NIELS BOHR AND CONTEMPORARY PHILOSOPHY BOSTON STUDIES IN THE PHILOSOPHY OF SCIENCE

Editor

ROBERT S. COHEN, Boston University

Editorial Advisory Board

THOMAS F. GLICK, Boston University ADOLF GRUNBAUM, University ofPittsburgh SAHOTRA SARKAR, Dibner Institute M.l. T. SYLVAN S. SCHWEBER, Brandeis University JOHN J. STACHEL, Boston University MARX W. WARTOFSKY, Baruch College of the City University ofNew York

VOLUME 153 NIELS BOHR AND CONTEMPORARY PHILOSOPHY

Edited by JAN FAYE Carlsberg Foundation, Copenhagen , Denmark

and

HENRY J. FOLSE Department ofPhilosophy, Loyola University, New Orleans, U. S. A.

...,~

Springer-Science+Business Media, B.Y. Library of Congress Cataloging-in-Publication Data

Niels Bohr and contemporary phIlosophy / edited by Jan Faye and Henry J. Folse. p. cm . -- (Boston studIes in the phi losophy of scIence; v. 153) Includes bIblIographIcal references and index.

1. Physlcs--Phl10sophy. 2 . PhIlosophy. Modern. I . Faye. Jan . II. Folse. Henry J .• 1945- III. Series. [DNLM : 1. Bohr. Niels Henrik DaVid. 1885-1962--Views on phIlosophy.] aC16.B65N493 1993 530' .01--dc20 DLC for Library of Congress 93-24825

ISBN 978-90-481-4299-6 ISBN 978-94-015-8106-6 (eBook) DOI 10.1007/978-94-015-8106-6

Printed on acid-free paper

All Rights Reserved © 1994 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1994. Softcover reprint ofthe hardcover 1st edition 1994 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechancial, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. TABLE OF CONTENTS

Preface Xl Introduction xiii

MARA BELLER & ARTHUR FINE / Bohr's Response to EPR I 1. EPR and Bohr's EPR 2 2. Incompleteness and Inconsistency 3 3. Simultaneous Position and Momentum in EPR 6 4. Bohr's Concept of Disturbance: EPR and Before 10 5. Ambiguity and Definition 16 6. Positivism and Its Puzzles 18 7. Locality and Separability 23 8. Concluding Remarks 27

CATHERINE CHEVALLEY / Niels Bohr's Words and the Atlantis of Kantianism 33 1. Introduction 33 2. Anschauung and Symbol- In Bohr's Interpretation of Quantum Mechanics 35 3. Anschauung and Symbol - The Philosophical Background 43 4. Conclusions 50 4.1. Original Interpretations of Quantum Mechanics 50 4.2. Bohr's Conception of Language 51 4.3. Changing Perspectives 53

JAMES T. CUSHING / A Bohmian Response to Bohr's Complementarity 57 1. Introduction 57 2. The Project of "Clarifying" Bohr's Views 58 3. Bohr's Complementarity 61 4. Bohmian Mechanics 63 4.1. Bohm's (1952) Theory 64 4.2. Recent Developments 70 5. Conclusions 72

v VI TABLE OF CONTENTS

DAVID FAVRHOLDT / Niels Bohr and Realism 77 I. Classical Concepts and Ordinary Language 77 2. What We Can Say About Reality 82 3. Subjective Idealism and Phenomenalism 84 4. A 'God's Eye View' of the World 86 5. Conclusion 94

JAN FAYE / Non-Locality or Non-Separability? A Defense of Bohr's Anti-Realist Approach to Quantum Mechanics 97 1. The Bohr-Einstein Debate in Retrospect 97 2. Non-Separability Anti-Realism 103 3. Non-Separability Realism 108 4. Non-Locality Realism 110 5. Non-Locality Anti-Realism 114

HENRY FOLSE / Bohr's Framework of Complementarity and the Realism Debate 119 I. Drawing the Battlelines: Bohr and Kant 120 2. Realism and the Atomic Description of Nature 123 3. Realism and Truth 127 4. Complementarity and the Realist Ideal of Understanding 134 5. Conclusion 137

JOHN HONNER / Description and Deconstruction: Niels Bohr and Modem Philosophy 141 I. Preamble 141 2. Bohr and the Philosophers 144 3. Derrida and Deconstruction 148 4. Bohr and the Description of Nature 151

CLIFFORD A. HOOKER / Bohr and the Crisis of Empirical Intelligibility: An Essay on the Depth of Bohr's Thought and Our Philosophical Ignorance 155 Part I. Bohr and the Kantian Legacy 155 1. Introduction 155 2. Reichenbach on Kant and Relativity Theory 158 Part II. Uniqueness and Rational Methodology: Newton and Kant 163 3. Kant, Newton and Rational Science 163 Part III. Bohr on Quantum Theory and Epistemology 174 TABLE OF CONTENTS VII

4. Bohr's Philosophical Lesson of Quantum Mechanics 174 5. Reichenbach and Bohr 178 6. Bohr's Conception of Intelligibility, Objectivity and Completeness 180 7. Einstein against Bohr 182 8. Bohr and Einstein versus Nature 185 9. Principled Ignorance, Adventures of Ideas and the Open Future 186 Appendix 1. Butts and Friedman on Kant's General Epistemological Framework 188 A. Overall Procedure 188 B. First Inference, to Metaphysical Principles of Pure Natural Science 189 C. Second Inference, to the Law of Universal Gravitation 194

DON HOWARD / What Makes a Classical Concept Classical? Toward a Reconstruction of Niels Bohr's Philosophy of Physics 201 1. Introduction 201 2. Objectivity and Unambiguous Description. Why Are Classical Concepts Important? 204 3. Instruments and Objects of Investigation. Where and How Are Classical Concepts to be Employed? 210 4. Of Mixtures and Pure Cases. What Makes a Classical Description Classical? 217 5. Does the Reconstruction Work? 223

PAUL HOYNINGEN-HUENE / Niels Bohr's Argument for the Irreducibility of Biology to Physics 231 1. Introduction 231 2. The Anti-Reductionist Claim 235 2.1. Explication of Concepts 236 2.2. The Relation of Bohr's Claim to Other Forms of Anti-Reductionism 238 3. The Argument for the Anti-Reductionist Claim 240 3.1. Bohr's Argument: An Analogical Inference 240 3.2. First Premise: Complementarity in Physics 241 3.3. Second Premise: Complementarity of Physics and Biology 249 3.4. Formal Reconstruction of Bohr's Argument 251 4. Critique of Bohr's Anti-Reductionist Argument 252 Vlll T ABLE OF CONTENTS

DAVID KAISER / Niels Bohr's Conceptual Legacy in Contemporary Particle Physics 257 1. Introduction 257 2.. Bohr and Particle Physics: A Brief History 258 3. The Compound Nucleus and Particle Physics Phenomenology 259 4. Questions of Ontology and Particle Physics Phenomenology 262 5. Bohr's Realism and Particle Physics 264 6. Conclusions 266

HENRY KRIPS / A Critique Of Bohr's Local Realism 269 I. Introduction 269 2. Instrumentalism 269 3. Bohr's Philosophy 270 4. Bohr and Local Realism 271 5. Critique ofLocal Realism 273

EDWARD MACKINNON / Bohr and the Realism Debates 279 I. Perspectives and Presuppositions 280 2. Coping with A Linguistic Crisis 282 3. Interpreting Quantum Mechanics 286 4. Realism in Perspective 290 4.1. Einsteinian Realism 291 4.2. Scientific Realism 293 5. Realism and Analysis 297

DUGALD MURDOCH / The Bohr-Einstein Dispute 303 1. Einstein's Opposition to the Copenhagen Interpretation 303 2. The EPR Argument 305 3. Bohr 's Reply to the EPR Paper 306 4. Einstein's Argument 308 5. Bohr's Reponse to Einstein's Argument 311 6. The Philosophical Background to Einstein's Argument 315 7. The Dispute in 1935, and Thirty Years On 318 8. The Physical Dispute Reconsidered 322

ULRICH ROSEBERG / Hidden Historicity: The Challenge of Bohr 's Philosophical Thought 325 1. The Problem 325 2. Reichenbach's Rational Reconstruction of the Development of Quantum Mechanics 327 T ABLE OF CONTENTS IX

3. A Rational Reconstruction of the Development of Quantum Mechanics in the Dialectic Tradition 329 3.1. Bohr's Research Program 329 3.2. A Physicist Becomes a Philosopher 332 4. Hidden Historicity in Bohr's Epistemological Reflections 337 5. Concluding Remarks 340

HENRY P. STAPP / Quantum Theory and the Place of Mind in Nature 345 1. Mind in the Physical Sciences 345 2. The Objective Wave-Function 346 3. Integrating Consciousness into Physical Science 347 4. Future Prospects for the Copenhagen Interpretation 349

References 353

Name Index 373 PREFACE

Since the Niels Bohr centenary of 1985 there has been an astonishing surge of publications on Bohr's philosophy. These contributions have appeared in a wide variety of different sources. While other volumes have collected a variety of essays on the many aspects of Bohr's work, hitherto there has been no col­ lection bringing the diversity of new philosophical interpretations between the covers of a single volume. Therefore, in this collection we have invited seven­ teen of today's best known authors who have helped shape this new round of discussions on Bohr's philosophy to address the question of Bohr's relation to issues currently discussed in contemporary philosophy of science. The sixteen previously unpublished papers included here reveal a surpris­ ing variety of different facets of Bohr as the natural philosopher whose ideas of complementarity shaped the final phase of the quantum revolution and influenced two generations of the century's leading physicists. Many of the questions discussed bear on the very active philosophical arena of realism versus anti-realism and the implications of the work stemming from the seminal contributions of John Bell. While our primary focus has been philo­ sophical, also discussed are important historical questions relating Bohr to Kant, neo-Kantians , and positivists. There is much on which the authors included here agree; but there are also polar disagreements, thus affording the reader an opportunity to compare and contrast new interpretations of Bohr as a philosopher. Indeed, the variety of differing opinions revealed in these papers assure us that the philosophical questions revolving around Bohr's "new viewpoint" will continue to be a subject of scholarly interest and discussion for years to come. It is our hope that this collection will interest all serious students of history and philosophy of science, as well as those readers interested in the foundations of physics and the philosophical implications of the quantum revolution. We would like to take this opportunity to acknowledge our appreciation to all of our contributors, as well as to Bob Cohen. General Editor of the Boston Studies series, and to Annie Kuipers, Acquisitions Editor of Kluwer Academic Publishers, for their advice and encouragement in helping us to bring out this collection. Our thanks are also do to Loyola University for financial assistance in preparing the final drafts and to Bror Bemild for allow­ ing us to use his photograph of Bohr appearing in the frontispiece.

Xl Niels Bohr, photograph taken in 1961 by Bror Bernild. INTRODUCTION

I.

More than any other single person Niels Bohr stands at the center of the momentous changes in physics that mark the quantum revolution. The theory of quantum mechanics together with the theories of special and general rela­ tivity are some of the greatest intellectual achievements in human history. But where the theories of relativity were more or less the product of a single individual, namely Albert Einstein, it took many physicists a long time to reach the final theory for the atom. This difference between the creation and the development of these theories may reflect the fact that quantum mechan­ ics departs to a even greater degree than relativity from the framework of classical mechanics. Although the theorie s of relativity expel concepts like absolute simultaneity, they still provide us with a deterministic description of the physical world as does classical mechanics. Quantum mechanic s, on the contrary, permits only an indeterministic description of microphy sical processes. The point of departure of Bohr's philosophy is an acceptance of this limitation , rather than the continued search for a description which would demand a classical deterministic account. Bohr was fond of justifying his interpretation of quantum mechanic s by appeal to the analogy with relativity. Relativity had taught physicists that physical properties such as length, duration, and velocity are ascribable only relative to a frame of reference, while the quantum revolution taught that physical properties can be attributed to an object only relative to an experi­ mental situation . Both revolutionary theorie s share a point of origin in an empirical discovery . In the case of relativity, it is the finite velocity of light; in the case of quantum theory, the discovery of the quantum of action. Both revolutions involved the recognition of limitations in the use of descriptive concepts as a consequence of an analysis of what can be empirically deter­ mined. Thus Bohr regarded both revolutions as teaching analogous epistemo­ logical lessons, and tried to use this fact to convince Einstein of the soundness of his interpretation of quantum mechanics. The quantum revolution can be analyzed into two periods, each of which is characterized by a new insight into how to describe physical attributes in the

xiii

J. Faye and H. J. Folse (eds.), Niels Bohr and Contemporary Philosophy, xiii-xxvii. © 1994 Kluwer Academic Publishers. XIV INTRODUCTION atomic domain. From Planck's introduction of the quantum in 1900 until the mid 1920's, physicists in the avant garde realized that a limit to the divisibil­ ity of classical dynamical properties such as energy had to be postulated in order to give an account of such phenomena as black-body radiation, the pho­ toelectric effect, and atomic spectra. This first insight became incorporated into Bohr's semi-classical atomic model of 1913, where he postulated that the energy with which an electron is bound to its nucleus can occur only in certain quantized states. This model was central to the research program of the next decade, until physicists eventually came to recognize that some non­ classical feature was still missing for a full description of quantum phenom­ ena. The indivisibility of the quantum of action on which Bohr's atomic model was based suggested to Bohr after 1927 that in interactions involving atomic systems one cannot dynamically separate the object from the agencies of observation. Nevertheless, an unambiguous description of the results of an observation requires predicating properties of the object, and this in turn implies that we must make an arbitrary distinction in our description between object and agencies of observation. Because this distinction does not reflect a physical situation, but is a precondition of unambiguous description, any properties attributed to the object cannot be understood to be properties pos­ sessed by the object independently of its interaction with the observing instruments. After 1927 when Bohr began to argue for the need for comple­ mentary descriptions, he began to stress that the indivisibility between observed object and observing systems in atomic physics was analogous to the unity of subject and object in the psychologist's attempt to provide descriptions of the observation of one's own consciousness . Bohr's refer­ ences in the context of psychology to the observing "subject" have been misread as defending the view that he holds that in the context of physics a description of observation must make reference to the observing subject qua conscious mind. The analogy Bohr wished to draw is on the epistemic level, namely that the distinction between the knowing subject and the known object is a logical demand for the unambiguous description of observation. Bohr's point is not to argue for the necessity of making a metaphysical dis­ tinction between inanimate physical systems and consciousness. This second major insight gained by the quantum revolution came with Heisenberg 's formulation of matrix mechanics and the subsequent derivation from it of the reciprocal limitation of the simultaneous measurement of precise values of certain physical properties. Bohr's reaction to Heisenberg's discovery was twofold. First he saw that the uncertainty principle limited not INTRODUCTION xv only the observability of these properties, but also the degree to which one could define unambiguously the classical descriptive concepts in a specific observational context. Second he realized that a consistent quantum theory would have to abandon the classical goal of providing both a space-time description and a causal account of the phenomena. By this time wave-particle dualism had already become characteristic of the description of the full range of phenomena involving both matter and radiation, but no one had any clear understanding of what that fact implied. While Bohr had been thinking about the limitations of the classical descrip­ tive concepts for most of this period, it was only now that he came to see the connection between the limitations of the uncertainty principle and wave-par­ ticle dualism. Thus he now recognized that both wave-particle dualism and the uncertainty principle were manifestations of a deeper underlying comple­ mentarity which restricted the Kantian scheme for the description of phenom­ ena in terms of the spatio-temporal forms of intuition and the concept of deterministic causal connections between phenomena. In saying that proper­ ties are "complementary" Bohr meant that they are incompatible but equally necessary for a full description of the quantum system. The picture of nature made possible by the classical mechanical account of the phenomena made use of simultaneous application of both of these modes of description. Bohr's new argument that they could be applied only in a complementary fashion, therefore, had profound consequences for our understanding of how the physicist's description of nature relates to the physical world. Understanding these consequences forms a central task for comprehending Bohr's unique viewpoint. Among these consequences are those which concern what Bohr called "the customary demand for visualization". Bohr is quite clear that we must "renounce" this classical demand, but his point is not that we must abandon using particle and wave "pictures". Rather, he argues that we must under­ stand their use in a different way from the literal interpretation they could be given in the classical framework. They cannot be altogether rejected, because it is only through such pictures that we are able to interpret experimental phe­ nomena as measurements determining the properties of atomic systems. However, while essential for this purpose, the proper understanding of the quantum description requires that we recognize that such pictures are "abstractions" or "idealizations" in the sense that they do not represent atomic objects as they exist 'in themselves'. Bohr regarded kinematic and dynamic properties as associated with the classical wave and particle pictures of quantum mechanical objects. Younger XVI INTRODUCTION physicists such as Heisenberg may have originally hoped that a consistent description could omit reference to particles and/or waves, but Bohr argued in his discussions with Heisenberg in early 1927 that it was necessary to pre­ serve both of these sets of concepts for an unambiguous description of the phenomena. For Bohr the necessity for the concepts of space, time, and causality lies in the nature of human experience and provides the continuity of empirical reference to evidential phenomena against a background of changing theory. These general concepts as refined into the physicist 's notions of the kinematic properties and the dynamical properties of momen­ tum and energy are "indispensable" for the physicist's understanding of nature. For those whose familiarity with Bohr's outlook derives from physics text­ books, "complementarity" is often associated with the relationship between wave and particle pictures. However, in its initial presentation in the Como paper of 1927, it is clear that Bohr intended complementarity to refer to the relationship between space-time description and the claims of causality. Thus the connection between wave-particle dualism and the complementarity of kinematic and dynamic properties remains a problematic issue both for the analysis of Bohr's philosophy and generally for the interpretation of quantum mechanics. The difficulties of understanding Bohr's position on this issue can be seen in the variance of interpretations including that of Murdoch (1987), who sees Bohr as retaining an ontological commitment to the reality of parti­ cles, in contrast with that of Beller (1992) who defends the view that in Como Bohr's "central message" was that the wave packet idea was "sufficient to resolve all the paradoxes of atomic structure". Following Bohr's initial presentation of his new viewpoint, the well­ known opposition of Einstein shaped the further development of Bohr's thinking in an essential way. Bohr had given the uncertainty principle not only an epistemic reading but an ontic interpretation as well, whereas Einstein would have liked to confine his reading to a purely epistemic inter­ pretation. This opposition reached its climax in 1935 with the publication of the now famous EPR thought-experiment. Here Einstein and his collaborators argued that quantum mechanics could not be a complete theory because one could conceive of different physical situations in which one was free to attribute either of two complementary properties to one quantum object on the basis of observations made on another object. Since there is no question of a physical interaction or "disturbance" with the latter object, it seems therefore that one is warranted in asserting that the real object has both prop­ erties, contrary to what Bohr had claimed. INTRODUCTION xvii

Bohr's reaction to this argument was to distinguish between two concep­ tions of "physical reality". Einstein regards it as meaningful to talk counter­ factually about the properties of an object apart from the circumstances of their empirical determination. However Bohr holds a concept of physical reality which demands that properties are well-defined only in the context of the description of a particular observational phenomenon. For this reason , the debate was transformed from a challenge to the uncertainty relations to the philosophical question of how physical theory attaches to nature . After EPR Bohr and Einstein did not take their debate to any new level. The dominant attitude was that the participants defended different "meta­ physical" theories of the nature of physical reality that could not be resolved by any further empirical research. Furthermore other attempts at interpreta­ tion or reformulation of the theory in terms of hidden variables all suffered the same fate of lacking any empirically detectable consequences. The later 1980's have seen a considerable shake up in this uneasy standoff that has prevailed between philosophy of nature and microphysics. To be sure, there is at least as much dissension (possibly more) as there has ever been. But in accord with an accelerating pace of publications in the area, there has grown an increasing felt sense ofneed for a philosophically satisfac­ tory account of the quantum description of microsystems. At least two forces have led to the breakup of what had been more or less the status quo since the time Bohr and Einstein debated the issues in the thir­ ties. The first of these is well known and the subject of a great deal of atten­ tion: the derivation of Bell-type inequalities and the experimental production of phenomena which seem to force a revision of the conjunction of a few very deeply entrenched fundamental assumptions about physical reality . A second , less well noticed, but nevertheless influential force, is the weight of shifting opinion about the views of Bohr and Einstein, and generally how the older generation of founding father physicists really saw the issues. The first of these two forces, Bell's theorem and its experimental tests, has transformed what was originally considered a "metaphysical" debate between Bohr and Einstein into a question of physics. Bell's work has been motivated by a natural extension of the debate over EPR. The results of the various experimental tests of Bell's theorem in favor of quantum mechanics have excluded the possibility of local "hidden variable" theories, on which the hope of many of those who sought to avoid the limitations of the quantum description was based. Of course there is no possibility of establishing the completeness of the quantum description iIl some final sense, but at least it now appears that certain of the conceptual revisions of the quantum revolu- XVlll INTRODUCTION tion are permanent features of any description of microphysical processes. While there remains considerable diversity of opinions on the significance of the violations of Bell's theorem, these developments have reawakened several themes prominent in Bohr's interpretation of quantum mechanics and his reply to EPR. These themes include both Bohr's insistence on the "indi­ visibility" of the interaction between the observed object and the agencies of observation, and his claims about the conditions for the well-defined predica­ tion of properties of physical systems. The second of these two trends in recent research in the philosophy of quantum physics does not tum on questions directly in physics. Both Einstein and Bohr have been the subject of intense scholarly publication of late. On the older, shallower, and historically unfounded view which finds prominent expression in Popper and was spread by a generation of his earnest disciples, Bohr dismisses quantum mysteries with a glib instrumentalist line: save the phenomena and renounce the attempt to form mechanical models of the behavior of microsystems. Any concept of "physical reality" is supposedly banned as irrelevant to empirical science. Meanwhile, on this story Einstein strangely appears as the reactionary defender of an antiquated determinism and a classical realist ontology of particles possessing determinate mechani­ cal properties. As we have seen above, in the earlier phase of their debate Bohr and Einstein tended to see their opposition in terms of indeterminism versus determinism, but after EPR the central issue became the question of the rela­ tionship between the quantum description and physical reality. Some work on Bohr, including for example that of Hooker (1972), Folse (1985), Honner (1987), and Murdoch (1987), has argued for the nontraditional view that Bohr should be termed a 'realist' because he does not doubt the objective reality of atomic systems and his arguments are based upon what he considered a dis­ covery about their real nature, namely that they had to be described as chang­ ing their classical mechanical states discontinuously. Other recent interpretations, including those of Fine (1986), Krips (1987), and Faye (1991), defend the traditional perception of Bohr as an 'anti-realist', but at least agree to the extent that it is today scholarly anachronistic to characterize Bohr's outlook as a form of instrumentalist phenomenalism . The work of Hooker (1972), Howard (1985), and Fine (1986) has shown that by 1935 Einstein's discontent with the quantum description had shifted from a rejection of indeterminism to the criticism that quantum theory was incompatible with what is now called 'separability'. Here our improved understanding of the basis of Einstein's objection connects directly with dis- INTRODUCTION XIX cussions of Bell's theorem, for a prime candidate for the classical presupposi­ tion which the quantum revolution requires abandoning is none other than this principle of separability. Therefore these new portraits of the old combat­ ants reveal a grasp of the issues more significant for revived efforts in the philosophy of nature than the old story that tells of the combat between realism and instrumentalism. Moreover our newer more historically well founded picture of the actual debates allows us to focus more sharply on what philosophy has to learn from Bell's results. Although this revisionism in the stories of the founding fathers has attracted some attention, it was the coincidence that the historical issue of realism/anti-realism reawoke from the dogmatic slumber of positivism at roughly the same time that Bell phenomena began to attract a great deal of attention that the two combined could put a great deal of pressure on the old impasse leading to the breakup of the status quo. Although Bohr's own con­ ception of his philosophy was not something that could be well expressed in terms of the traditional philosophical categories of realism and idealism, his philosophy is pertinent to these debates. Many of today's philosophers are in effect searching for a mean between the unacceptable extremes of a naive metaphysical realism and an impoverished instrumentalist rejection of the notion of physical reality. It is in just this philosophical no man's land that Bohr's philosophy may well be located.

II.

In this volume we have asked each contributor to address issues in Bohr's philosophy as they relate to problems discussed in philosophy at the present time. It is our belief, that though much remains to be said about the historical development of quantum physics and its interpretation, the real significance for Bohr's views to philosophers generally lies in the fact that his insights and arguments touch on many issues at the forefront of today's discussions in philosophy of science and epistemology. Given the current concern in philosophy of science with issues surrounding the question of realism, it is not surprising that many contributors to this volume have sought to relate Bohr's position to the debate between realists and their opponents. Among these, Edward MacKinnon argues that although Bohr is "methodologically" on the side of the anti-realists, his position has much in common with the common sense realism of Donald Davidson. McKinnon considers Bohr's realism "paradoxical" in the sense that though he is not an anti-realist, he is anti-ontological. Bohr would argue that the xx INTRODUCTION attempt to make ontological claims, transgress the limits that he wants to set on the proper use of our descriptive concepts. The physicist's use of the for­ malism of quantum mechanics requires no ontological commitments. Nevertheless, Bohr's view has certain affinities to Davidson's in that the physical models and the language in terms of which Bohr describes them carry with them unavoidable ontological commitments to the existence of various kinds of physical objects. Among those inherent in Bohr's discourse are those that include a commitment to the reality of microsystems. Another contribution concerned with realism is David Favrholdt's charac­ terization of Bohr as a realist who rejected the classicalimage of scientific knowledge as a "God's eye view" of nature. Bohr's argument for the neces­ sity of classical concepts turns on the need to use ordinary language to make description unambiguous by marking a sharp distinction between observing system and object. But its use to construct an image mirroring how nature would look to an omniscient being is now blocked because the discovery of a finite quantum of action in all observations implies that scientific knowledge is not concerned with how nature is, but rather how it must be described, i.e. about what we can unambiguously say about it. Favrholdt is concerned to dis­ tinguish Bohr's view from the subjective idealism of Berkeley or Machian phenomenalism, which some have misread in Bohr's statements. Favrholdt calls attention to Bohr's reminder that 'reality' also is a concept, the limits of the correct use of which we must also learn to modify with the growth of knowledge . Henry Folse continues the discussion of how Bohr's position relates to the philosophers' debates over realism. The boundaries between realist and anti­ realist views have shifted in the historical move of philosophical discussion from Kantian concerns with the conditions for the applicability of concepts, to debates over the reality of atoms, to questions of semantics, correspon­ dence and representation. In these different cases Bohr can be seen as allied with the 'realist' side because he defends the view that 'understanding the phenomena' requires more than empirically adequate predictability. But his identification with the realist side is never total, for in each case Bohr attacks positions traditionally allied to realism . Folse's categorization of Bohr as a realist rests on his claim that talk about complementary phenomenal appear­ ances of atomic systems requires essential reference to atomic systems con­ sidered apart from the experimental phenomena in which they are said to be 'observed'. If we abandon the classical conception of knowledge based on mechanistic representationalism and the ideal of visualizability, we can hold that the application of such complementary descriptions permitted by INTRODUCTION XXI quantum mechanics expresses our knowledge of what these objects are like. Thus Folse concludes that Bohr ought to be considered a 'realist' because he is committed ontologically to the independent reality of the objects of atomic physics as well as to the belief that quantum theory enables us to communi­ cate whatever it is possible to know about them. Reflecting a similar concern with locating a position for Bohr between the realism of classical physics and an anti-realist instrumentalism, Henry Krips sees Bohr as arguing against the Enlightenment ideal of a ' universal realism ' in favor of a 'local realism' restricted to representing reality through this or that conceptual scheme in different contexts and unable to attain the classical goal of a single 'universal' picture of reality. Krips concludes that because of the weakness of Bohr's claim that we are restricted to the 'classical con­ cepts' , he cannot make good any argument for the necessity of the move to a 'local realism' and that therefore Enlightenment ideals of objectivity need not yet be banished from physics. Krips thinks that Bohr's view of classical con­ cepts is insupportable, but he does not attack the weaknesses of Bohr's argu­ ment directly; instead he hopes to refute Bohr by showing the possibility of a 'meaningful non-classical mode of description ' which 'Bohr tells us cannot exist'. This non-classical mode of description takes the density operator as representing an objective probability inhering in the atomic object. The Bohr-Einstein debate provides a popular avenue of approach for ana­ lyzing the philosophical impact of Bohr's viewpoint. Dugald Murdoch follows this route (already pursued by Fine, 1986 and Howard, 1985) to trace the debate up to the choice between rejecting the completeness of the quantum description (Einstein) or the principle of separability (Bohr). Murdoch holds that Bohr rejected separability as a semantic principle about what we can say about atomic systems, as well as ontically, about the physical reality thus described. But Bohr's position in this regard is based on a verificationist semantics (from pragmatist rather than positivist sources) and the conse­ quences of an ontic non-separability are far from clear. Murdoch concludes that these facts make Einstein's rejection of completeness philosophically the preferable route, but this would force in physics some sort of ensemble inter­ pretation of quantum mechanics. However, so-called 'orthodox' ensemble interpretations are all in deep difficulties (as Einstein well knew) and seem to be fatally wounded by Bell phenomena. So the only hope for Einstein's philo­ sophically more attractive position is to be found in 'non-orthodox ensemble interpretations', but these seem 'far-fetched'. Thus Murdoch alters the usual perception of Bohr as the clear victor, concluding that while Einstein's objec­ tion was based on philosophically more cogent arguments, Bohr's position is XXll INTRODUCTION stronger from the point of view of existing physical theory, but that is, of course, a state of affairs which could change in the future. The Bohr-Einstein debate appears in Jan Faye's contribution in the form of the debate between anti-realists and realists. What distinguishes Bohr from Einstein, in Faye's view, are alternative semantic theories; Einstein allows statements with empirical truth conditions that are in principle unverifiable to be meaningful, whereas Bohr's prohibits it. However, it is a mistake to hope that EPR and Bell type experiments can allow an empirical determination of the correct semantics. Faye defends his 'objective anti-realist' interpretation of Bohr with respect to the various possibilities for responding to the recogni­ tion that Bell's theorem requires abandoning either separability or locality. Whether the realist takes the route of abandoning locality or abandoning sep­ arability, he remains in the position of making claims which are in principle unverifiable. An anti-realist may opt for non-locality, but then he must present empirical evidence for such non-locality, which at present does not exist. Thus the anti-realist who defends non-separability is in the strongest position, and this, Faye claims, is essentially Bohr's view. It is through the heritage of the EPR challenge that the Bohr-Einstein debate continues to echo through today's discussions of the philosophical consequences of quantum physics. However, Mara Beller and Arthur Fine warn against reading contemporary concerns into the actual historical exchange between Bohr and Einstein. Although it is common to see the ulti­ mate divide between the protagonists in terms of rival conceptions of ' physi­ cal reality', Beller and Fine argue that Bohr essentially agreed with the EPR criterion of reality. Bohr tried to undercut EPR's reasoning by finding an "ambiguity" in the "no disturbance condition", but Bohr's "physical realiza­ tion of the EPR case . .. involves mechanical effects not present in EPR" and so cannot successfully refute EPR's conclusion . The real significance of EPR for understanding Bohr's thought lies in the fact that though he could origi­ nally defend the need for complementary descriptions by appeal to a "robust physical disturbance" in the quantum interaction, after EPR Bohr could main­ tain his view only by taking refuge in a positivistic verificationist semantics. Although there is tension between the more realistic outlook in Bohr's appeal to interaction and the positivistic outlook of verificationism, Beller and Fine conclude that by reinterpreting Bohr's talk about "exchange of momentum", it is possible to effect a reconciliation between the two. Neverthele ss, because of deep philosophical difficulties inherent in such a positivistic defense, it can hardly be said that "Bohr got the better of Einstein" in the final act of their momentous confrontation. INTRODUCTION xxiii

Don Howard also attempts to sharpen the focus of our image of the differ­ ences separating Bohr and Einstein, but he seizes on Bohr's controversial claims concerning the indispensable status of 'classical concepts'. Howard asks whether the distinction between that which must be described through the framework of quantum mechanics and that which must be described classi­ cally can coincide with the distinction between observed object and observing instruments. Howard argues against this coincidence and develops instead the thesis that, on the one hand, the proper quantum mechanical description of a measurement interaction corresponds to what is treated as a 'pure case' in the formalism in which observed and observing systems are assigned a single 'inseparable' state. On the other hand, the objective description of a measure­ ment outcome requires that, as Bohr was wont to urge, an arbitrary but neces­ sary distinction must be made between observing and observed systems. This requires describing the observational interaction as though the two systems were in separate classical states, and corresponds in the formalism to a 'mixture' appropriate to the particular measurement being performed. Thus the crucial distinction between classical and quantum descriptions in reflected formally in the distinction between mixtures and pure cases. Over the years many scholars have written on the similarities between Bohr and Kant, and this continues to be a subject of interest in several of the articles included here. Cliff Hooker sees both Bohr and Einstein as challeng­ ing the ideal of intelligibility embodied in classical physics, which on Kant's analysis upholds a criterion of 'intelligibility' which requires that the descrip­ tion of phenomena meet certain conditions of uniqueness. Hooker employs his ground breaking (1972) analysis of the Bohr-Einstein debate to see how these challenges differ from one another and in what way Bohr's entails a more radical challenge to the classical ideal of intelligibility. Hooker first develops a contrast between Newtonian and Kantian ideals of theoretical understanding. He then argues that Einstein's departure from Kant, with Reichenbach's, is more closely allied with Newton's ideal of intelligibility, while Bohr's challenge can be seen as a development of Kant's ideal in response to what has been discovered in the quantum revolution. Bohr argues that it is the challenge to the subject/object distinction posed by the quantum of action which implies that the Kantian criteria can be met only in a comple­ mentary manner. Each challenge has its weaknesses, and Hooker concludes with a reminder of the "depths of our ignorance" with respect to an ideal of intelligibility consonant with contemporary physics. Catherine Chevalley strives to shed light on Bohr's discourse by consider­ ing the nineteenth century discussion of 'Anschaulichkeit' and 'Symbol' XXIV INTRODUCTION deriving from Kant's third Critique. The study of the relation between lan­ guage and reality from Kant, through Goethe, Humboldt, and Hemholtz came to elevate the 'symbolic' presentation in scientific knowledge to a level Kant had accorded to the'intuitive'. She shows how this largely forgotten discus­ sion shaped Bohr's choice of terminology, such that in his mature works his choice of words was such as to associate 'anschaulichkeit' ('intuition') with the classical mode of description, while the quantum description became associated with 'Symbol' . Bohr's 'epistemological lesson', teaching that we need to combine complementary descriptions, can be seen as a way of com­ pensating for what is lost in the quantum revolution with the passing of the classical anschaulich mode of presentation in favor of an understanding of the phenomena which is purely 'symbolic'. Chevalley suggests, but does not explore, the possibility that this conception of complementary modes of description may illuminate issues of concern to philosophers involved with relating 'analytical' and 'continental' traditions in philosophy. Indeed, Bohr's concern with setting the limits to the unambiguous use of concepts in the description of nature echoes the general concern of philoso­ phers with the relationship between language and the world described by it, and this is a view shared by 'continental' philosophers as well as those in the analytic tradition. John Hanner employs the metaphor of the scientist as 'reading the book of nature' to relate Bohr's philosophical concerns with lan­ guage to the deconstructivist concern with the interpretation of text. In this regard Honner explores the relation of Bohr's views on the description of nature to the critique of objectivity found in postmodernist philosophers such as Derrida. Bohr's conception of 'complementarity' has something in common with Derrida's notion of 'supplementarity' but while both reject Enlightenment conceptions of 'objectivity', Bohr's complementarity doctrine allows him to be more optimistic about achieving 'objectivity' in the descrip­ tion of nature than is possible according to the deconstructivist critique of language. Ulrich Riiseberg considers Bohr's philosophy as carrying within it a 'hidden historicity' reflecting its historical development. Roseberg contrasts Reichenbach's analytical notion of the history of science as a reconstruction which masks rather than reveals the history of the science it seeks to recon­ struct, with his own dialectical notion of the history of science based on the Hegelian tradition. Roseberg sees in Bohr's philosophical viewpoint of com­ plementarity the dialectical play of ideas that the historical Niels Bohr lived through. This 'hidden historicity' is lost from sight in the dominant analytical mode of doing history of science thus obscuring Bohr's view; Roseberg's INTRODUCTION xxv suggestion is that if it were made manifest in a dialectical reconstruction of Bohr's thought, it would become clear. Bohr's interpretation is of course based on the so-called 'orthodox' quantum mechanical formalism. James Cushing shows that if one is willing to make slight modifications in that formalism, as has been done by David Bohm, then it is possible to provide an account of atomic processes which retains a deterministic ontology that Bohr opposes. Cushing argues that Bohm's description of physical reality is quite similar to Bohr's in that he also emphasizes that the observed values are dependent on the whole experi­ mental context and that atomic physics must describe the interaction between the observing instruments and atomic systems. But in contrast to Bohr, Bohm believes that the experiments disturb the object such that its classical trajec­ tory, while real, is unobservable. This disturbance is a consequence of the non-local field which Bohm associates with the otherwise classical particle. Henry Stapp agrees with Cushing that, by providing a classical realist interpretation, David Bohm's contribution has put the lie to Bohr's claim that such an interpretation of quantum mechanics is impossible. For this reason, as well as because of demands placed on physics by quantum cosmology and by Bell's work, Stapp sees Bohr's influence as loosing its grip on the minds of today's physicists. Bohr's position is best seen as only the "right face" that is the "first step of an about face" in the physicist's conception of nature. According to Stapp, Bohr took the fact that the formalism predicts measure­ ment outcomes to imply that quantum theory is about what we can 'say' or 'know' about nature; the state function is basically an epistemological con­ struct. However Stapp advocates the full "ontologicalization" of the state function along lines suggested by Heisenberg's reference to the actualization of objective potentialities. The result will be a conception of nature in terms of entities that are more "idealike" than "matterlike", as in the classical con­ ception. This new perspective allows for the possibility of integrating con­ sciousness into the physical sciences. Though no one now possesses such a theory, Stapp is optimistic about progress following this line of thought. Bohr's philosophy has of course had its greatest impact among those con­ cerned with paradoxes in the interpretation of quantum mechanics, but it has by no means been exclusively confined to this matter. David Kaiser explores the generally ignored area of Bohr's work in nuclear physics and how his unique philosophical approach shaped and continues to influence develop­ ments in contemporary particle physics. Kaiser shows how Bohr's model of the compound nucleus implied methodological postulates which continue to guide the analysis of particle interactions in high energy physics. But these XXVI INTRODUCTION postulates for how to analyzes interactions raise ontological questions con­ cerning elementary particles which are quite different from those of elemen­ tary quantum mechanics. As both realist and anti-realist interpreters of Bohr agree, the independent reality of atoms, electrons, and protons is not in dispute, but what is at issue is the conditions necessary for meaningful asser­ tions about them. Thus the statistical nature of the predictions of elementary quantum mechanics raises ontological questions about the existence only of properties of entities, not the entities themselves. However, Kaiser shows that at least in some cases, the evidence for the existence of elementary particles of particular kinds is itself statistical in nature, thus ironically reintroducing modem-day analogues to the questions about the existence of the supposed elementary constituents of matter that perturbed earlier periods of natural phi­ losophy. Going beyond the domain of physics, Bohr carried his epistemological lesson into the areas of psychology and biology; the latter is discussed by Paul Hoyningen-Huene. Bohr's interest in biology was shaped by the Kantian tradition which opposed mechanistic and teleological accounts . While earlier writers like Folse (1990) and Faye (1991) have seen Bohr's primary conclu­ sion in this area to be his claim that mechanistic and teleological descriptions must be used complementarily in order to give a full description of the bio­ logical phenomena, Hoyningen-Huene relates Bohr's position to the philo­ sophical question of whether biological laws are reducible to physics. Bohr's defense of epistemological, ontological, and methodological anti-reduction­ ism is based on the biological analogue to the complementarity that exists in quantum physics. On Hoyningen-Huene's analysis, complementarity pro­ vides a means for defending the view that even though different aspects of one phenomenon exhibit a relation of 'theoretical irreconcilability', they are not necessarily in direct contradiction. When description of these different aspects entails such a complementary relation, as is the relation between the physicists' description of vital processes in the terms of molecular biology and the purposive accounts of much traditional biology, then Bohr holds that neither can be reduced to the other. However, Hoyningen-Huene criticizes Bohr's conclusion for having failed to show that there are some biological functions the descriptions of which are theoretically irreconcilable with the descriptions of molecular biology. The reader will search in vain for a single common vision of Bohr running throughout all of these contributions. But we believe that the essays of this collection do permit us to make some interesting generalizations. In the first place the Bohr who emerges from these pages holds a considerably more INTRODUCTION XXVII subtle and deeper position than has often been attributed to him. Like all great originators in human thought, his views will no doubt continue to inspire different rival interpretations. But this fact should hardly lead us to despair over ever hoping to make sense of Bohr's position. And the essays included here testify to this. Second, although there is no one interpretation offered here, many points of agreement do unite our contributors. Virtually all are agreed on the central­ ity of Bohr's controversial view with regard to the classical concepts. Virtually all see Bohr as attempting to revise our understanding of scientific knowledge by restricting or altering the application of these concepts. No one sees Bohr as denying the reality of atomic objects , but all see him as con­ cerned with how they must be described. None gives a 'subjectivist' or 'phe­ nomenalist' reading of Bohr's position. All are agreed that Bohr's 'epistemological lesson' concerns under what conditions we can say what­ ever it is that we can say about the world. Third, in spite of much mutual agreement (or non-disagreement) there also remain deep philosophical differences separating the various views of Bohr presented here. Many of our contributors see Bohr's position - no matter how subtle or profound - as ultimately an unsatisfactory response to the problems it was designed to solve, although these critical conclusions are reached for a variety of different reasons . Moreover, even among those who do see Bohr's position as satisfactory and defensible, there remain serious disagreements on what that position is and what philosophical consequences follow from it. Nonetheless it is our hope that by bringing these various different images of Bohr together in one place, we can highlight the relevance of Bohr's view­ point for contemporary philosophical problems.