n terms of the 'degree of g e n e t i c v a r i a b i l i t y 1 r a t h e r than of ' s u r v i v a l , ' the p r i n c i p l e of the ' s u r v i v a l of the f i t t e s t ' i s thus not t a u t o l o g i c a l and e m p i r i c a l l y vacuous. As i n the case of ' f i t n e s s ' , Manser claims t h a t another key term of the theory of n a t u r a l s e l e c t i o n , namely 'environment,' a l s o s u f f e r s from being c i r c u l a r l y d e f i n e d . He argues t h a t i f a s p e c i e s which has been r e l a t i v e l y constant i n numbers s t a r t s to d e c l i n e without there being any obvious e x t e r n a l cause such as human dep r e d a t i o n s , the e x p l a n a t i o n may be sought i n a change i n the environment. 1 7 1 Yet t h i s change i s only postulated because there has been a change in numbers or i n some other aspect of the population. Thus, "what i s to be counted as a change i n the environment depends on a change i n the population; here again no independent d e f i n i t i o n i s possible, and thus no explanation i s provided by using the term." 1 I t may well be true that a change i n the environment i s postulated because a change in the population has been observed, but i t does not then follow that what i s to be counted as a change i n the environment depends on a change i n the population. Rather, I would hold that the change i n the population provides us with an invaluable clue or indicati o n that there might possibly have been a change i n the environment. Thus, the term 'environment' does not suffer from having a c i r c u l a r d e f i n i t i o n , though a change i n the environment might not have been noticed had there not been a change i n the population. 2 Furthermore, since i t i s b i o l o g i c a l l y possible ( i f not p r a c t i c a l l y the case sometimes) that a decline i n the population without any obvious external cause i s not brought about by a change i n the environment, and conversely, that a change i n the environment i s not always followed by a change in the numbers or i n some other aspect of the population, the d e f i n i t i o n of the term 'environment' i s by no means c i r c u l a r . Manser (1965) , p. 26. 2A s i m i l a r point i s made by Flew (1966), p. 70. 172 S u r e l y , some changes i n the environment are e a s i l y d e t e c t e d independent of any change i n the p o p u l a t i o n , f o r example, a change from an a r i d t o wet environment. I t i s o n l y i n those cases where the change i s not so d r a s t i c o r c l e a r - c u t t h a t the change might not have been n o t i c e d had t h e r e not been a change i n the numbers or some other aspect of the p o p u l a t i o n . But t h i s does not show t h a t 'environment' i s t h e r e f o r e a c i r c u l a r term; i t merely shows t h a t 'environment' i s a r a t h e r complex and, to some extent, r e l a t i v i s t i c term. For example, what counts as an environment (and subsequently an environmental change) f o r animals does not always count as an environment (and subsequently an environmental change) f o r i n s e c t s or p l a n t s . A s l i g h t and unimportant change i n the environment f o r animals might be a very s i g n i f i c a n t change f o r i n s e c t s or p l a n t s , so s i g n i f i c a n t t h a t the l a t t e r might have to undergo e v o l u t i o n i n order to adapt to t h i s new environment. 1 7.2.2 P r e d i c t a b i l i t y and F a l s i f i a b l l i t y The most s e r i o u s c r i t i c i s m of the theory of n a t u r a l s e l e c t i o n i s the c l a i m t h a t there i s no way of c o n f i r m i n g or ^ I n c i d e n t a l l y , as a p o i n t of i n t e r e s t , i f one adopts Quine's a s s e r t i o n t h a t " i t i s m i s l e a d i n g to speak of the e m p i r i c a l content of an i n d i v i d u a l statement" o u t s i d e the theory of which i t i s a p a r t , then Manser's attempt to show the a n a l y t i c i t y of the h y p o t h e s i s , i . e . "Whenever th e r e i s a change i n the environment, there i s a subsequent change i n the p o p u l a t i o n , " i n i s o l a t i o n from the theory of n a t u r a l s e l e c t i o n of which i t i s a p a r t , seems f u t i l e . See Quine (1963). 173 f a l s i f y i n g i t . In a crude form, the c r i t i c i s m runs l i k e t h i s : The c o n f i r m a t i o n (or f a l s i f i c a t i o n ) o f a s c i e n t i f i c theory c o n s i s t s i n making p r e d i c t i o n s on the b a s i s of the theory, t h a t i s , i n deducing o b s e r v a t i o n statements about the f u t u r e from the premises of the theory. Should these p r e d i c t i o n s t u r n out to be t r u e , then we have c o n f i r m a t o r y evidence f o r the theory; should they t u r n out to be f a l s e , then the theory i s f a l s i f i e d . However, one cannot make such p r e d i c t i o n s on the b a s i s o f the s y n t h e t i c t h e o r y . For example, no one can p r e d i c t what man w i l l evolve i n t o next, or whether indeed he w i l l go on e v o l v i n g r a t h e r than becoming e x t i n c t . Moreover, even i f one c o u l d make such p r e d i c t i o n s , t h e r e i s no way of p u t t i n g them t o the t e s t , s i n c e o b v i o u s l y such p r e d i c t i n g s would r e f e r t o phenomena many m i l l i o n of years i n the f u t u r e . Consequently, the s y n t h e t i c theory i s i n p r a c t i c e , i f not a l s o i n p r i n c i p l e , u n t e s t a b l e . 1 T h i s i s e x a c t l y the l i n e o f argument taken up by S c r i v e n who uses i t to support h i s c l a i m t h a t e v o l u t i o n a r y theory can o n l y e x p l a i n but not p r e d i c t . He argues t h a t s i n c e we cannot p r e d i c t environmental changes and s i n c e "both e x t e n s i v e and l o c a l c a t a s t r o p h e s w i l l p l a y a l a r g e p a r t i n deter m i n i n g the s u r v i v o r s r e g a r d l e s s of t h e i r c h a r a c t e r i s t i c s , " 2 " i t i s q u i t e c l e a r t h a t we cannot p r e d i c t which organisms w i l l s u r v i v e . " 3 N e i t h e r can we p r e d i c t t h a t "a p a r t i c u l a r l i t t e r from an a l b i n o s t r a i n w i l l c o n t a i n a n o n a l b i n o , " 4 though we know t h a t a c e r t a i n percentage o f the l i t t e r s w i l l c o n t a i n n o n a l b i n o s . Thus, i t i s o n l y p o s s i b l e to make some h y p o t h e t i c a l p r o b a b i l i t y p r e d i c t i o n s , but not a c t u a l p r e d i c t i o n s , on the b a s i s of e v o l u t i o n a r y theory, and l-Ruse (1969) , p. 329. 2 S c r i v e n (1959a), p. 478. 3 I b i d . 4 I b i d . , p. 479. 174 " i t should be noted t h a t these ' p r e d i c t i o n s ' are not e a s i l y f a l s i f i e d by o b s e r v a t i o n , s i n c e they o n l y a s s e r t the l i k e l i h o o d of a c e r t a i n outcome." 1 I t h i n k S c r i v e n ' s argument needs some c l a r i f i c a t i o n . His r e j e c t i o n o f p r e d i c t i o n s based on e v o l u t i o n a r y theory — w h i c h are h y p o t h e t i c a l p r o b a b i l i t y p r e d i c t i o n s — a s genuine s c i e n t i f i c p r e d i c t i o n s r e s t s on two separate grounds: the f i r s t i s t h e i r h y p o t h e t i c a l ' i f - t h e n ' c h a r a c t e r , and the second i s t h e i r p r o b a b i l i s t i c or S t a t i s t i c a l c h a r a c t e r . In my view, n e i t h e r ground i s s u b s t a n t i a l enough t o c o n s t i t u t e an a l l - e m b r a c i n g reason t o d e p r i v e these p r e d i c t i o n s of t h e i r s c i e n t i f i c genuineness. 7.2.2.1 H y p o t h e t i c a l P r e d i c t i o n s S c r i v e n i s a b s o l u t e l y r i g h t when he s t a t e s t h a t , not knowing the f u t u r e changes of environment, we cannot make a c t u a l p r e d i c t i o n s of d i s c r e t e , f u t u r e e v o l u t i o n a r y events. Any such p r e d i c t i o n we make i s h y p o t h e t i c a l i n form. However, there seems t o be no e s s e n t i a l d i f f e r e n c e i n t h i s r e s p e c t between t h e o r i e s i n p h y s i c s , p a r t i c u l a r l y c l a s s i c a l Newtonian mechanics, and e v o l u t i o n a r y theory. Newton's F i r s t Law of Motion i s an e x c e l l e n t example i n support of t h i s p o i n t . I t s t i p u l a t e s t h a t " I f a body i s not acted upon by a f o r c e , then i t w i l l move w i t h constant v e l o c i t y i n a s t r a i g h t l i n e i n one d i r e c t i o n f o r e v e r . " But s i n c e i t i s 1 I b i d . / p. 478. 175 p h y s i c a l l y , though not i n p r i n c i p l e , i m p o s s i b l e to have a body t h a t i s not a c t e d upon by a f o r c e , no a c t u a l p r e d i c t i o n r e g a r d i n g any p a r t i c u l a r body can be a t t a i n e d on the b a s i s of the F i r s t Law. T h i s , however, i s not tantamount to s a y i n g t h a t an h y p o t h e t i c a l p r e d i c t i o n i n the form of the F i r s t Law i s not a genuine s c i e n t i f i c p r e d i c t i o n . For the h y p o t h e t i c a l p r e d i c t i o n i s indeed t e s t a b l e , at l e a s t i n p r i n c i p l e , i f we c o u l d f i n d a body t h a t i s not acted upon by a f o r c e and observe whether i t a c t u a l l y moves with constant v e l o c i t y i n a s t r a i g h t l i n e i n one d i r e c t i o n f o r e v e r . To be sure, i t i s e x p e r i m e n t a l l y , i f not p h y s i c a l l y , i m p o s s i b l e to c a r r y out a t e s t of the F i r s t Law i n the above manner, but t h i s by ho means s t r i p s the F i r s t Law of i t s s c i e n t i f i c n e s s . ( I n c i d e n t a l l y , the F i r s t Law, i n c o n j u n c t i o n w i t h o t h e r p h y s i c a l laws, i s amenable to experimental t e s t s . ) Analogously, e v o l u t i o n a r y theory i s genuinely s c i e n t i f i c even though the theory makes mostly h y p o t h e t i c a l but not a c t u a l p r e d i c t i o n s , as i t i s p r a c t i c a l l y i m p o s s i b l e to s p e c i f y each and every one of the numerous r e l e v a n t i n i t i a l c o n d i t i o n s needed f o r p r e d i c t i n g a c t u a l , s p e c i f i c f u t u r e e v o l u t i o n a r y events. In a l a s t - d i t c h e f f o r t to salvage h i s c r i t i c i s m , S c r i v e n might i n s i s t t h a t there i s a s i g n i f i c a n t d i s a n a l o g y between the above two kinds of p r e d i c t i o n s . For i n Newtonian mechanics, every event i s determined, though o f t e n we are 176 p r a c t i c a l l y or t e c h n i c a l l y i n c a p a b l e of l o c a t i n g i t s cause. But environmental changes such as earthquakes are never p r e d i c t a b l e , not even i n p r i n c i p l e . T h i s argument, however, i s v e r y dubious s i n c e i t i s simply f a l s e t h a t environmental changes are i n p r i n c i p l e u n p r e d i c t a b l e . True, as s t a t e d e a r l i e r , i t would be extremely d i f f i c u l t , i f not p r a c t i c a l l y i m p o s s i b l e , to f o r e c a s t , f o r i n s t a n c e , the weather twenty years ahead, s i n c e numerous f a c t o r s have to be a c c u r a t e l y s p e c i f i e d b e f o r e any such p r e d i c t i o n i s p o s s i b l e . Yet t h i s , I t h i n k , i s a p r a c t i c a l or t e c h n i c a l handicap, but not a t h e o r e t i c a l one. Thus, there i s no l o g i c a l d i s p a r i t y between these two k i n d s of p r e d i c t i o n s . And i f a p r e d i c t i o n based on Newton's F i r s t Law of Motion i s g e n u i n e l y s c i e n t i f i c , so i s a p r e d i c t i o n based on e v o l u t i o n a r y theory. 7.2.2.2 P r o b a b i l i s t i c or S t a t i s t i c a l P r e d i c t i o n s Now l e t us t u r n our a t t e n t i o n to the problem of making a c t u a l p r e d i c t i o n s on the b a s i s of e v o l u t i o n a r y t h e o r y . When S c r i v e n t a l k s about a c t u a l p r e d i c t i o n s based on e v o l u t i o n a r y theory, he i s always r e f e r r i n g to d i s c r e t e , i n d i v i d u a l f u t u r e events. However, e v o l u t i o n a r y t h e o r y does not d e a l w i t h i n d i v i d u a l s or i n d i v i d u a l events, but w i t h p o p u l a t i o n s or s e t s of events. Darwin h i m s e l f admits i n a l e t t e r t o H. G. Bronn, "You put v e r y w e l l and v e r y f a i r l y t h a t I can i n no one i n s t a n c e e x p l a i n the course of 177 m o d i f i c a t i o n i n any p a r t i c u l a r ^ i t a l i c s mine} i n s t a n c e . " 1 As Darwin formulated i t , e v o l u t i o n a r y theory d i d not permit v e r i f i c a t i o n or f a l s i f i c a t i o n by the simple deduction of a s i n g l e o b s e r v a t i o n a l consequence and checking i t . Given e v o l u t i o n a r y theory and the k i n d of data a v a i l a b l e to a b i o l o g i s t , no p r e c i s e i n f e r e n c e s are p o s s i b l e . One c o u l d not p r e d i c t how an e x t a n t s p e c i e s would evolve or r e t r o d i c t how an e x t i n c t s p e c i e s d i d e v o l v e . At b e s t , o n l y a range of some more or l e s s p o s s i b l e outcomes c o u l d be generated. T h i s i n h e r e n t l y p r o b a b i l i s t i c or s t a t i s t i c a l nature of e v o l u t i o n a r y theory has long been r e c o g n i z e d and emphasized by C. S. P e i r c e whose acc u r a t e understanding of i t i s n i c e l y expressed i n the f o l l o w i n g q u o t a t i o n : The Darwinian c o n t r o v e r s y i s , i n l a r g e p a r t , a q u e s t i o n of l o g i c . Mr. Darwin proposed to apply the s t a t i s t i c a l method to b i o l o g y . The same t h i n g has been done i n a w i d e l y d i f f e r e n t branch of s c i e n c e , the theory of gases. Though unable to say what the movements of any p a r t i c u l a r molecule of gas would be on a c e r t a i n h y p o thesis r e g a r d i n g the c o n s t i t u t i o n of t h i s c l a s s of b o d i e s , C l a u s i u s and Maxwell were y e t a b l e , by the a p p l i c a t i o n of the d o c t r i n e of p r o b a b i l i t i e s , to p r e d i c t t h a t i n the long run such and such a p r o p o r t i o n of the molecules would, under gi v e n circumstances, a c q u i r e such and such v e l o c i t i e s ; t h a t there would take p l a c e , every second, such and such a number of c o l l i s i o n s , e t c . ; and from these p r o p o s i t i o n s they were able t o deduce c e r t a i n p r o p e r t i e s of gases, e s p e c i a l l y i n regard to t h e i r heat-r e l a t i o n s . In l i k e manner, Darwin while unable to say what the o p e r a t i o n of v a r i a t i o n and n a t u r a l s e l e c t i o n i n every i n d i v i d u a l case w i l l be, demonstrates t h a t i n the long run they w i l l adapt animals to t h e i r c i r c u m s t a n c e s . Whether or not e x i s t i n g animal forms are due to such a c t i o n , or what p o s i t i o n the theory ought to take, forms the s u b j e c t of a d i s c u s s i o n i n which q u e s t i o n s of f a c t and q u e s t i o n s of l o g i c are c u r i o u s l y i n t e r l a c e d . 2 Darwin & Seward ( 1 9 0 3 ) , V o l . 1 , pp. 1 7 2 - 1 7 3 , quoted i n H u l l ( 1 9 7 3 ) , p. 3 2 . 2 P e i r c e ( 1 8 7 7 ) , p. 1 - 1 5 , quoted i n H u l l ( 1 9 7 3 ) , p. 3 3 . 178 A l s o on the t o p i c of the i n h e r e n t l y s t a t i s t i c a l , nature of e v o l u t i o n a r y theory o r , more s p e c i f i c a l l y , the concepts of 'adaptation' and ' f i t n e s s 1 , G h i s e l i n w r i t e s : A d a p t a t i o n s , l i k e the k i n e t i c energy of gaseous m o l e c u l e s , i s c onceived of i n s t a t i s t i c a l terms. Our c o n c e p t i o n of a d a p t a t i o n corresponds to a tendency of organisms to enter i n t o p a r t i c u l a r kinds of r e l a t i o n s h i p s w i t h t h e i r s u rroundings. The k i n e t i c energy of a gas i s measurable by i t s p r e s s u r e , but not d e f i n e d i n terms of i t , f o r i t i s p o s s i b l e f o r the i n d i v i d u a l molecules to so c o i n c i d e i n t h e i r movements t h a t they would e x e r t no p r e s s u r e . S i m i l a r l y , w e l l-adapted organisms d i e and become e x t i n c t i n s p i t e of t h e i r f i t n e s s : f o r example, f l i e s s t r u c k by l i g h t n i n g . And t h e r e i s no reason to suppose t h a t a r e l a t i v e l y i l l - a d a p t e d group cannot p o s s i b l y s u r v i v e i n s p i t e of the i m p r o b a b i l i t y of i t s doing so. To speak of a d a p t a t i o n , then, i m p l i e s t h a t one d e a l s i n g e n e r a l i t i e s , t h a t one d i s c u s s e s matters of a h y p o t h e t i c a l n a t u r e , or t h a t one has made a s t a t i s t i c a l e stimate or i n f e r e n c e , as when one a t t r i b u t e s the disappearance of a group to a d a p t a t i o n r e l a t i v e l y i n f e r i o r to t h a t of another one. In p r i n c i p l e , one may estimate or c a l c u l a t e a d a p t a t i o n , but i t may not be p o s s i b l e to p r o v i d e an a b s o l u t e l y p r e c i s e measurement of i t . 1 I f e v o l u t i o n a r y theory i s i n h e r e n t l y s t a t i s t i c a l , the c r i t i c might ask, then how can i t p o s s i b l y be f a l s i f i e d s i n c e a s t a t i s t i c a l h y p o t h e s i s or theory i s not f a l s i f i e d by a s i n g l e or a few d i s c o n f i r m i n g i n s t a n c e s ? And i f t h a t i s the case, then s t a t i s t i c a l p r e d i c t i o n s on the b a s i s of e v o l u t i o n a r y theory do not appear t o be g e n u i n e l y s c i e n t i f i c . I t h i n k when the c r i t i c complains t h a t a s t a t i s t i c a l h y p o t hesis or theory cannot p o s s i b l y be f a l s i f i e d , they mean t h a t i t cannot be c o n c l u s i v e l y f a l s i f i e d , and they are a b s o l u t e l y r i g h t . However, the l a c k of c o n c l u s i v e n e s s does not imply t h a t we may not have good evidence f o r r e j e c t i n g a ^ G h i s e l i n (1966), p. 149. 179 s t a t i s t i c a l h y p o t h e s i s . S t a t i s t i c a l hypotheses, by t h e i r v ery s t a t i s t i c a l n a ture, are not f a l s i f i e d or v e r i f i e d by i n d i v i d u a l events. Thus, the h y p o t h e s i s t h a t "30% of the c i g a r e t t e smoking p o p u l a t i o n w i l l develop lung cancer b e f o r e the age of 60" w i l l be f a l s i f i e d i f r e s e a r c h e s show t h a t 5% or 2 0% of a r e l e v a n t l y l a r g e 1 c i g a r e t t e smoking p o p u l a t i o n are s u f f e r i n g from lung cancer. I f a c t u a l p r e d i c t i o n s of e v o l u t i o n a r y theory are e s s e n t i a l l y s t a t i s t i c a l , j u s t l i k e p r e d i c t i o n s of the k i n e t i c theory of gases, then why do c r i t i c s l i k e S c r i v e n p e r s i s t e n t l y r e j e c t the p r e d i c t i v e power of e v o l u t i o n a r y theory f o r i t s f a i l u r e to make a c t u a l p r e d i c t i o n s about i n d i v i d u a l organisms or events? I s t r o n g l y f e e l t h a t W i l l i a m s has probably the b e s t and most i n s i g h t f u l s o l u t i o n to t h i s p u z z l e , worthy of q uoting i n i t s e n t i r e t y : In f i n e , the problem i s not t h a t e v o l u t i o n a r y t h e o r y d i f f e r s from gas theory i n i t s a b i l i t y t o make a c t u a l ( i . e . h y p o t h e t i c o - d e d u c t i v e ) p r e d i c t i o n s ; the problem i s , r a t h e r , t h a t w h i l e the p h y s i c a l l y u n s o p h i s t i c a t e d person expects the p r e d i c t i o n s of gas theory to be about temperature, p r e s s u r e , e t c . , the b i o l o g i c a l u n s o p h i s t i c a t e d person expects the p r e d i c t i o n s of e v o l u t i o n a r y t h e o r y to be about i n d i v i d u a l organisms. The reason f o r t h i s d i f f e r e n c e i n e x p e c t a t i o n s i s t h a t we expect p r e d i c t i o n s about the p r o p e r t i e s t h a t we d i r e c t l y e x p e r i e n c e . We cannot see i n d i v i d u a l molecules and so we do not ask f o r p r e d i c t i o n s about t h e i r speed; we can f e e l temperature and p ressure and so we t r y to f i n d p r e d i c t i o n s about them. S i m i l a r l y we do not see c l a n s [ s e t s or groups] and so we do not ask about t h e i r p r o p e r t i e s ; we do see i n d i v i d u a l organisms and so we ask f o r p r e d i c t i o n s about t h e i r p r o p e r t i e s . The apparent d i f f e r e n c e i n the types 1-There i s of course no a b s o l u t e way of j u d g i n g whether the s i z e of the examined p o p u l a t i o n i s ' r e l e v a n t l y l a r g e ' enough, but i t w i l l s u f f i c e f o r our purpose to adopt whatever standard t h a t i s s t a t i s t i c a l l y a c c e p t a b l e . 180 of p r e d i c t i o n p r o v i d e d by e v o l u t i o n a r y theory and gas theory i s caused by the d i f f e r e n c e i n our r e l a t i o n s h i p to the two t h e o r i e s : gas theory g i v e s p r e d i c t i o n s about p r o p e r t i e s t h a t we p e r s o n a l l y e x p e r i e n c e , and we do not care t h a t i t cannot p r e d i c t the speed of a s p e c i f i c i n d i v i d u a l molecule; but e v o l u t i o n a r y theory g i v e s p r e d i c t i o n s about p r o p e r t i e s of a b s t r a c t c o n s t r u c t s which we do not p e r s o n a l l y experience, and we f i n d i t u n s a t i s f y i n g t h a t i t cannot make p r e d i c t i o n s about the th i n g s we do p e r s o n a l l y e x p e r i e n c e . There i s no p h i l o s o p h i c a l d i f f e r e n c e between the two t h e o r i e s ; the d i f f e r e n c e i s p s y c h o l o g i c a l [ i t a l i c s mine] . 1 7.2.2.3 I n d u s t r i a l Melanism Having d e a l t so f a r w i t h the p r e d i c t a b i l i t y and f a l s i f i a b i l i t y of e v o l u t i o n a r y theory i n a predominantly t h e o r e t i c a l manner, l e t us now examine i n d e t a i l the famous c o n f i r m i n g i n s t a n c e of the theory, namely the phenomenon of i n d u s t r i a l melanism. T h i s phenomenon of i n d u s t r i a l melanism, t h a t i s , an i n c r e a s e i n frequency of dark c o l o u r e d moths i n the i n d u s t r i a l r e g i o n s of Europe d u r i n g the past hundred y e a r s , i s , i n Dobzhansky's words, "perhaps the most s t r i k i n g and lo n g e s t known example of e v o l u t i o n a r y changes." 2 P o p u l a t i o n s of s e v e r a l d i f f e r e n t and u n r e l a t e d s p e c i e s of moths have changed c o l o u r over e s s e n t i a l l y the same p e r i o d of time. Before the change o c c u r r e d , the moths of these s p e c i e s were u n i f o r m l y l i g h t , p a l e gray or w h i t i s h i n c o l o u r . At v a r i o u s p. 25, ^Williams (1973), p. 530 2Dobzhansky (1963), p. 104, quoted i n Manser (1965), 181 times and places, which varied with the species concerned, individ u a l s with dark coloured wings and body appeared, f i r s t as i s o l a t e d freaks, and l a t e r with increasing frequency u n t i l they dominated the populations of cert a i n areas which were i n every case regions of extreme i n d u s t r i a l i z a t i o n , such as Ruhr Valley in-Germany and the English Midlands. I t has been suggested that t h i s evolutionary change i s due to the fact that the main enemies of the moths are birds which prey on them. The moths evolved protective camouflage against t h e i r surroundings, for example, trees, to avoid t h e i r predators. Before the advent of i n d u s t r i a l i z a t i o n , the trees were comparatively clean and hence l i g h t in colour, so there was a sele c t i v e pressure against the melanic mutants which stood out more conspicuously than t h e i r l i g h t counterparts. But as a r e s u l t of the ef f e c t s of i n d u s t r i a l p o l l u t i o n i n which smoke and soot from the factory chimneys darkened the ground and the tree trunks, there has been a selec t i v e pressure against the l i g h t moths i n these i n d u s t r i a l i z e d regions. However, i t has been observed that the l i g h t moths are coming back i n those areas which have introduced smokeless zones. 1 I t c e r t a i n l y appears that t h i s phenomenon of i n d u s t r i a l melanism i s an excellent confirming case of the theory of natural sele c t i o n . For i n both instances, that i s , aft e r i n d u s t r i a l i z a t i o n and aft e r introducing smokeless zones, i t J-Cook, Askew & Bishop (1970) , p. 1155. 182 i s the b e t t e r - a d a p t e d moths, the dark and l i g h t ones r e s p e c t i v e l y , which have s u r v i v e d , and the change of c o l o u r has o c c u r r e d because the desideratum of a d a p t i v e advantage has changed. Yet Manser t h i n k s otherwise. He argues t h a t the account g i v e n of the change of c o l o u r i s . . . o n l y a d e s c r i p t i o n i n s l i g h t l y t h e o r y - l a d e n terms which g i v e s the i l l u s i o n of an e x p l a n a t i o n i n the f u l l s c i e n t i f i c sense. I f no mutant forms had o c c u r r e d and the s p e c i e s had become e x t i n c t as a r e s u l t of the change of circumstances, i t would not have been adaptable. We cannot use the account to p r e d i c t what w i l l happen when a new f e a t u r e occurs i n the environment of a d i f f e r e n t s p e c i e s , or even i f t h e r e i s another change i n the environment of the o r i g i n a l moths. A l l we can do i s , a f t e r t here has been time f o r the s t a t e of a f f a i r s t o become s t a b l e a g a i n , say whether the s p e c i e s i n q u e s t i o n was or was not a d a p t a b l e . 1 In other words, he i s c l a i m i n g t h a t the c o l o u r change of the moths opens up no "way of t e s t i n g the theory. I t i s o n l y a f t e r the change had o c c u r r e d t h a t we can say t h a t the melanic form was the b e t t e r - a d a p t e d form. I f no change had o c c u r r e d , we would then say i t was the l i g h t form which was the b e t t e r - a d a p t e d . No matter what the evidence, whether the moths be l i g h t or dark, the theory of n a t u r a l s e l e c t i o n can c l a i m i t as c o n f i r m a t o r y support. However, he concludes, such i n d i f f e r e n c e t o evidence i s not a t r u e s i g n of a genuine s c i e n t i f i c t heory. Is the above melanism example r e a l l y a t y p i c a l case of f i t t i n g the theory to the f a c t s as Manser has charged? I 2 t h i n k not and I seem t o be i n good company. There i s no xManser (1965), pp. 25-26. 2See, f o r example, Ruse (1969), p. 340 and K e t t l e w e l l (1959). 183 evidence to substantiate the charge that t h i s melanism case i s one i n which the facts are manipulated to f i t the theory, for Kettlewell has car r i e d out experiments which c l e a r l y show that birds destroy many more mutants than l i g h t moths on lichen-covered trees i n the non-industrial areas and more l i g h t than melanic moths on soot-covered trees i n the i n d u s t r i a l regions.^ In other words, these experiments support the hypothesis that wing- and body-colour can confer adaptive advantage and, coupled with the change i n tree-colour, that the phenomenon of i n d u s t r i a l melanism i s a genuine confirming case of evolutionary theory. As a matter of f a c t , the theory could e a s i l y have been f a l s i f i e d by these experiments i f a change i n tree-colour had not been followed by an adaptive change i n the moths to accomodate the new environment; i n t h i s case, a change i n the colour of t h e i r wings and body. Incidentally, evolutionary theory, as suggested by both Ruse 2 and Williams, 3 does make other f a l s i f i a b l e predictions which are subject to empirical t e s t . 7.2.3 Experimental Reproducibility and A r t i f i c i a l Selection Before discussing whether the theory of natural selection can be v e r i f i e d or f a l s i f i e d by experiments i n a r t i f i c i a l surroundings, l e t us f i r s t draw a d i s t i n c t i o n between natural selection and a r t i f i c i a l s e l e c t i o n . The 1See Kettlewell (1955). 2See Ruse (1969), pp. 347-352. 3See Williams (1973), pp. 518-524. e s s e n t i a l d i f f e r e n c e between the two i s t h a t a r t i f i c i a l s e l e c t i o n , as opposed t o n a t u r a l s e l e c t i o n , i m p l i e s t h a t there i s a r a t i o n a l human agent t r y i n g to i n f l u e n c e the f i n a l product, whether i t be the r e t a i n i n g or removal of c e r t a i n c h a r a c t e r i s t i c s . O b v i o u s l y , one can have n a t u r a l s e l e c t i o n without a r t i f i c i a l s e l e c t i o n , but s i n c e human plans o f t e n f a i l and p r i z e animals t u r n out to be i n f e r t i l e , a r t i f i c i a l s e l e c t i o n i s probably u s u a l l y accompanied by some n a t u r a l s e l e c t i o n . Bearing t h i s d i s t i n c t i o n i n mind, l e t us t r y to answer the f o l l o w i n g q u e s t i o n s . F i r s t , can the r e s u l t s of a r t i f i c i a l s e l e c t i o n , which i s done p u r e l y f o r p r a c t i c a l or a e s t h e t i c reasons without thought of the u n d e r l y i n g theory, n e v e r t h e l e s s have i m p l i c a t i o n s f o r e v o l u t i o n a r y theory? Secondly, can experiments i n v o l v i n g a r t i f i c i a l s e l e c t i o n be de v i s e d which w i l l throw l i g h t on the theory of n a t u r a l s e l e c t i o n ? T h i r d l y , can n a t u r a l s e l e c t i o n be achieved i n a r t i f i c i a l surroundings, and i f so, how can t h i s be used to co n f i r m e v o l u t i o n a r y theory? I s h a l l f o l l o w very c l o s e l y Ruse's e x c e l l e n t answers to these q u e s t i o n s . 1 With r e f e r e n c e to the f i r s t q u e s t i o n , Ruse r i g h t l y t h i n k s t h a t i t i s undeniable t h a t a r t i f i c i a l s e l e c t i o n i s r e l e v a n t t o e v o l u t i o n a r y theory. Darwin h i m s e l f devoted the f i r s t chapter of h i s O r i g i n of Species to a d i s c u s s i o n of such i n s t a n c e s of a r t i f i c i a l s e l e c t i o n , a r g u i n g t h a t they xRuse (1969), pp. 341-347 and Ruse (1973b), pp. 102-105. 185 s t r o n g l y supported h i s theory. For example, about one i n s t a n c e of the pigeon breeder's a r t he wrote: A l t o g e t h e r at l e a s t a score of pigeons might be chosen, which, i f shown to an o r n i t h o l o g i s t , and he were t o l d t h a t they were w i l d b i r d s , would c e r t a i n l y , I t h i n k , be ranked by him as w e l l - d e f i n e d s p e c i e s . Moreover, I do not b e l i e v e t h a t any o r n i t h o l o g i s t would p l a c e the E n g l i s h c a r r i e r , the s h o r t - f a c e d tumbler, the r u n t , the barb, pouter, and f a n t a i l i n the same genus . . . .! Yet, s i m u l t a n e o u s l y , Darwin was a b l e to Show t h a t i n f a c t a l l these d i f f e r e n t breeds of domestic pigeon are descended from one and o n l y one s p e c i e s of w i l d pigeon. Because of t h i s , he argued t h a t s i n c e a r t i f i c i a l s e l e c t i o n can have so d r a s t i c an e f f e c t on the c h a r a c t e r s of a p o p u l a t i o n of organisms, i t i s reasonable to suppose t h a t n a t u r a l s e l e c t i o n can have a s i m i l a r e f f e c t : Can the p r i n c i p l e of s e l e c t i o n , which we have seen i s so potent i n the hands of man, apply i n nature? I t h i n k we s h a l l see t h a t i t can a c t most e f f e c t u a l l y . 2 What these cases of a r t i f i c i a l s e l e c t i o n show i s t h a t , even though the d i f f e r e n c e s between the s u c c e s s f u l and u n s u c c e s s f u l organisms may be very s l i g h t , the cumulative e f f e c t of t h i s d i f f e r e n t i a l r e p r o d u c t i o n can be very g r e a t . Hence, gi v e n d i f f e r e n t i a l s e l e c t i o n , there i s no need to hypothesize the e x i s t e n c e of l a r g e v a r i a t i o n s to account f o r l a r g e o v e r a l l changes. However, s i n c e n a t u r a l s e l e c t i o n i s a l s o a type of d i f f e r e n t i a l r e p r o d u c t i o n , i t t h e r e f o r e seems l e g i t i m a t e t o argue f o r t h i s c o n c l u s i o n f o r n a t u r a l s e l e c t i o n l-Darwin (1859), pp. 22-23. 2 I b i d . , p. 80. 186 on t h e b a s i s o f a r t i f i c i a l s e l e c t i o n . I n o t h e r words, a r t i f i c i a l s e l e c t i o n shows t h a t a l l k i n d s o f s e l e c t i o n ( i n c l u d i n g n a t u r a l s e l e c t i o n ) c o u l d l e a d t o l a r g e changes even though o n l y s m a l l v a r i a t i o n s a r e i n v o l v e d . Y e t , Ruse c a u t i o n s u s , t h i s does not show e i t h e r t h a t n a t u r a l s e l e c t i o n e x i s t s o r t h a t i f i t does e x i s t , i n what d i r e c t i o n such s e l e c t i o n p o i n t s . T h i s b r i n g s us t o t h e second q u e s t i o n , whether o r not a r t i f i c i a l s e l e c t i o n can be used t o throw l i g h t on e v o l u t i o n a r y t h e o r y ? I t seems f a i r l y o b v i o u s , Ruse h o l d s , t h a t i t can i f t h e e x p e r i m e n t e r can g i v e good r e a s o n s f o r s u g g e s t i n g t h a t h i s method of s e l e c t i o n i s analogous t o a s e l e c t i v e f o r c e w h i c h might be found i n the w i l d . A good example o f an a t t empt t o use a r t i f i c i a l s e l e c t i o n t o s i m u l a t e n a t u r a l s e l e c t i o n would be t h e e x p e r i m e n t s c a r r i e d out by Thoday and a s s o c i a t e s on s m a l l p o p u l a t i o n s o f D r o s o p h i l a w i t h r e g a r d t o t h e d i s p u t e whether a l l o p a t r i c o r g e o g r a p h i c a l s p e c i a t i o n i s t h e o n l y method of s p e c i a t i o n , o r t h a t s y m p a t r i c s p e c i a t i o n , i . e . s p e c i a t i o n w i t h o u t g e o g r a p h i c i s o l a t i o n , a l s o e x i s t s . The r e s u l t s o f t h e s e e x p e r i m e n t s tended t o s u b s t a n t i a t e t h e l a t t e r c l a i m b u t r e s e r v a t i o n s have t o be made, the most i m p o r t a n t b e i n g t h a t the e x p e r i m e n t a l p o p u l a t i o n s might be t o o s m a l l (about 40 members i n each) t o be t r u e i m i t a t i o n s o f the f r e e l y - i n t e r b r e e d i n g p o p u l a t i o n s i n t h e w i l d . P r o b a b l y most s e l e c t i o n e x p e r i m e n t s a r e l i k e T hoday 1s i n t h a t one must make q u a l i f i c a t i o n s about the c o n c l u s i o n s 187 one can draw, which lea d s us to the t h i r d q u e s t i o n , namely what evidence can one g i v e of the e x i s t e n c e of n a t u r a l s e l e c t i o n by u s i n g c a p t i v e p o p u l a t i o n s i n a r t i f i c i a l surroundings? In other words, i f we do not f i r s t e s t a b l i s h the e x i s t e n c e of n a t u r a l s e l e c t i o n , then do not a l l o f our an a l o g i e s from a r t i f i c i a l s e l e c t i o n become f u t i l e ? In Ruse's o p i n i o n , there c e r t a i n l y seems no reason, i n p r i n c i p l e , why n a t u r a l s e l e c t i o n should not occur and be s t u d i e d i n c a p t i v e p o p u l a t i o n s i n a r t i f i c i a l surroundings. Given organisms' geometric c a p a c i t y f o r p o p u l a t i o n i n c r e a s e , then i f one r e s t r i c t s t h i s c a p a c i t y by r e g u l a t i n g food or space or something e l s e , then many are going t o d i e without r e p r o d u c i n g . Moreover, success and f a i l u r e w i l l be a f u n c t i o n of the c h a r a c t e r s possessed by the o r g a n i s m s - - i t w i l l not be a f u n c t i o n of the experimenter's d e c i s i o n t o l e t some succeed and ot h e r s f a i l , as i t would be were a r t i f i c i a l s e l e c t i o n o c c u r r i n g . An e x c e l l e n t example of an experiment i n v o l v i n g t h i s k i n d of n a t u r a l s e l e c t i o n i n a c a p t i v e p o p u l a t i o n i s g i v e n by Dobzhansky. 1 P o p u l a t i o n s of D r o s o p h i l a melanogaster were kept i n c a g e s — t h e p o p u l a t i o n s c o n s i s t i n g of mixtures of the w i l d type ( i . e . f l i e s w i t h normal-length wings) and of a mutant, ' v e s t i g i a l ' ( i . e . f l i g h t l e s s f l i e s w i t h stubby wings). When no s p e c i a l adjustments were made to the environment, the w i l d type i n c r e a s e d r a p i d l y , p o i n t i n g to the f a c t , not onl y t h a t n a t u r a l s e l e c t i o n was o c c u r r i n g , but t h a t the w i l d type 1Dobzhansky (1951), p. 90. 188 was f i t t e r . However, one might suspect t h a t i n some circumstances, the v e s t i g i a l would prove to be f i t t e r , and t h i s turned out to be the case. On s m a l l oceanic i s l a n d s , a l a c k of wings c o u l d be a d a p t a t i v e l y advantageous, f o r winged i n s e c t s would tend t o be blown out to the sea and dest r o y e d . Experiments confirmed t h i s s u p p o s i t i o n , f o r when the c a p t i v e p o p u l a t i o n s were kept i n p l a c e s where wind c o u l d blow members away, i t was the v e s t i g i a l which proved f i t t e r and i n c r e a s e d i n number. I completely agree w i t h Ruse t h a t these experiments undoubtedly supply evidence, not onl y of the e x i s t e n c e of n a t u r a l s e l e c t i o n , but a l s o o f the e f f e c t s of the s o r t s of s e l e c t i o n we might expect to f i n d i n nature. In f a c t , the hypothesis t h a t the l a c k of wings might be a d a p t a t i v e l y advantageous i n windy c o n d i t i o n s would have been f a l s i f i e d i f the v e s t i g i a l mutants d i d not prove f i t t e r i n these circumstances. However, a c c o r d i n g to Ruse, some q u a l i f i c a t i o n s must be made. F i r s t , the s u b j e c t s of these experiments are r e s t r i c t e d to r a p i d l y b reeding organisms. Second, one must r e c o g n i z e t h a t some organisms j u s t r e f u s e to breed i n c a p t i v i t y whereas they would i n nature, and c o n v e r s e l y , some normally r e p r o d u c t i v e l y i s o l a t e d organisms h y b r i d i z e madly i n c a p t i v i t y , producing f u l l y f i t o f f s p r i n g . O b viously, the scope f o r n a t u r a l s e l e c t i o n experiments i s l i m i t e d to those organisms which behave i n c a p t i v i t y more or l e s s as they do i n the w i l d . F i n a l l y , i t i s very d i f f i c u l t , i f not i m p o s s i b l e , t o simulate n a t u r a l c o n d i t i o n s e x a c t l y . 189 As a r e s u l t , when very weak s e l e c t i v e f o r c e s are being c o n s i d e r e d , t h e r e i s a s t r o n g p o s s i b i l i t y t h a t some f e a t u r e which i s a f u n c t i o n of the a r t i f i c i a l i t y of the surroundings w i l l become, s i g n i f i c a n t and d i s t o r t the r e s u l t s . In other words, such experiments can n e c e s s a r i l y c o n s i d e r o n l y s e l e c t i v e f o r c e s s t r o n g e r than the minimum found i n nature (in any case, the l i m i t a t i o n s of time w i l l probably c o n f i n e the experimenter's a t t e n t i o n t o r e l a t i v e l y s t r o n g s e l e c t i v e f o r c e s ) . In my o p i n i o n , however, these q u a l i f i c a t i o n s do not hamper the c o n f i r m a t o r y v a l u e of these experiments. They onl y show t h a t some p r a c t i c a l , experimental l i m i t a t i o n s are to be expected. Yet t h i s i s not u n i q u e l y t r u e of experiments i n v o l v i n g n a t u r a l s e l e c t i o n i n a r t i f i c i a l s urroundings, but i n most, i f not a l l , experiments, be i t p h y s i c a l , c h e m i c a l , or b i o l o g i c a l . For i n s t a n c e , j u s t as b i o l o g i s t s choose organisms which behave i n c a p t i v i t y more or l e s s as they do i n nature f o r n a t u r a l s e l e c t i o n experiments, chemists, i n l i k e manner, use water a t room temperature, but not a t -5°C when i t i s i c e and not a t 105°C when i t i s steam, f o r experiments demonstrating the s o l u b i l i t y o f a substance. F u r t h e r , i t i s a pronounced f a c t among s c i e n t i s t s t h a t i t i s extremely d i f f i c u l t , i f not t o t a l l y i m p o s s i b l e , to s i m u l a t e n a t u r a l c o n d i t i o n s e x a c t l y . Margins of e r r o r are a f a c t , not a v i c e , of s c i e n t i f i c experiments. A l l we can hope f o r i s to minimize, but never e l i m i n a t e , these e r r o r s . 190 At t h i s p o i n t , the c r i t i c might argue t h a t although such experiments show t h a t n a t u r a l s e l e c t i o n occurs and indeed a t times show how i t o c c u r s , they n e v e r t h e l e s s f a i l to e x p l a i n (and confirm) the main aim of e v o l u t i o n a r y s t u d i e s , namely to show (with c o n v i n c i n g evidence) the major e v o l u t i o n a r y paths which p a s t organisms have trodden. However, as Ruse has c o r r e c t l y p o i n t e d out, t h i s i s to misconceive the problem. One must draw a d i s t i n c t i o n between a theory of e v o l u t i o n and a phylogehy. A theory of e v o l u t i o n t e l l s us about the mechanism of e v o l u t i o n ; a p h y l o g e n e t i c d e s c r i p t i o n t e l l s us about the course of e v o l u t i o n . 1 Evidence f o r one i s not n e c e s s a r i l y evidence f o r the o t h e r . S e l e c t i o n experiments are designed to t e s t the t h e o r y of e v o l u t i o n by n a t u r a l s e l e c t i o n and i t passes them w i t h f l y i n g c o l o u r s . The evidence f o r p h y l o g e n i es i s much more scanty and c o n t r o v e r s i a l , but i t i s mistaken to t h i n k t h a t t h i s a u t o m a t i c a l l y i m p l i e s t h a t t h e r e can be l i t t l e evidence f o r e v o l u t i o n a r y theory. ! c f . sec. 4.1.1.2. 191 7.3 Is E v o l u t i o n a r y Theory a M e t a p h y s i c a l  Research Programme? Popper, l i k e Hempel, i s a f i r m b e l i e v e r i n the symmetry t h e s i s . T h i s i s e v i d e n t from h i s statement t h a t s i n c e "Darwinism does not r e a l l y p r e d i c t the e v o l u t i o n of v a r i e t y , i t t h e r e f o r e cannot e x p l a i n i t . " 1 Yet, u n l i k e Hempel, he does not regard Darwinism or e v o l u t i o n a r y theory as a t e s t a b l e s c i e n t i f i c theory, and thus not a s u p p o r t i n g case f o r the symmetry t h e s i s . Rather, he maintains t h a t Darwinism i s a meta p h y s i c a l r e s e a r c h programme—a p o s s i b l e framework f o r t e s t a b l e s c i e n t i f i c t h e o r i e s . I i n t e n d to show i n t h i s s e c t i o n t h a t Darwinism i s indeed a t e s t a b l e s c i e n t i f i c theory which i s not a counterexample to the symmetry t h e s i s . By 'Darwinism,' Popper i s r e f e r r i n g to modern 1neo-Darwinism' 2 which c o n s i s t s e s s e n t i a l l y of the f o l l o w i n g assumptions: (1) The g r e a t v a r i e t y of the forms of l i f e on e a r t h o r i g i n a t e from very few forms, perhaps even from a s i n g l e organism: there i s an e v o l u t i o n a r y t r e e , an e v o l u t i o n a r y h i s t o r y . (2) There i s an e v o l u t i o n a r y theory which e x p l a i n s t h i s . I t c o n s i s t s i n the main of the f o l l o w i n g hypotheses: (a) H e r e d i t y : the o f f s p r i n g reproduce the parent organisms Popper (1974), p. 136. See Popper (1974), pp. 135-136. 192 f a i r l y f a i t h f u l l y . (b) V a r i a t i o n : there are (perhaps among others) 'small' v a r i a t i o n s . The most important of these are the ' a c c i d e n t a l ' and h e r e d i t a r y mutations. (c) N a t u r a l s e l e c t i o n : there are v a r i o u s mechanisms by which not onl y the v a r i a t i o n s but the whole h e r e d i t a r y m a t e r i a l i s c o n t r o l l e d by e l i m i n a t i o n . Among them are mechanisms which al l o w o n l y 'small' mutations to spread; 'big' mutations are as a r u l e e l i m i n a t e d . (d) V a r i a b i l i t y : although v a r i a t i o n s i n some s e n s e — t h e presence of d i f f e r e n t c o m p e t i t o r s — a r e f o r obvious reasons p r i o r to s e l e c t i o n , i t may w e l l be the case t h a t v a r i a b i l i t y — t h e scope of v a r i a t i o n — i s c o n t r o l l e d by n a t u r a l s e l e c t i o n ; f o r example, w i t h r e s p e c t to the frequency as w e l l as the s i z e of v a r i a t i o n s . Let us now t u r n to the arguments which Popper uses to support h i s c l a i m t h a t Darwinism i s a meta p h y s i c a l r e s e a r c h programme. In Popper's o p i n i o n , Darwinism i s metap h y s i c a l because i t i s not t e s t a b l e . One might t h i n k t h a t i t i s . I t seems to a s s e r t t h a t i f ever on some p l a n e t we f i n d l i f e which s a t i s f i e s c o n d i t i o n s (a) and (b), then (c) w i l l come i n t o p l a y and b r i n g about i n time a r i c h v a r i e t y of d i s t i n c t forms. Darwinism, however, does not a s s e r t as much as t h i s . For assume t h a t we f i n d l i f e on Mars c o n s i s t i n g of e x a c t l y three s p e c i e s of b a c t e r i a w i t h a g e n e t i c o u t f i t s i m i l a r to th a t of t e r r e s t r i a l s p e c i e s . Is Darwinism r e f u t e d ? By no means. We s h a l l say t h a t these three s p e c i e s were the only forms among the many mutants which were s u f f i c i e n t l y w e l l a d j u s t e d to s u r v i v e . And we s h a l l say the same i f there i s onl y one sp e c i e s (or none). Thus Darwinism does not r e a l l y p r e d i c t the e v o l u t i o n 193 of v a r i e t y . I t t h e r e f o r e cannot e x p l a i n i t . At b e s t , i t can p r e d i c t the e v o l u t i o n of v a r i e t y under "favour a b l e c o n d i t i o n s . " But i t i s h a r d l y p o s s i b l e to d e s c r i b e i n g e n e r a l terms what favo u r a b l e c o n d i t i o n s a r e — e x c e p t t h a t , i n t h e i r presence, a v a r i e t y of forms w i l l emerge.1 Popper claims t h a t he has taken Darwinian theory almost at i t s b e s t — a l m o s t i n i t s most t e s t a b l e form. In other f i e l d s , he a s s e r t s , i t s p r e d i c t i v e or e x p l a n a t o r y power i s s t i l l more d i s a p p o i n t i n g . Take 'adaptation' as an example. At f i r s t s i g h t n a t u r a l s e l e c t i o n appears to e x p l a i n i t , and i n a way i t does, but i t i s h a r d l y a s c i e n t i f i c way. For t o say t h a t a s p e c i e s now l i v i n g i s adapted to i t s environment i s , i n f a c t , almost t a u t o l o g i c a l . Indeed we use the terms "adaptation" and " s e l e c t i o n " i n such a way t h a t we can say t h a t , i f the s p e c i e s were not adapted, i t would have been e l i m i n a t e d by n a t u r a l s e l e c t i o n . S i m i l a r l y , i f a s p e c i e s has been e l i m i n a t e d i t must have been i l l adapted to the c o n d i t i o n s . Adaptation or f i t n e s s i s d e f i n e d by modern e v o l u t i o n i s t s as s u r v i v a l v a l u e , and can be measured by a c t u a l success i n s u r v i v a l : there i s h a r d l y any p o s s i b i l i t y o f t e s t i n g a theory as f e e b l e as t h i s . 2 Popper makes the same a c c u s a t i o n elsewhere when he says t h a t . . . the t r o u b l e about e v o l u t i o n a r y theory i s i t s t a u t o l o g i c a l , or almost t a u t o l o g i c a l c h a r a c t e r : the d i f f i c u l t y i s t h a t Darwinism and n a t u r a l s e l e c t i o n , though extremely important, e x p l a i n e v o l u t i o n by 'the s u r v i v a l of the f i t t e s t 1 (a term due to Herbert Spencer). Yet there does not seem to be much d i f f e r e n c e , i f any, between the a s s e r t i o n 'those t h a t s u r v i v e are the f i t t e s t ' and the t a u t o l o g y 'those t h a t s u r v i v e are those t h a t s u r v i v e 1 . For we have, I am a f r a i d , no o t h e r c r i t e r i o n of f i t n e s s than a c t u a l s u r v i v a l , so t h a t we conclude from the f a c t t h a t some Popper (1974), p. 136. 2 I b i d . , p. 137. 194 organisms have s u r v i v e d t h a t t hey were the f i t t e s t , o r those b e s t adapted t o the c o n d i t i o n s o f l i f e . 7.3.1 A C r i t i c a l A p p r a i s a l o f Popper's Arguments L e t us ta k e a c l o s e r l o o k a t the Mars example. The g i s t o f t h e example, as I u n d e r s t a n d Popper, i s t h a t Darwinism seems t o a s s e r t t h a t i f on Mars we f i n d l i f e w h ich s a t i s f i e s c o n d i t i o n s (2a) and ( 2 b ) , then (2c) w i l l come i n t o p l a y and b r i n g about i n time a r i c h v a r i e t y o f d i s t i n c t forms. But Popper does n o t t h i n k Darwinism a s s e r t s as much as t h i s . I n h i s o p i n i o n , Darwinism would not r e g a r d i t s e l f as r e f u t e d even i f we c o u l d o n l y f i n d on Mars t h r e e s p e c i e s o f b a c t e r i a w i t h a g e n e t i c o u t f i t s i m i l a r t o t h a t o f t e r r e s t r i a l s p e c i e s . A D a r w i n i a n , Popper m a i n t a i n s , would argue t h a t t h e s e t h r e e s p e c i e s were the o n l y forms among t h e many mutants w h i c h were s u f f i c i e n t l y w e l l a d j u s t e d t o s u r v i v e . And i t won't h e l p e i t h e r t o say t h a t e v o l u t i o n a r y t h e o r y can p r e d i c t the e v o l u t i o n o f v a r i e t y under ' f a v o u r a b l e c o n d i t i o n s ' ; f o r i t i s h a r d l y p o s s i b l e t o d e s c r i b e i n g e n e r a l terms what t h e s e f a v o u r a b l e c o n d i t i o n s a r e — e x c e p t t h a t , i n t h e i r p r e s e n c e , a v a r i e t y o f forms w i l l emerge. In s h o r t , I don't agree w i t h Popper's i n t e r p r e t a t i o n o f e v o l u t i o n a r y t h e o r y a t a l l . I n my o p i n i o n , e v o l u t i o n a r y t h e o r y can and does i n d e e d p r e d i c t the e v o l u t i o n o f v a r i e t y under ' f a v o u r a b l e c o n d i t i o n s ' ; and i t i s p o s s i b l e t o d e s c r i b e i n g e n e r a l terms what t h e s e ' f a v o u r a b l e c o n d i t i o n s ' a r e , 1 P o p p e r (1973), pp. 241-242. 195 independent of the f a c t t h a t i n t h e i r presence a v a r i e t y of forms w i l l emerge. Let us assume, f o r i n s t a n c e , we have knowledge t h a t a c e r t a i n environment has constant droughts and f l o o d s a l t e r n a t i n g f o r c e r t a i n p e r i o d s of time. Under these 'unfavourable c o n d i t i o n s , ' then s p e c i e s of animals which can only s u r v i v e on land or i n water would not be able to s u r v i v e . In view of these c o n d i t i o n s , e v o l u t i o n a r y theory would of course not p r e d i c t the e v o l u t i o n of a v a r i e t y of forms; f o r only those s p e c i e s which are g e n e t i c a l l y adapted to l i v e both on l a n d and i n water would be able to s u r v i v e . But i f environmental changes are i n g e n e r a l f a v o u r a b l e to the s u r v i v a l of both land and marine s p e c i e s , then e v o l u t i o n a r y theory would p r e d i c t the e v o l u t i o n of a v a r i e t y of forms. In g e n e r a l , I maintain t h a t we can independently determine whether changes i n the environmental c o n d i t i o n s are f a v o u r a b l e to the e v o l u t i o n of a v a r i e t y of forms or not without having t o r e f e r to the r e s u l t whether these v a r i e t i e s of forms d i d come about or not. Now l e t us go back to the Mars example. Assume t h a t environmental c o n d i t i o n s on Mars, as f a r as we can observe, are not favourable to the s u r v i v a l of most t e r r e s t r i a l s p e c i e s of b a c t e r i a . Taking t h i s i n t o c o n s i d e r a t i o n , e v o l u t i o n a r y theory would not p r e d i c t a v a r i e t y of b a c t e r i a l s p e c i e s w i t h a g e n e t i c o u t f i t s i m i l a r to t h a t of t e r r e s t r i a l s p e c i e s . Thus, i f we c o u l d only f i n d three such b a c t e r i a l s p e c i e s on Mars, i t i s completely w i t h i n our e x p e c t a t i o n and Darwinism i s not r e f u t e d 196 at a l l . But assume now t h a t environmental c o n d i t i o n s on Mars are, to our knowledge, favourable to the s u r v i v a l of most t e r r e s t r i a l s p e c i e s o f b a c t e r i a , then e v o l u t i o n a r y theory would indeed p r e d i c t the coming-into-existence of a v a r i e t y of s p e c i e s o f b a c t e r i a with a g e n e t i c o u t f i t s i m i l a r t o t h a t of t e r r e s t r i a l s p e c i e s . Yet i f we c o u l d only f i n d three or even l e s s such s p e c i e s o f b a c t e r i a on Mars, i s Darwinism thereby r e f u t e d ? There are two p o s s i b l e answers to t h i s q u e s t i o n . F i r s t , on o b s e r v i n g t h a t there are only three s p e c i e s o f b a c t e r i a on Mars, we might t r y to f i n d out why such i s the case. Suppose we d i s c o v e r t h a t Mars has a high d e n s i t y of gamma-radiation , which k i l l s most b a c t e r i a l s p e c i e s except the three which are found to be immune to gamma-radiation. I f t h i s i s the case, then we would have to conclude t h a t our i n i t i a l p r e d i c t i o n of a v a r i e t y of b a c t e r i a l s p e c i e s was mistaken as we had not i n c l u d e d the gamma-radiation f a c t o r i n c o n s i d e r a t i o n . I f we had known t h i s a d d i t i o n a l i n f o r m a t i o n beforehand, we would not have p r e d i c t e d the e v o l u t i o n of a v a r i e t y of b a c t e r i a l s p e c i e s with a g e n e t i c o u t f i t s i m i l a r to t h a t of t e r r e s t r i a l s p e c i e s . Thus, although our i n i t i a l p r e d i c t i o n was mistaken as i t had not taken the r e l e v a n t gamma-radiation f a c t o r i n t o c o n s i d e r a t i o n , Darwinism i t s e l f i s not r e f u t e d . But what i f we c o u l d not f i n d any a d d i t i o n a l i n f o r m a t i o n conducive to the emergence of only three s p e c i e s o f b a c t e r i a on Mars when c o n d i t i o n s are appa r e n t l y favourable t o the coming-into-being of a v a r i e t y of b a c t e r i a l s p e c i e s ? I f t h a t i s the 197 case, then we would have no choice but to admit t h a t Darwinism i s r e f u t e d . And f o r the very f a c t t h a t Darwinism can be r e f u t e d , i t i s , by Popper's own c r i t e r i o n o f f a l s i f i a b i l i t y , a g e n u i n e l y t e s t a b l e s c i e n t i f i c theory. As a r e s u l t , Darwinism i s not a met a p h y s i c a l r e s e a r c h programme as Popper h o l d s i t to be. 19 8 CHAPTER EIGHT C o n c l u s i o n 8.1 F i r s t S u b - T h e s i s — E v e r y Adequate E x p l a n a t i o n  i s a P o t e n t i a l P r e d i c t i o n I t h i n k I have shown t h a t t h e p a r e s i s c a s e and c a s e s s i m i l a r t o i t do not c o n s t i t u t e genuine c o u n t e r e x a m p l e s t o the f i r s t s u b - t h e s i s o f t h e symmetry t h e s i s . F o r t h e s e c a s e s can be t r e a t e d as i n c o m p l e t e D-N e x p l a n a t i o n s 1 i f _ we have good r e a s o n s t o b e l i e v e t h a t f u r t h e r i n v e s t i g a t i o n would p r o v i d e t h e means t o t e l l us why, f o r i n s t a n c e , some u n t r e a t e d l a t e n t s y p h i l i t i c s d e v e l o p p a r e s i s w h i l e o t h e r s do n o t . T h i s i s b e s t i l l u s t r a t e d by the c a s e o f r e c o v e r y from s t r e p t o c o c c u s i n f e c t i o n upon t r e a t m e n t by p e n i c i l l i n — a case t h a t Hempel h i m s e l f r e g a r d s as a t y p i c a l example o f an I-S e x p l a n a t i o n , f o r i t i s now p o s s i b l e i n p r i n c i p l e t o o f f e r D-N e x p l a n a t i o n s o f such c u r e s as t h e n a t u r e o f b a c t e r i a l r e s i s t a n c e t o p e n i c i l l i n seems now t o be u n d e r s t o o d . 2 I n so a r g u i n g , however, I am not c l a i m i n g t h a t " a l l -'-See Hempel ( 1 9 6 5 c ) , s e c . 4.2 f o r a d i s c u s s i o n o f t h e d i f f e r e n t k i n d s o f i n c o m p l e t e e x p l a n a t i o n s . T h i s i l l u s t r a t i o n i s borrowed from Salmon (1977a), n. 15, p. 164. 199 i n d u c t i v e - s t a t i s t i c a l e x p l a n a t i o n s are i n c o m p l e t e " 1 — a charge m i s t a k e n t l y r a i s e d a g a i n s t Hempel by Salmon, f o r Hempel has never s u b s c r i b e d to such a c l a i m . 2 What I a c t u a l l y m a i n t a i n i s t h a t i n cases where we do have good grounds t o b e l i e v e t h a t f u r t h e r i n v e s t i g a t i o n and r e s e a r c h would p r o v i d e the m i s s i n g l i n k s r e q u i r e d f o r a D-N e x p l a n a t i o n , then we can t r e a t these cases as incomplete D-N e x p l a n a t i o n s . Once f i t t e d i n t o the D-N schema, these cases would no longer c o n s t i t u t e counterexamples to the f i r s t s u b - t h e s i s which i s t r i v i a l l y t r u e f o r D-N e x p l a n a t i o n s . But what i f c r i t i c s l i k e S c r i v e n argue t h a t the p a r e s i s case, and e v o l u t i o n a r y theory i n p a r t i c u l a r , 3 are i n h e r e n t l y s t a t i s t i c a l and t h e r e f o r e do not f i t the D-N schema as incomplete D-N e x p l a n a t i o n s ? Would these cases now be harmful to the f i r s t s u b - t h e s i s ? I t h i n k not, as they w i l l now f a l l i n t o the scope of my proposed m o d i f i e d I-S model of e x p l a n a t i o n i n which the f i r s t s u b - t h e s i s i s kept i n t a c t , f o r to e x p l a i n an event i n t h i s model i s to p r o v i d e the b e s t p o s s i b l e grounds we c o u l d have had f o r making p r e d i c t i o n s concerning i t . Salmon expresses t h i s i d e a b e a u t i f u l l y : To e x p l a i n an event i s to show to what degree i t was to be expected, and t h i s degree may be t r a n s l a t e d i n t o p r a c t i c a l p r e d i c t i v e behavior such as wagering on i t . In some cases the e x p l a n a t i o n w i l l show t h a t the explanandum event was not to be expected, but t h a t does Salmon (1977a), p. 155. 2See Hempel (1965c), p. 417. 3 I n f a c t , as I have argued i n sec. 7.2.2.2, e v o l u t i o n a r y theory should be t r e a t e d as a s t a t i s t i c a l t heory. 200 not destroy the symmetry of explanation and prediction. The symmetry consists i n the fact that the explanatory facts constitute the f u l l e s t possible basis for making a prediction of whether or not the event would occur. To explain an event i s not to predict i t ex post facto, but a complete explanation does provide complete grounds for r a t i o n a l prediction concerning that event. 1 isalmon (1971), p. 79. 201 8•2 Second Sub-Thesis--Every Adequate P r e d i c t i o n  i s a P o t e n t i a l E x p l a n a t i o n The s t r e n g t h of the barometer case (and cases s i m i l a r to i t ) as a counterexample to the second s u b - t h e s i s l i e s i n the c l a i m t h a t p r e d i c t i o n s can be made on the b a s i s of i n d i c a t o r laws, but subsumption under these laws does not p r o v i d e any e x p l a n a t i o n whatsoever. Such a c l a i m , however, i s untenable, f o r I have shown t h a t e x p l a n a t i o n s i n v o k i n g i n d i c a t o r laws are g e n u i n e l y e x p l a n a t o r y a f t e r a l l . Thus, the second s u b - t h e s i s i s not undercut by the barometer case and cases s i m i l a r to i t . But the same cannot be s a i d of S c h e f f l e r ' s counterexample which i n v o l v e p r e d i c t i v e i n f e r e n c e s from observed f i n i t e sample to unobserved f i n i t e samples without i n v o k i n g any k i n d of laws at a l l . These cases do seem to pr e s e n t problems f o r the second s u b - t h e s i s . Yet, as Hempel h i m s e l f admits, t h e r e i s a l i m i t to the scope of h i s two covering-law models; t h a t they "represent the l o g i c a l s t r u c t u r e of two important types of p r e d i c t i v e i n f e r e n c e i n e m p i r i c a l s c i e n c e , but not t h a t these two are the o n l y t y p e s . " 1 Thus, S c h e f f l e r ' s cases do not present counterexamples to the second s u b - t h e s i s s i n c e they i n v o l v e p r e d i c t i v e i n f e r e n c e s t h a t f a l l o u t s i d e the scope of Hempel's covering-law models. 1Hempel (1963) , p. 119. 202 8.3 Summary In sum, I have shown t h a t t h e r e i s i n d e e d a symmetry between e x p l a n a t i o n and p r e d i c t i o n , though the form the symmetry t a k e s between D-N e x p l a n a t i o n s and p r e d i c t i o n s i s not the same as the form i t e x h i b i t s between I-S e x p l a n a t i o n s and p r e d i c t i o n s . T h i s c o n c l u s i o n i s s u b s t a n t i a l l y d i f f e r e n t from Hempel's o r i g i n a l e x p o s i t i o n o f t h e symmetry t h e s i s w h i c h makes no d i s t i n c t i o n between the two forms the symmetry t a k e s . 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