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 ofthe City University ofNew York

VOLUME 153

NIELS BOHR ANDCONTEMPORARY PHILOSOPHY

Edited byJAN FAYE

Carlsberg Foundat ion, Copenhagen , Denmark

and

HENRY J. FOLSEDepartment 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 HenryJ. 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 onphIlosophy.]aC16.B65N493 1993530' .01--dc20DLCfor Library of Congress 93-24825

Printed on acid-free paper

All Rights Reserved© 1994 Springer Science+Business Media Dordrecht

Originally published by Kluwer Academic Publishers in 1994.

Softcover reprint of the hardcover 1st edition 1994No 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.

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

TABLE OF CONTENTS

Preface Xl

Introduction xiii

MARA BELLER & ARTHUR FINE / Bohr's Response to EPR I1. EPR and Bohr's EPR 22. Incompleteness and Inconsistency 33. Simultaneous Position and Momentum in EPR 64. Bohr's Concept of Disturbance: EPR and Before 105. Ambiguity and Definition 166. Positivism and Its Puzzles 187. Locality and Separability 238. Concluding Remarks 27

CATHERINE CHEVALLEY / Niels Bohr's Words and the Atlantisof Kantianism 33

1. Introduction 332. Anschauung and Symbol- In Bohr's Interpretation

of Quantum Mechanics 353. Anschauung and Symbol - The Philosophical Background 434. Conclusions 50

4.1. Original Interpretations of Quantum Mechanics 504.2. Bohr's Conception of Language 514.3. Changing Perspectives 53

JAMES T. CUSHING / A Bohmian Response to Bohr'sComplementarity 57

1. Introduction 572. The Project of "Clarifying" Bohr's Views 583. Bohr's Complementarity 614. Bohmian Mechanics 63

4.1. Bohm's (1952) Theory 644.2. Recent Developments 70

5. Conclusions 72

v

VI TABLE OF CONTENTS

DAVID FAVRHOLDT / Niels Bohr and Realism 77I. Classical Concepts and Ordinary Language 772. What We Can Say About Reality 823. Subjective Idealism and Phenomenalism 844. A 'God's Eye View' of the World 865. Conclusion 94

JAN FAYE / Non-Locality or Non-Separability? A Defense of Bohr'sAnti-Realist Approach to Quantum Mechanics 97

1. The Bohr-Einstein Debate in Retrospect 972. Non-Separability Anti-Realism 1033. Non-Separability Realism 1084. Non-Locality Realism 1105. Non-Locality Anti-Realism 114

HENRY FOLSE / Bohr's Framework of Complementarity andthe Realism Debate 119

I. Drawing the Battlelines: Bohr and Kant 1202. Realism and the Atomic Description of Nature 1233. Realism and Truth 1274. Complementarity and the Realist Ideal of Understanding 1345. Conclusion 137

JOHN HONNER / Description and Deconstruction: Niels Bohr andModem Philosophy 141

I. Preamble 1412. Bohr and the Philosophers 1443. Derrida and Deconstruction 1484. Bohr and the Description of Nature 151

CLIFFORD A. HOOKER / Bohr and the Crisis of EmpiricalIntelligibility : An Essay on the Depth of Bohr's Thought and OurPhilosophical Ignorance 155

Part I. Bohr and the Kantian Legacy 1551. Introduction 1552. Reichenbach on Kant and Relativity Theory 158

Part II. Uniqueness and Rational Methodology: Newton and Kant 1633. 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 1745. Reichenbach and Bohr 1786. Bohr's Conception of Intelligibility, Objectivity and

Completeness 1807. Einstein against Bohr 1828. Bohr and Einstein versus Nature 1859. Principled Ignorance, Adventures of Ideas and the Open Future 186

Appendix 1. Butts and Friedman on Kant's General EpistemologicalFramework 188

A. Overall Procedure 188B. First Inference, to Metaphysical Principles of Pure Natural

Science 189C. Second Inference, to the Law of Universal Gravitation 194

DON HOWARD / What Makes a Classical Concept Classical? Toward aReconstruction of Niels Bohr's Philosophy of Physics 201

1. Introduction 2012. Objectivity and Unambiguous Description. Why Are Classical

Concepts Important? 2043. Instruments and Objects of Investigation. Where and How Are

Classical Concepts to be Employed? 2104. Of Mixtures and Pure Cases. What Makes a Classical

Description Classical? 2175. Does the Reconstruction Work? 223

PAUL HOYNINGEN-HUENE / Niels Bohr's Argument for theIrreducibility of Biology to Physics 231

1. Introduction 2312. The Anti-Reductionist Claim 235

2.1. Explication of Concepts 2362.2. The Relation of Bohr's Claim to Other Forms of

Anti-Reductionism 2383. The Argument for the Anti-Reductionist Claim 240

3.1. Bohr's Argument: An Analogical Inference 2403.2. First Premise: Complementarity in Physics 2413.3. Second Premise: Complementarity of Physics and

Biology 2493.4. 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 ContemporaryParticle Physics 257

1. Introduction 2572.. Bohr and Particle Physics: A Brief History 2583. The Compound Nucleus and Particle Physics Phenomenology 2594. Questions of Ontology and Particle Physics Phenomenology 2625. Bohr's Realism and Particle Physics 2646. Conclusions 266

HENRY KRIPS / A Critique Of Bohr's Local Realism 269I. Introduction 2692. Instrumentalism 2693. Bohr's Philosophy 2704. Bohr and Local Realism 2715. Critique of Local Realism 273

EDWARD MACKINNON / Bohr and the Realism Debates 279I. Perspectives and Presuppositions 2802. Coping with A Linguistic Crisis 2823. Interpreting Quantum Mechanics 2864. Realism in Perspective 290

4.1. Einsteinian Realism 2914.2. Scientific Realism 293

5. Realism and Analysis 297

DUGALD MURDOCH / The Bohr-Einstein Dispute 3031. Einstein's Opposition to the Copenhagen Interpretation 3032. The EPR Argument 3053. Bohr 's Reply to the EPR Paper 3064. Einstein's Argument 3085. Bohr's Reponse to Einstein's Argument 3116. The Philosophical Background to Einstein's Argument 3157. The Dispute in 1935, and Thirty Years On 3188. The Physical Dispute Reconsidered 322

ULRICH ROSEBERG / Hidden Historicity: The Challenge of Bohr 'sPhilosophical Thought 325

1. The Problem 3252. Reichenbach's Rational Reconstruction of the Development

of Quantum Mechanics 327

T ABLE OF CONTENTS IX

3. A Rational Reconstruction of the Development of QuantumMechanics in the Dialectic Tradition 3293.1. Bohr's Research Program 3293.2. A Physicist Becomes a Philosopher 332

4. Hidden Historicity in Bohr's Epistemological Reflections 3375. Concluding Remarks 340

HENRY P. STAPP / Quantum Theory and the Place of Mind in Nature 3451. Mind in the Physical Sciences 3452. The Objective Wave-Function 3463. Integrating Consciousness into Physical Science 3474. 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 ofpublications on Bohr's philosophy. These contributions have appeared in awide variety of different sources. While other volumes have collected a varietyof essays on the many aspects of Bohr's work, hitherto there has been no col­lection bringing the diversity of new philosophical interpretations between thecovers of a single volume. Therefore, in this collection we have invited seven­teen of today's best known authors who have helped shape this new round ofdiscussions on Bohr 's philosophy to address the question of Bohr's relation toissues currently discussed in contemporary philosophy of science.

The sixteen previously unpublished papers included here reveal a surpris­ing variety of different facets of Bohr as the natural philosopher whose ideasof complementarity shaped the final phase of the quantum revolution andinfluenced two generations of the century's leading physicists . Many of thequestions discussed bear on the very active philosophical arena of realismversus anti-realism and the implications of the work stemming from theseminal contributions of John Bell. While our primary focus has been philo­sophical, also discussed are important historical questions relating Bohr toKant, neo-Kantians , and positivists .

There is much on which the authors included here agree; but there are alsopolar disagreements, thus affording the reader an opportunity to compare andcontrast new interpretations of Bohr as a philosopher. Indeed, the variety ofdiffering opinions revealed in these papers assure us that the philosophicalquestions revolving around Bohr's "new viewpoint" will continue to be asubject of scholarly interest and discussion for years to come. It is our hopethat this collection will interest all serious students of history and philosophyof science, as well as those readers interested in the foundations of physicsand the philosophical implications of the quantum revolution.

We would like to take this opportunity to acknowledge our appreciation toall of our contributors, as well as to Bob Cohen. General Editor of the BostonStudies series, and to Annie Kuipers, Acquisitions Editor of KluwerAcademic Publishers, for their advice and encouragement in helping us tobring out this collection. Our thanks are also do to Loyola University forfinancial assistance in preparing the final drafts and to Bror Bemild for allow­ing 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 themomentous changes in physics that mark the quantum revolution. The theoryof quantum mechanics together with the theories of special and general rela­tivity are some of the greatest intellectual achievements in human history.But where the theories of relativity were more or less the product of a singleindividual, namely Albert Einstein, it took many physicists a long time toreach the final theory for the atom. This difference between the creation andthe development of these theories may reflect the fact that quantum mechan­ics departs to a even greater degree than relativity from the framework ofclassical mechanics. Although the theorie s of relativity expel concepts likeabsolute simultaneity, they still provide us with a deterministic description ofthe physical world as does classical mechanics. Quantum mechanic s, on thecontrary, permits only an indeterministic description of microphy sicalprocesses. The point of departure of Bohr's philosophy is an acceptance ofthis limitation , rather than the continued search for a description which woulddemand a classical deterministic account.

Bohr was fond of justifying his interpretation of quantum mechanic s byappeal to the analogy with relativity . Relativity had taught physicists thatphysical properties such as length, duration, and velocity are ascribable onlyrelative to a frame of reference, while the quantum revolution taught thatphysical properties can be attributed to an object only relative to an experi­mental situation . Both revolutionary theorie s share a point of origin in anempirical 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. Bothrevolutions involved the recognition of limitations in the use of descriptiveconcepts as a consequence of an analysis of what can be empirically deter­mined. Thus Bohr regarded both revolutions as teaching analogous epistemo­logical lessons, and tried to use this fact to convince Einstein of thesoundness of his interpretation of quantum mechanics.

The quantum revolution can be analyzed into two periods, each of which ischaracterized 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 themid 1920's, physicists in the avant garde realized that a limit to the divisibil­ity of classical dynamical properties such as energy had to be postulated inorder to give an account of such phenomena as black-body radiation, the pho­toelectric effect, and atomic spectra. This first insight became incorporatedinto Bohr's semi-classical atomic model of 1913, where he postulated that theenergy with which an electron is bound to its nucleus can occur only incertain quantized states. This model was central to the research program ofthe next decade, until physicists eventually came to recognize that some non­classical feature was still missing for a full description of quantum phenom­ena.

The indivisibility of the quantum of action on which Bohr's atomic modelwas based suggested to Bohr after 1927 that in interactions involving atomicsystems one cannot dynamically separate the object from the agencies ofobservation. Nevertheless, an unambiguous description of the results of anobservation requires predicating properties of the object, and this in turnimplies that we must make an arbitrary distinction in our description betweenobject and agencies of observation. Because this distinction does not reflect aphysical situation, but is a precondition of unambiguous description, anyproperties attributed to the object cannot be understood to be properties pos­sessed by the object independently of its interaction with the observinginstruments. After 1927 when Bohr began to argue for the need for comple­mentary descriptions, he began to stress that the indivisibility betweenobserved object and observing systems in atomic physics was analogous tothe unity of subject and object in the psychologist's attempt to providedescriptions of the observation of one's own consciousness . Bohr's refer­ences in the context of psychology to the observing "subject" have beenmisread as defending the view that he holds that in the context of physics adescription of observation must make reference to the observing subject quaconscious mind. The analogy Bohr wished to draw is on the epistemic level,namely that the distinction between the knowing subject and the knownobject is a logical demand for the unambiguous description of observation.Bohr's point is not to argue for the necessity of making a metaphysical dis­tinction between inanimate physical systems and consciousness .

This second major insight gained by the quantum revolution came withHeisenberg 's formulation of matrix mechanics and the subsequent derivationfrom it of the reciprocal limitation of the simultaneous measurement ofprecise values of certain physical properties. Bohr's reaction to Heisenberg'sdiscovery 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 onecould define unambiguously the classical descriptive concepts in a specificobservational context. Second he realized that a consistent quantum theorywould have to abandon the classical goal of providing both a space-timedescription and a causal account of the phenomena .

By this time wave-particle dualism had already become characteristic ofthe description of the full range of phenomena involving both matter andradiation, but no one had any clear understanding of what that fact implied.While Bohr had been thinking about the limitations of the classical descrip­tive concepts for most of this period, it was only now that he came to see theconnection between the limitations of the uncertainty principle and wave-par­ticle dualism. Thus he now recognized that both wave-particle dualism andthe uncertainty principle were manifestations of a deeper underlying comple­mentarity which restricted the Kantian scheme for the description of phenom­ena in terms of the spatio-temporal forms of intuition and the concept ofdeterministic causal connections between phenomena . In saying that proper­ties are "complementary" Bohr meant that they are incompatible but equallynecessary for a full description of the quantum system. The picture of naturemade possible by the classical mechanical account of the phenomena madeuse of simultaneous application of both of these modes of description . Bohr'snew argument that they could be applied only in a complementary fashion,therefore, had profound consequences for our understanding of how thephysicist's description of nature relates to the physical world. Understandingthese consequences forms a central task for comprehending Bohr's uniqueviewpoint.

Among these consequences are those which concern what Bohr called "thecustomary demand for visualization". Bohr is quite clear that we must"renounce" this classical demand, but his point is not that we must abandonusing particle and wave "pictures". Rather, he argues that we must under­stand their use in a different way from the literal interpretation they could begiven in the classical framework. They cannot be altogether rejected, becauseit is only through such pictures that we are able to interpret experimental phe­nomena as measurements determining the properties of atomic systems.However, while essential for this purpose, the proper understanding of thequantum description requires that we recognize that such pictures are"abstractions" or "idealizations" in the sense that they do not representatomic objects as they exist ' in themselves' .

Bohr regarded kinematic and dynamic properties as associated with theclassical wave and particle pictures of quantum mechanical objects . Younger

XVI INTRODUCTION

physicists such as Heisenberg may have originally hoped that a consistentdescription could omit reference to particles and/or waves, but Bohr arguedin his discussions with Heisenberg in early 1927 that it was necessary to pre­serve both of these sets of concepts for an unambiguous description of thephenomena. For Bohr the necessity for the concepts of space, time, andcausality lies in the nature of human experience and provides the continuityof empirical reference to evidential phenomena against a background ofchanging theory. These general concepts as refined into the physicist 'snotions of the kinematic properties and the dynamical properties of momen­tum and energy are "indispensable" for the physicist's understanding ofnature.

For those whose familiarity with Bohr's outlook derives from physics text­books, "complementarity" is often associated with the relationship betweenwave and particle pictures. However, in its initial presentation in the Comopaper of 1927, it is clear that Bohr intended complementarity to refer to therelationship between space-time description and the claims of causality. Thusthe connection between wave-particle dualism and the complementarity ofkinematic and dynamic properties remains a problematic issue both for theanalysis of Bohr's philosophy and generally for the interpretation of quantummechanics . The difficulties of understanding Bohr's position on this issue canbe seen in the variance of interpretations including that of Murdoch (1987),who sees Bohr as retaining an ontological commitment to the reality of parti­cles, in contrast with that of Beller (1992) who defends the view that in ComoBohr's "central message" was that the wave packet idea was "sufficient toresolve all the paradoxes of atomic structure" .

Following Bohr's initial presentation of his new viewpoint , the well­known opposition of Einstein shaped the further development of Bohr'sthinking in an essential way. Bohr had given the uncertainty principle notonly an epistemic reading but an ontic interpretation as well, whereasEinstein would have liked to confine his reading to a purely epistemic inter­pretation. This opposition reached its climax in 1935 with the publication ofthe now famous EPR thought-experiment. Here Einstein and his collaboratorsargued that quantum mechanics could not be a complete theory because onecould conceive of different physical situations in which one was free toattribute either of two complementary properties to one quantum object onthe basis of observations made on another object. Since there is no questionof a physical interaction or "disturbance" with the latter object, it seemstherefore that one is warranted in asserting that the real object has both prop­erties, contrary to what Bohr had claimed.

INTRODUCTION xvii

Bohr's reaction to this argument was to distinguish between two concep­tions of "physical reality". Einstein regards it as meaningful to talk counter­factually about the properties of an object apart from the circumstances oftheir empirical determination. However Bohr holds a concept of physicalreality which demands that properties are well-defined only in the context ofthe description of a particular observational phenomenon. For this reason , thedebate was transformed from a challenge to the uncertainty relations to thephilosophical 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 "meta­physical" theories of the nature of physical reality that could not be resolvedby any further empirical research. Furthermore other attempts at interpreta­tion or reformulation of the theory in terms of hidden variables all sufferedthe same fate of lacking any empirically detectable consequences.

The later 1980's have seen a considerable shake up in this uneasy standoffthat has prevailed between philosophy of nature and microphysics. To besure, there is at least as much dissension (possibly more) as there has everbeen. But in accord with an accelerating pace of publications in the area,there has grown an increasing felt sense ofneed for a philosophically satisfac­tory account of the quantum description of microsystems.

At least two forces have led to the breakup of what had been more or lessthe status quo since the time Bohr and Einstein debated the issues in the thir­ties. The first of these is well known and the subject of a great deal of atten­tion: the derivation of Bell-type inequalities and the experimental productionof phenomena which seem to force a revision of the conjunction of a fewvery deeply entrenched fundamental assumptions about physical reality . Asecond , less well noticed, but nevertheless influential force , is the weight ofshifting opinion about the views of Bohr and Einstein, and generally how theolder generation of founding father physicists really saw the issues.

The first of these two forces, Bell's theorem and its experimental tests, hastransformed what was originally considered a "metaphysical" debate betweenBohr and Einstein into a question of physics. Bell's work has been motivatedby a natural extension of the debate over EPR. The results of the variousexperimental tests of Bell's theorem in favor of quantum mechanics haveexcluded the possibility of local "hidden variable" theories, on which thehope of many of those who sought to avoid the limitations of the quantumdescription was based. Of course there is no possibility of establishing thecompleteness of the quantum description iIl some final sense , but at least itnow 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 ofthe violations of Bell's theorem, these developments have reawakenedseveral themes prominent in Bohr 's interpretation of quantum mechanics andhis reply to EPR. These themes include both Bohr's insistence on the "indi­visibility" of the interaction between the observed object and the agencies ofobservation, and his claims about the conditions for the well-defined predica­tion of properties of physical systems.

The second of these two trends in recent research in the philosophy ofquantum physics does not tum on questions directly in physics. Both Einsteinand Bohr have been the subject of intense scholarly publication of late. Onthe older, shallower , and historically unfounded view which finds prominentexpression in Popper and was spread by a generation of his earnest disciples,Bohr dismisses quantum mysteries with a glib instrumentalist line: save thephenomena and renounce the attempt to form mechanical models of thebehavior of microsystems . Any concept of "physical reality" is supposedlybanned as irrelevant to empirical science. Meanwhile, on this story Einsteinstrangely appears as the reactionary defender of an antiquated determinismand a classical realist ontology of particles possessing determinate mechani­cal properties .

As we have seen above, in the earlier phase of their debate Bohr andEinstein tended to see their opposition in terms of indeterminism versusdeterminism, but after EPR the central issue became the question of the rela­tionship between the quantum description and physical reality. Some work onBohr, including for example that of Hooker (1972), Folse (1985), Honner(1987), and Murdoch (1987), has argued for the nontraditional view that Bohrshould be termed a 'realist' because he does not doubt the objective reality ofatomic systems and his arguments are based upon what he considered a dis­covery about their real nature, namely that they had to be described as chang­ing their classical mechanical states discontinuously. Other recentinterpretations, including those of Fine (1986), Krips (1987), and Faye(1991), defend the traditional perception of Bohr as an 'anti-realist', but atleast agree to the extent that it is today scholarly anachronistic to characterizeBohr's outlook as a form of instrumentalist phenomenalism .

The work of Hooker (1972), Howard (1985), and Fine (1986) has shownthat by 1935 Einstein's discontent with the quantum description had shiftedfrom a rejection of indeterminism to the criticism that quantum theory wasincompatible with what is now called ' separability' . Here our improvedunderstanding 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 presupposi­tion which the quantum revolution requires abandoning is none other thanthis principle of separability . Therefore these new portraits of the old combat­ants reveal a grasp of the issues more significant for revived efforts in thephilosophy of nature than the old story that tells of the combat betweenrealism and instrumentalism. Moreover our newer more historically wellfounded picture of the actual debates allows us to focus more sharply on whatphilosophy has to learn from Bell's results.

Although this revisionism in the stories of the founding fathers hasattracted some attention, it was the coincidence that the historical issue ofrealism/anti-realism reawoke from the dogmatic slumber of positivism atroughly the same time that Bell phenomena began to attract a great deal ofattention that the two combined could put a great deal of pressure on the oldimpasse leading to the breakup of the status quo. Although Bohr's own con­ception of his philosophy was not something that could be well expressed interms of the traditional philosophical categories of realism and idealism, hisphilosophy is pertinent to these debates . Many of today's philosophers are ineffect searching for a mean between the unacceptable extremes of a naivemetaphysical realism and an impoverished instrumentalist rejection of thenotion of physical reality. It is in just this philosophical no man's land thatBohr's philosophy may well be located.

II.

In this volume we have asked each contributor to address issues in Bohr'sphilosophy as they relate to problems discussed in philosophy at the presenttime. It is our belief, that though much remains to be said about the historicaldevelopment of quantum physics and its interpretation, the real significancefor Bohr's views to philosophers generally lies in the fact that his insights andarguments touch on many issues at the forefront of today's discussions inphilosophy of science and epistemology .

Given the current concern in philosophy of science with issues surroundingthe question of realism, it is not surprising that many contributors to thisvolume have sought to relate Bohr's position to the debate between realistsand their opponents. Among these, Edward MacKinnon argues that althoughBohr is "methodologically" on the side of the anti-realists, his position hasmuch in common with the common sense realism of Donald Davidson.McKinnon considers Bohr's realism "paradoxical" in the sense that thoughhe 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 seton the proper use of our descriptive concepts. The physicist's use of the for­malism of quantum mechanics requires no ontological commitments.Nevertheless, Bohr's view has certain affinities to Davidson's in that thephysical models and the language in terms of which Bohr describes themcarry with them unavoidable ontological commitments to the existence ofvarious kinds of physical objects . Among those inherent in Bohr's discourseare those that include a commitment to the reality of microsystems.

Another contribution concerned with realism is David Favrholdt's charac­terization of Bohr as a realist who rejected the classicalimage of scientificknowledge as a "God's eye view" of nature. Bohr's argument for the neces­sity of classical concepts turns on the need to use ordinary language to makedescription unambiguous by marking a sharp distinction between observingsystem and object. But its use to construct an image mirroring how naturewould look to an omniscient being is now blocked because the discovery of afinite quantum of action in all observations implies that scientific knowledgeis 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 dis­tinguish Bohr's view from the subjective idealism of Berkeley or Machianphenomenalism, which some have misread in Bohr's statements. Favrholdtcalls attention to Bohr's reminder that 'reality' also is a concept, the limits ofthe correct use of which we must also learn to modify with the growth ofknowledge .

Henry Folse continues the discussion of how Bohr's position relates to thephilosophers' debates over realism. The boundaries between realist and anti­realist views have shifted in the historical move of philosophical discussionfrom Kantian concerns with the conditions for the applicability of concepts,to debates over the reality of atoms, to questions of semantics, correspon­dence and representation. In these different cases Bohr can be seen as alliedwith the 'realist' side because he defends the view that 'understanding thephenomena' requires more than empirically adequate predictability. But hisidentification with the realist side is never total, for in each case Bohr attackspositions traditionally allied to realism . Folse's categorization of Bohr as arealist rests on his claim that talk about complementary phenomenal appear­ances of atomic systems requires essential reference to atomic systems con­sidered apart from the experimental phenomena in which they are said to be'observed'. If we abandon the classical conception of knowledge based onmechanistic representationalism and the ideal of visualizability, we can holdthat 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 heis committed ontologically to the independent reality of the objects of atomicphysics as well as to the belief that quantum theory enables us to communi­cate whatever it is possible to know about them.

Reflecting a similar concern with locating a position for Bohr between therealism of classical physics and an anti-realist instrumentalism, Henry Kripssees Bohr as arguing against the Enlightenment ideal of a ' universal realism 'in favor of a 'local realism' restricted to representing reality through this orthat conceptual scheme in different contexts and unable to attain the classicalgoal of a single 'universal' picture of reality. Krips concludes that because ofthe weakness of Bohr 's claim that we are restricted to the 'classical con­cepts' , he cannot make good any argument for the necessity of the move to a'local realism' and that therefore Enlightenment ideals of objectivity need notyet be banished from physics. Krips thinks that Bohr 's view of classical con­cepts is insupportable, but he does not attack the weaknesses of Bohr's argu­ment directly; instead he hopes to refute Bohr by showing the possibility of a'meaningful non-classical mode of description ' which 'Bohr tells us cannotexist'. This non-classical mode of description takes the density operator asrepresenting an objective probability inhering in the atomic object.

The Bohr-Einstein debate provides a popular avenue of approach for ana­lyzing the philosophical impact of Bohr's viewpoint. Dugald Murdochfollows this route (already pursued by Fine, 1986 and Howard, 1985) to tracethe debate up to the choice between rejecting the completeness of the quantumdescription (Einstein) or the principle of separability (Bohr). Murdoch holdsthat Bohr rejected separability as a semantic principle about what we can sayabout atomic systems, as well as ontically, about the physical reality thusdescribed. But Bohr's position in this regard is based on a verificationistsemantics (from pragmatist rather than positivist sources) and the conse­quences of an ontic non-separability are far from clear. Murdoch concludesthat these facts make Einstein's rejection of completeness philosophically thepreferable route, but this would force in physics some sort of ensemble inter­pretation of quantum mechanics. However, so-called 'orthodox' ensembleinterpretations are all in deep difficulties (as Einstein well knew) and seem tobe fatally wounded by Bell phenomena. So the only hope for Einstein's philo­sophically more attractive position is to be found in 'non-orthodox ensembleinterpretations', but these seem 'far-fetched' . Thus Murdoch alters the usualperception of Bohr as the clear victor, concluding that while Einstein's objec­tion 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, ofcourse, a state of affairs which could change in the future .

The Bohr-Einstein debate appears in Jan Faye's contribution in the form ofthe debate between anti-realists and realists. What distinguishes Bohr fromEinstein, in Faye's view, are alternative semantic theories; Einstein allowsstatements with empirical truth conditions that are in principle unverifiable tobe meaningful, whereas Bohr's prohibits it. However, it is a mistake to hopethat EPR and Bell type experiments can allow an empirical determination ofthe correct semantics. Faye defends his 'objective anti-realist' interpretationof Bohr with respect to the various possibilities for responding to the recogni­tion that Bell's theorem requires abandoning either separability or locality.Whether the realist takes the route of abandoning locality or abandoning sep­arability, he remains in the position of making claims which are in principleunverifiable. An anti-realist may opt for non-locality, but then he mustpresent empirical evidence for such non-locality, which at present does notexist. Thus the anti-realist who defends non-separability is in the strongestposition , and this, Faye claims, is essentially Bohr 's view.

It is through the heritage of the EPR challenge that the Bohr-Einsteindebate continues to echo through today 's discussions of the philosophicalconsequences of quantum physics . However , Mara Beller and Arthur Finewarn against reading contemporary concerns into the actual historicalexchange between Bohr and Einstein . Although it is common to see the ulti­mate divide between the protagonists in terms of rival conceptions of ' physi­cal reality', Beller and Fine argue that Bohr essentially agreed with the EPRcriterion of reality. Bohr tried to undercut EPR's reasoning by finding an"ambiguity" in the "no disturbance condition", but Bohr's "physical realiza­tion of the EPR case . .. involves mechanical effects not present in EPR" andso cannot successfully refute EPR's conclusion . The real significance of EPRfor understanding Bohr's thought lies in the fact that though he could origi­nally defend the need for complementary descriptions by appeal to a "robustphysical disturbance" in the quantum interaction, after EPR Bohr could main­tain his view only by taking refuge in a positivistic verificationist semantics.Although there is tension between the more realistic outlook in Bohr 's appealto interaction and the positivistic outlook of verificationism, Beller and Fineconclude that by reinterpreting Bohr's talk about "exchange of momentum ",it is possible to effect a reconciliation between the two. Neverthele ss, becauseof deep philosophical difficulties inherent in such a positivistic defense , it canhardly be said that "Bohr got the better of Einstein" in the final act of theirmomentous confrontation.

INTRODUCTION xxiii

Don Howard also attempts to sharpen the focus of our image of the differ­ences separating Bohr and Einstein, but he seizes on Bohr's controversialclaims concerning the indispensable status of 'classical concepts'. Howardasks whether the distinction between that which must be described through theframework of quantum mechanics and that which must be described classi­cally can coincide with the distinction between observed object and observinginstruments. Howard argues against this coincidence and develops instead thethesis that, on the one hand, the proper quantum mechanical description of ameasurement interaction corresponds to what is treated as a 'pure case' in theformalism in which observed and observing systems are assigned a single'inseparable' state. On the other hand, the objective description of a measure­ment outcome requires that, as Bohr was wont to urge, an arbitrary but neces­sary distinction must be made between observing and observed systems. Thisrequires describing the observational interaction as though the two systemswere in separate classical states, and corresponds in the formalism to a'mixture' appropriate to the particular measurement being performed. Thusthe crucial distinction between classical and quantum descriptions in reflectedformally in the distinction between mixtures and pure cases.

Over the years many scholars have written on the similarities betweenBohr and Kant, and this continues to be a subject of interest in several of thearticles included here. Cliff Hooker sees both Bohr and Einstein as challeng­ing the ideal of intelligibility embodied in classical physics, which on Kant'sanalysis upholds a criterion of 'intelligibility' which requires that the descrip­tion of phenomena meet certain conditions of uniqueness. Hooker employshis ground breaking (1972) analysis of the Bohr-Einstein debate to see howthese challenges differ from one another and in what way Bohr's entails amore radical challenge to the classical ideal of intelligibility . Hooker firstdevelops a contrast between Newtonian and Kantian ideals of theoreticalunderstanding. He then argues that Einstein's departure from Kant, withReichenbach'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 inresponse to what has been discovered in the quantum revolution. Bohr arguesthat it is the challenge to the subject/object distinction posed by the quantumof action which implies that the Kantian criteria can be met only in a comple­mentary manner. Each challenge has its weaknesses, and Hooker concludeswith a reminder of the "depths of our ignorance" with respect to an ideal ofintelligibility consonant with contemporary physics.

Catherine Chevalley strives to shed light on Bohr's discourse by consider­ing the nineteenth century discussion of 'Anschaulichkeit' and 'Symbol'

XXIV INTRODUCTION

deriving from Kant's third Critique. The study of the relation between lan­guage and reality from Kant, through Goethe, Humboldt, and Hemholtz cameto elevate the 'symbolic' presentation in scientific knowledge to a level Kanthad accorded to the' intuitive' . She shows how this largely forgotten discus­sion shaped Bohr's choice of terminology, such that in his mature works hischoice of words was such as to associate 'anschaulichkeit' ('intuition') withthe classical mode of description, while the quantum description becameassociated with 'Symbol' . Bohr's 'epistemological lesson' , teaching that weneed to combine complementary descriptions, can be seen as a way of com­pensating for what is lost in the quantum revolution with the passing of theclassical anschaulich mode of presentation in favor of an understanding ofthe phenomena which is purely 'symbolic'. Chevalley suggests , but does notexplore, the possibility that this conception of complementary modes ofdescription may illuminate issues of concern to philosophers involved withrelating 'analytical' and 'continental' traditions in philosophy .

Indeed, Bohr's concern with setting the limits to the unambiguous use ofconcepts in the description of nature echoes the general concern of philoso­phers 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 theanalytic tradition. John Hanner employs the metaphor of the scientist as'reading the book of nature' to relate Bohr's philosophical concerns with lan­guage to the deconstructivist concern with the interpretation of text. In thisregard Honner explores the relation of Bohr's views on the description ofnature to the critique of objectivity found in postmodernist philosophers suchas Derrida. Bohr's conception of 'complementarity' has something incommon with Derrida's notion of 'supplementarity' but while both rejectEnlightenment conceptions of 'objectivity', Bohr's complementarity doctrineallows him to be more optimistic about achieving 'objectivity' in the descrip­tion of nature than is possible according to the deconstructivist critique oflanguage.

Ulrich Riiseberg considers Bohr's philosophy as carrying within it a' hidden historicity' reflecting its historical development. Roseberg contrastsReichenbach's analytical notion of the history of science as a reconstructionwhich masks rather than reveals the history of the science it seeks to recon­struct, with his own dialectical notion of the history of science based on theHegelian tradition. Roseberg sees in Bohr's philosophical viewpoint of com­plementarity the dialectical play of ideas that the historical Niels Bohr livedthrough. This 'hidden historicity' is lost from sight in the dominant analyticalmode 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 ofBohr '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 willingto make slight modifications in that formalism, as has been done by DavidBohm, then it is possible to provide an account of atomic processes whichretains a deterministic ontology that Bohr opposes. Cushing argues thatBohm's description of physical reality is quite similar to Bohr's in that healso emphasizes that the observed values are dependent on the whole experi­mental context and that atomic physics must describe the interaction betweenthe observing instruments and atomic systems. But in contrast to Bohr, Bohmbelieves that the experiments disturb the object such that its classical trajec­tory, while real, is unobservable. This disturbance is a consequence of thenon-local field which Bohm associates with the otherwise classical particle.

Henry Stapp agrees with Cushing that, by providing a classical realistinterpretation , David Bohm's contribution has put the lie to Bohr 's claim thatsuch an interpretation of quantum mechanics is impossible. For this reason,as well as because of demands placed on physics by quantum cosmology andby Bell's work, Stapp sees Bohr's influence as loosing its grip on the mindsof today's physicists . Bohr 's position is best seen as only the "right face" thatis 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 measure­ment outcomes to imply that quantum theory is about what we can 'say' or'know' about nature; the state function is basically an epistemological con­struct. However Stapp advocates the full "ontologicalization" of the statefunction along lines suggested by Heisenberg's reference to the actualizationof objective potentialities. The result will be a conception of nature in termsof entities that are more "idealike" than "matterlike", as in the classical con­ception. This new perspective allows for the possibility of integrating con­sciousness into the physical sciences. Though no one now possesses such atheory, Stapp is optimistic about progress following this line of thought.

Bohr's philosophy has of course had its greatest impact among those con­cerned with paradoxes in the interpretation of quantum mechanics, but it hasby no means been exclusively confined to this matter. David Kaiser exploresthe generally ignored area of Bohr's work in nuclear physics and how hisunique philosophical approach shaped and continues to influence develop­ments in contemporary particle physics. Kaiser shows how Bohr's model ofthe compound nucleus implied methodological postulates which continue toguide the analysis of particle interactions in high energy physics. But these

XXVI INTRODUCTION

postulates for how to analyzes interactions raise ontological questions con­cerning elementary particles which are quite different from those of elemen­tary quantum mechanics. As both realist and anti-realist interpreters of Bohragree, the independent reality of atoms, electrons, and protons is not indispute, but what is at issue is the conditions necessary for meaningful asser­tions about them. Thus the statistical nature of the predictions of elementaryquantum mechanics raises ontological questions about the existence only ofproperties of entities, not the entities themselves . However, Kaiser shows thatat least in some cases, the evidence for the existence of elementary particlesof particular kinds is itself statistical in nature, thus ironically reintroducingmodem-day analogues to the questions about the existence of the supposedelementary constituents of matter that perturbed earlier periods of natural phi­losophy.

Going beyond the domain of physics, Bohr carried his epistemologicallesson into the areas of psychology and biology; the latter is discussed byPaul Hoyningen-Huene . Bohr's interest in biology was shaped by the Kantiantradition which opposed mechanistic and teleological accounts . While earlierwriters like Folse (1990) and Faye (1991) have seen Bohr's primary conclu­sion in this area to be his claim that mechanistic and teleological descriptionsmust be used complementarily in order to give a full description of the bio­logical phenomena, Hoyningen-Huene relates Bohr's position to the philo­sophical question of whether biological laws are reducible to physics . Bohr 'sdefense of epistemological, ontological, and methodological anti-reduction­ism is based on the biological analogue to the complementarity that exists inquantum physics. On Hoyningen-Huene's analysis, complementarity pro­vides a means for defending the view that even though different aspects ofone phenomenon exhibit a relation of 'theoretical irreconcilability', they arenot necessarily in direct contradiction. When description of these differentaspects entails such a complementary relation, as is the relation between thephysicists' description of vital processes in the terms of molecular biologyand the purposive accounts of much traditional biology, then Bohr holds thatneither can be reduced to the other. However, Hoyningen-Huene criticizesBohr's conclusion for having failed to show that there are some biologicalfunctions the descriptions of which are theoretically irreconcilable with thedescriptions of molecular biology.

The reader will search in vain for a single common vision of Bohr runningthroughout all of these contributions. But we believe that the essays of thiscollection do permit us to make some interesting generalizations. In the firstplace 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 allgreat originators in human thought, his views will no doubt continue toinspire different rival interpretations. But this fact should hardly lead us todespair over ever hoping to make sense of Bohr's position. And the essaysincluded here testify to this.

Second, although there is no one interpretation offered here, many pointsof agreement do unite our contributors. Virtually all are agreed on the central­ity of Bohr's controversial view with regard to the classical concepts.Virtually all see Bohr as attempting to revise our understanding of scientificknowledge by restricting or altering the application of these concepts. No onesees Bohr as denying the reality of atomic objects , but all see him as con­cerned with how they must be described. None gives a 'subjectivist' or 'phe­nomenalist' reading of Bohr's position. All are agreed that Bohr's' epistemological lesson' concerns under what conditions we can say what­ever it is that we can say about the world.

Third, in spite of much mutual agreement (or non-disagreement) there alsoremain deep philosophical differences separating the various views of Bohrpresented here . Many of our contributors see Bohr's position - no matter howsubtle or profound - as ultimately an unsatisfactory response to the problemsit was designed to solve, although these critical conclusions are reached for avariety of different reasons . Moreover, even among those who do see Bohr'sposition as satisfactory and defensible, there remain serious disagreements onwhat that position is and what philosophical consequences follow from it.Nonetheless it is our hope that by bringing these various different images ofBohr together in one place, we can highlight the relevance of Bohr's view­point for contemporary philosophical problems.