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ON THEORIES ON THEORIES Logical Empiricism and the Methodology of Modern Physics WILLIAM DEMOPOULOS Edited with a Foreword and Afterword by Michael Friedman HARVARD UNIVERSITY PRESS Cambridge, Mas sa chu setts & London, England • 2022 Copyright © 2022 by the President and Fellows of Harvard College All rights reserved Printed in the United States of Amer i ca First printing 9780674269729 (EPUB) 9780674269712 (PDF) The Library of Congress has cataloged the printed edition as follows: Names: Demopoulos, William, author. | Friedman, Michael, 1947– editor. Title: On theories : logical empiricism and the methodology of modern physics / William Demopoulos ; edited with a foreword and afterword by Michael Friedman. Description: Cambridge, Mas sa chu setts : Harvard University Press, 2022. | Includes bibliographical references and index. Identifiers: LCCN 2021020465 | ISBN 9780674237575 (cloth) Subjects: LCSH: Empiricism. | Physics— Philosophy. | Quantum theory. | Science— Philosophy. Classification: LCC B816 .D46 2021 | DDC 171 / .2— dc23 LC rec ord available at https:// lccn . loc . gov / 2021020465 CONTENTS Editor’s Foreword vii Introduction 1 1 Logical Empiricist and Related Reconstructions of Theoretical Knowledge 18 1.1 The Partial Interpretation Account of Theories 18 1.2 Carnap on Ramsey Sentences and the Explicit Definition of Theoretical Terms 20 1.3 A Proposal of David Lewis and Two Theorems of John Winnie 24 1.4 Putnam’s Model- Theoretic Argument 32 1.5 Ramsey on Russell’s Analy sis of Matter and the Partial Interpretation of Theories 39 1.6 Constructive Empiricism and Partial Interpretation 53 2 Molecular Real ity 64 2.1 The Molecular Hypothesis 64 2.2 Molecular Real ity and Brownian Motion 65 vi Contents 2.3 The Nature and Status of Perrin’s “Connecting Link” 78 2.4 Perrin’s Argument for Molecular Real ity 83 2.5 Thomson and the Constitution of Cathode Rays 93 3 Poincaré on the Theories of Modern Physics 100 3.1 Poincaré on “True Relations” 100 3.2 Robustness versus Consilience 110 3.3 Poincaré and Scientific Realism 113 3.4 Russell and Poincaré 116 4 Quantum Real ity 121 4.1 Bohr on the Primacy of Classical Concepts 121 4.2 Complementarity, Completeness, and Einstein’s Local Realism 139 4.3 Bell’s Theorem and Einstein’s Local Realism 154 4.4 Quantum Mechanics and Real ity 172 Editor’s Afterword 187 Notes 205 Bibliography 225 Acknowl edgments 235 Index 239 EDITOR’S FOREWORD William (“Bill”) Demopoulos died on May 29, 2017, a fter an approxi- mately ten- year b attle with lymphoma, leaving b ehind the manuscript On Theories. The argument of the manuscript was essentially complete, although the text needed some obvious editorial revisions, especially in connection with the Bibliography. More seriously, however, Demopoulos felt that a final chapter was needed which would perspicuously explain the relationship between the fourth chapter on quantum real ity and the earlier chapters, particularly the previous two chapters covering molecular real ity and Poincaré’s reactions to it. Unfortunately, t here was not enough time left, at the end, for Demopoulos to make much pro gress on this final chapter, which would have served as the counterpart of his In- troduction (formerly entitled “Overview of This Study”) at the begin- ning. So, when time was clearly growing shorter, Demopoulos and I agreed that, if needed, I would do my best to prepare the manuscript for publication in accordance with his wishes and see it through to press. All of Demopoulos’s close friends agreed that the last de cade of his life was extraordinarily productive. It culminated, in par tic u lar, with the collection of essays Logicism and Its Philosophical Legacy (2013) and the pre sent book, On Theories, which, for its part, represents the culmina- tion of his work in the philosophy of physical science. During this pe- riod, Demopoulos and I spoke at length by telephone almost every viii editor’s Foreword weekend. And it was from this experience, together with regular visits to London, Ontario, that I derived my appreciation of what he was at- tempting to do in On Theories. As I was considering what I could reasonably accomplish in the wake of his death, however, it quickly became clear that I was in no position to write the final chapter that Demopoulos might have written had he lived. He did not leave b ehind even a short draft of this chapter, and, more importantly, my own philosophical style is very dif er ent from his. What I could reasonably attempt, first, was to locate Demopoulos’s argument against the background of his more re- cent work in the philosophy of science (as represented in Logicism and Its Philosophical Legacy) and, second, by locating his discussion of quantum real ity against the background of his ongoing work on the interpretation of quantum mechanics throughout his c areer. The pre sent Foreword is devoted to the first; the concluding Afterword to the second. Logicism and Its Philosophical Legacy, not surprisingly, focuses cen- trally on views of the relationship between logic and mathem atics in the classical logicist tradition that begins with Frege and continues with Rus- sell, Ramsey, and Carnap. Two of that book’s essays, however, “Carnap’s Thesis” (chapter 2) and “On Extending ‘Empiricism, Semantics, and Ontology’ to the Realism- Instrumentalism Controversy” (chapter 3), in- troduce new material concerning the relationship between a priori mathematical and empirical physical theories that is especially relevant to the argument of On Theories. “Carnap’s Thesis” proposes a novel understanding of Carnap’s distinction between a priori and empirical theories drawing on Demopoulos’s earlier work on neo- logicism and Hume’s Princi ple, as discussed in De- mopoulos’s impor tant edited volume Frege’s Philosophy of Mathe matics (1995). Already here Demopoulos had suggested that Hume’s Princi ple is not best understood as an analytic truth but rather as a criterion of identity, which, in good Fregean style, can be seen as an analy sis of the concept of natu ral number but not as a reductive analytic definition of this concept within pure logic; this, in the end, is the lesson of the failure of Axiom V of the Grundgesetze. The resulting novel understanding of editor’s Foreword ix Carnap’s distinction then follows as a kind of corollary. The criterion of identity provided by Hume’s Princi ple applies equally to applied as well as pure arithmetic, insofar as any sortal concept whatsoever, and, in par tic u lar, any empirical sortal concept (such as “ people in this room”) figures in an account of the application of the concept of number (via Hume’s Princi ple) to any appropriate empirical domain. By contrast, criteria of identity for empirical concepts in physics, such as temporal simultaneity and physical spatial congruence, are constrained by clearly empirical rather than purely logical factors. Temporal simultaneity, for example, is determined by empirical methods of mea surem ent (using light signals and the like), and so, of course, is physical spatial congruence (as in Einstein’s appeal to practically rigid bodies). Here we find the germ of the asymmetry Demopoulos emphasizes between physical theories and logico- mathematical theories in the work of Carnap. But the r eally decisive asymmetry, from the point of view of On Theories, emerges in the following chapter 3 of “On Extending ‘Em- piricism, Semantics, and Ontology’ to the Realism- Instrumentalism Controversy” (Demopoulos 2013) (which had previously appeared in the Journal of Philosophy in 2011). Demopoulos is here concerned, among other t hings, with replying to an objection to Carnap’s paper raised by Penelope Maddy in Second Philosophy: A Naturalistic Method (2007). The objection is that Carnap cannot provide a reasonable interpretation of the question of whether atoms and molecules exist, since we begin with a simpler language for recording scientific observations (say, the “ thing language”), and we must then extend this language to contain theoretical terms (say, “atom” or “electron”) before we can even raise the question of the existence of such t hings. But, according to Carnap (Maddy’s objection goes), all choices of language or linguistic framework must be purely pragmatic or conventional, which is precisely the mark of an external rather than internal question. This (the objection con- cludes) flies in the face of the fact that the surprising evidence marshalled by Einstein and Jean Perrin based on the phenomenon of Brownian motion provides a perfectly good internal answer to the question of