Scientific Explanation and the Causal Structure of the World - Wesley C. Salmon(Cut)

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Scie11tific Explanati and the Causal Structure

Princeton University Press Princeton, New Jersey

Copyright © 1984 by Princeton University Press Published by Princeton University Press. 41 William Street, Princeton, New Jersey 08540 In the United KinRdom: Princeton University Press, Guildford, Surrey All Rights Reserved Library of Congress Cataloging in Publication Data ,,·ill be found on the last printed page of this book ISBN (cloth) 0-691-07293-0 (LPE) 0-691-10170-1 This book has been composed in Linotron Times Roman Clothbound editions of Princeton University Press books are printed on acid-free paper. and binding materials are chosen fnr strength and durability. Paperbacks, although satisfactory for personal collections, are not usually suitable for librmy rehinding Printed in the United States of America by Princeton University Press, Princeton, New Jersey

To

Car) G. Hempel Philosophical Master aka

Peter Hempel Colleague and Friend

Contents

PREFACE ACKNOWLEDGMENTS

I.

Scientific Explanation: Three General Conceptions

2.

3.

4.

X Ill

3

4

Explanation versus Description Other Types of Explanation Laplacian Explanation Three Basic Conceptions An Outline of Strategy

21

Statistical Explanation and Its Models

24

Inductive-Statistical Explanation The Statistical-Relevance Approach

%

Objective Homogeneity

48

Epistemic Relativization Randomness Homogeneity Some Philosophical Applications Some Philosophical Reflections

48 55 60

The Three Conceptions Revisited

84

The Epistemic Conception The Modal Conception The Ontic Conception How These Concertions Answer a Fundamental Question Conclusions

5.

ix

9 11 15

27

72

76

84 Ill

121 124 133

Causal Con nec: tion s

135

Basic Problems Two Basic Concepts Processes The 'At-At' Theory of Causal Propagation

135 137 139

147

viii

J

Contents

Causal Forks and Common Cause~ Conjunctive Forks Interactive Forks Relations between Conjunctive and Interactive Porks Perfect Forks The Causal Structure of the World Concluding Remarks

158 158 168 174

7.

Probabilistic Causality The Sufficiency/Necessity View Statistical Relevance and Probabilistic Causality Causality and Positive Relevance Causal Processes and Propensities

184 185 190 192 202

8.

Theoretical Explanation Causal Connections and Common Causes Explanatory versus Inferential Principles The Common Cause Principle and Molecular Reality The Explanatory Power of Theories Empiricism and Realism

206 207 211 213 227 229

9.

The Mechanical Philosophy

239 239 242 259 267 276

6.

Logic versus Mechanisms Explanation in Quantum Mechanics Explanation and Understanding The Causal/Mechanical Model The Final Contrast BIBLIOGRAPHY INDEX

i

177

178 182

280 295

Preface

ALTHOUGH there is no doubt in my mind that scientific understanding of the world and what transpires within it represents a marvelous intellectual achievement, the chief purpose of this book is not to laud scientific understanding, but, rather. to try to get clear on what it is. Our aim is to understand scientific understanding. We sec ure scientific understanding by providing scientific explanations ; thus our main concern will be with the nature of scientific explanation. The attempt to find an adequate characterizati0n of scientific explanation is a problem that has perplexed scientists and philosophers for several millennia. As we shall see in chapter I. its roots go back at least to Greek antiquity, and it continues to be an object of considerable attention today. There are, it seems to me, three basic conceptions that have figured significantly in the discussions for twenty-five hundred years. We shall examine them in some detail. One particular view has played a dominant role, at least since the middle of the twentieth century, in the philosophical literature on scientific explanation. It has often been called-somewhat inappropriately I thinkthe "covering law model. " Its crucial tenet is that explanations are arguments. Another view, which has enjoyed some philosophical support, has a good deal of common-sense appeal. It takes scientific explanations to be essentially causal. A major aim of our investigations will be to draw a sharp contrast between these two conceptions . A third conception, which inseparably associates explanation with necessitation , will be judged scientifically anachronistic . It will be my contention that the ' received view' (the inferential conception) is infected with severe diniculties that. in the end, render it untenable . The causal conception also suffers from serious problemsnamely, philosophical perplexities c0nceming the nature of causality- hut l believe they can be overcome. This can be done only if we are able to provide an adequate philosophical treatment of fundamental causal concepts, as [ attempt to do in chapters 5-· 7. The distinction between these two conception s of scientific explanation is by no means trivial or merely verbal. As l indicate in the final section of the last chapter, the transition from the one conception to the other demands nothing less than a radical gestalt switch in our thought about

x

I Preface

scien tific explanation. I shall, of course, attempt to provide stron g phililci lljillira ll rllrlil 'fllinn frrr 11111lr in g lllf' shifl .

Alth ough much modern work on scientific explauatlon has been rather formal and technical--often treating various quasi-formal 'models' in great detail-! shall dwell extensively upon less formal considerations. There are two reasons for this emphasis. In the first place, I have been convinced for some time that many recent philosophical discussions of scientific exp lanation suffer from a lack of what Rudolf Carnap called ''clarification of the explicandum.'' As Camap has vividly shown, precise philosophical exp lications of important concepts can egregiously miss the mark if we do not have a sound prior informal grasp of the concept we are endeavoring to explicate. It is impossible, I think, to read chapters 1, 2, and 4 of Camap's clatsic, Logical Foundations of Probability (1950), without seeing the profound importance of such preliminary clarifications . Because of my · strong conviction that the concept of scien tific explanation needs similar elucidation, chapters I, 4, and 9 of thi s book arc large ly devoted to this task. In the second place, I have earnestly attempted to make the discussion accessible to a wide group of readers-philosophers, scientists, and other individuals who have serious interests in science or philosophy . Every effort has been made to illustrate the philosophical points with examples that have either historical or contemporary significance from a wide range of scientific disciplines . At the same time, a number of examples have deliberately been drawn from such sources as Scientific American and Science, for I hope this book will be intelligible to those who find such literature rewarding. No particular specialized knowledge of any scientific di scipline is presupposed . In addition, I have tried to supply enough philosophical background to obviate any need for prior familiarity with the philosophical literature on scientific explanation . It is my special hope that some parts of this book--for example, the discussions of causality-will be useful to philosophers who work in areas outside of philosophy of science. I do not mean to suggest that this book is easy; it is not. For the most part, however, it should be comprehensible to intelligent readers, more or less regardless of backgrou nd , who are willing to expend some effort upon it. Unavoidably, some parts are rather technical- in particular, the details of the S-R basis in chapter 2 and the treatment of randomness and objective homogeneity in chapter 3. These can be skimmed or omitted on first reading by those who are not interested in such techn icali ties for their own sake. Th is book had its real beginnings when , around 197~. I began to recognize the serious shortcomings and limitations of the statis tical -relevance

Prerace 1

x1

model or scientific ex pl anation-as propounded in (Sa lmon et al ., 1971 )-and the need to come to grips with th e explanatory force of scientific thcoric!i. At hi ~ kind in vitation, I promi~ed to con tribu te a paper entitled "Theoretical Explanation" to a sy mposium on explanation organized by Stephan Komer at Bristol; the proceedings are published in (Korner. 1975). As sometimes happens when the title is written before the paper, the content of the paper did not fit the title very we ll ; all of my efforts to deal with theories got bogged down in problems of causality. As chapters 5-R of the present book will show, I now believe that causal explanation is the key to theoretical explanation. No adequate account of theoretical explanation could be developed withou t extensive work on causality itself. The actual writing of the book began during a visit to the Department of History and Philosophy of Science at the University of Melbourne , where I was invited to give a seminar on scientific explanation in 1978 . Suffice it to say that every chapter bears the marks. explicitly or implicitly, of stimulating interactions with Australian philosophers . I am indeed grateful for the personal and intellectual hospitality accorded me durin g a splendid three-month visit. There are several individuals to whom 1 owe special thanks for substantive help in developing the ideas in this book. Nancy Cartwright pointed out a serious flaw in an earlier attempt to explicate the propagation of causal influence , and stimulated me to improve my account. Alberto Coffa made me see the crucial role of epistcmic relativization in Car I G. Hempel 's theory of inductive explanation, and made me realize the indispensable character of objective homogeneity for the type of theory I was attempting to develop. Clark Glymour called to my attention the epoch-making work of Jean Perrin on Avogadro's number and directed me to Mary Jo Nye's superb account of that work (1972). Paul Humphreys offered substantial aid in dealing with objective homogeneity, probabilistic causality, and propensities . Patrick Maher and Richard Otte noticed a fatal flaw in an earlier explication of causal interaction and forced me to correct that account. Otte also provided valuable critiques of various aspects of probabilistic causality. Hugh Melior, more than anyone else, led me to appreciate the importance of clarifying the explicandum in discussions of scientific explanation . Bas van Fraassen made me realize that there must be two kinds of causal forks, conjun ctive and interactive. Philip van Bretzel first called to my attention the direct relationship between causal forks and causal interactions . 1 cannot beg in to detail the ways in whi ch my ideas on scientific explanation have been influenced by the wri tings of Bertrand Russcll--especially The Anaf.ysis of Matter ( 1927), Mysticism and Logic (1929). and Human Knowled~e. Its Scope anrl Limits (1948)--as well as Hans Rei -

xii

I Preface

chenbach ' s posthumous work, The Direction of Time (1956). Although neither of these authors dealt explicitly with scientific explanation at any length, both provided gold mines of material directly relevant to the subject. My main intellectual debt-like that of anyone who has worked on philosophical theories of scientific explanation in recent years-is to Car! G. Hempel, who has taught all of us how to think clearly and deeply about the subject. If we have failed to learn the lesson , the fault is not his. Although I disagree on many points with his views, his method of philosophizing seems to me peerless, and I can think of no more worthy goal than to try to emulate it. Many of the ideas discussed in this book were published in preliminary fom1 in various artic les that have appeared since 1975. Specific citations will be found under Acknow ledgments.

Acknowledgments

I AM GRATEFUL to Scientific American for permission to use the excerpts from "Quantum Theory and Reality'' by Bemard d 'Espagnat, Copyright © 1979 by Scientific American, Inc . These excerpts appear in chapters 1 and 9; specific page references are given at the end of each quotation . I shou ld like to express my sincere appreciation to the following publishers for permission to adapt material from my previously published articles for use in this book: Portions of the material in chapters 1 and 4 were included , in a much abbreviated fonn, in "Comets, Pollen, and Dreams: Some Reflection s on Scientific Explanation,'' in Robert McLaughlin, ed., What? Where? When? Why?, pp. 155- 178. Copyright© 1982 by D. Rei del Publishing Company, Dordrecht, Holland . The section "Epistemic Relativization" in chapter 3 is adapted from "Indeterminism and Epistemic Relativization," Philosophy of Science 44, pp . I 99-202. Copyright © J977 by the Philosophy of Science Association. East Lansing, Michigan. The sections "Randomness" and "Homogenei ty" in chapter 3 are drastically revised versions of material that appeared in "Objectively Homogeneous Reference Classes," Synthese 36, pp. 399--414. Copyright© 1977 by D. Rei del Publishing Company, Dordrecht, Holland. The section ''The Epistemic Conception'' in chapter 4 includes material adapted from'' A Third Dogma of Empiricism.'' in Robert Butts and Jaakko Hintikka, eds., Basic Problems in Methodology and Linguistics, pp. 149166. Copyright© 1977 by D. Reidel Publishing Company, Dorclrecht. It also contains material from "Hempel's Conception of Inducti ve Inference in Inductive-Statistical Explanation,'' Philosophy nf Science 44, pp. 180185. Copyright © 1977 by the Philosophy of Science Association. East Lansing, Michigan. Some of the basic ideas in chapters 5 and 6 were presented in a preliminary and much-condensed version in "Causality : Production and Propagation," in Peter Asquith and Ronalct N. Giere, eels., PSA 1980, pp. 4969. Copyright © 1982 by the Philosophy of Science Association, East Lansing, Michigan. The section "The 'At-At' Theory of Causal Propagation" in chapter 5 is an adaptation of "An 'At-At' Theory of Causal

· xiv

I Acknowledgments

Influence," Philosophy of Science 44, pp. 215- 219 . Copyright© 1977 by the Philosophy of Science Association, East Lansing, Michigan . Most of the material in chapter 7 was drawn from "Probabilistic Causality," Pacific Philosophical Quarterly 61, pp. 50-74. Copyright© 1980 by the University of Southern California. • Some of the material in chapters 8 and 9 was anticipated in a much less complete fonn in "Further Reflections," in Robe1t McL1ughlin, ed., What? Where? When? Why ?, pp. 231 --280 (especially pp. 260- 278) . Copyright © 1982 by D. Rei del Publishing Company, Dordrecht, Holland . I should like to express special thanks to the University of Melbourne for providing me with a research appointment in September through November 1978, and to the National Science Foundation for support of my research on causality and scientific explanation. This material is based upon work supported by the National Science Foundation under Grant No. GS-42056 and Grant No . SOC-7809146. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation.

Scientific Explanation and the Causal Structure of the World

1

Scientific Explanation: Three General Conceptions

MoDERN SCIENCE provides us with extensive knowledge about the world in which we live . We know that our universe originated in a 'hig bang· several thousand million years ago, and that it has been expanding ever since. We know that th ere was a fairly severe drought in northeastern Arizona during the last few years of the fourteenth century A. D. , and that large settlements were abandoned at that time . We know that Halley's comet moves in a roughly elliptical orbit, and that it will return to perihelion in 1986. We know that-barring nuclear holocaust-the human population of the earth will continue to increase for some time to come. We know that the planet Uranus has rings, that E. coli live in the norn1al human intestinal tract, that copper is an electrical conductor, and that the surface temperature of Venus is high . As these examples show, science provides knowledge of what has happened in the past, what will happen in the future, and what is happening now in regions that we are not observing at the present moment. It encompasses knowledge of both particular facts and general regularities. In none of these instances, of course, is our scientific knowledge to be regarded as certain or incorrigible; nevertheless, the physical, biological, and social sciences furnish impressive bodies of knowledge about what goes on in the world, and we have every reaso11 to believe that these sciences will continue to grow at a prodigious rate. Such knowledge- valuable as it may be for intellectual or practical purposes-is not fully satisfy ing . Not only do we desire to know what happens; we also want to understand why. Moreover, it is widely acknowledged today that sc ience can provide explanations of natural phenomena: indeed, to many philosophers and scient ists, this is the primary goal of scientific activity . Scientific explanations can be given for such particular occurrences as the appearance of Hallcy's cornet in 1759 or the crash of a DC-10 jet airliner in Chicago in 1979, as well as such general features of the world as the nearly el lipti cal orbits of planets or the electrical conductivity of copper. The chief aim of this book is to try to discover just what scientific understanding of this sort consists in. Before undertaking the ta~k at hand. it may be u~eful to make a remark about the basic strategy. During the last thirty-six years (since I 94R). a

4

I Scientific Exp lanation



number of quasi-forma l 'models' of scientific explanation have appeared in the philosophical literature , and a good deal of attention has been devoted to the in ves tigation of their formal properties. Quite a few are such familiar frie nds that philosophers often refer to them merely by their initials . We sha ll meet many of tt;em in the ensuing chapters: deductive-nomological (D-N), deductive-statistical (D-S), inductive-statistical (J-S), statisticalrelevance (S-R), decluctive-nomologic
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