CONJECTURES AND REFUTATIONS -i- By the same Author The Open Society and Its Enemies Vol. I: The Spell of Plato Vol. II: The High Tide of Prophecy: Hegel, Marx, and the Aftermath The Poverty of Historicism The Logic of Scientific Discovery -ii- CONJECTURES AND REFUTATIONS The Growth of Scientific Knowledge by KARL R. POPPER BASIC BOOKS, Publishers NEW YORK LONDON -iii- © Karl R. Popper 1962 Manufactured in the United States of America -iv- TO F. A. VON HAYEK -v- Experience is the name every one gives to their mistakes. OSCAR WILDE -vi- PREFACE THE ESSAYS and lectures of which this book is composed are variations upon one very simple theme--the thesis that we can learn from our mistakes. They develop a theory of knowledge and of its growth. It is a theory of reason that assigns to rational arguments the modest and yet important role of criticizing our often mistaken attempts to solve our problems. And it is a theory of experience that assigns to our observations the equally modest and almost equally important role of tests which may help us in the discovery of our mistakes. Though it stresses our fallibility it does not resign itself to scepticism, for it also stresses the fact that knowledge can grow, and that science can progress--just because we can learn from our mistakes. The way in which knowledge progresses, and especially our scientific knowledge, is by unjustified (and unjustifiable) anticipations, by guesses, by tentative solutions to our problems, by conjectures. These conjectures are controlled by criticism; that is, by attempted refutations, which include severely critical tests. They may survive these tests; but they can never be positively justified: they can neither be established as certainly true nor even as 'probable' (in the sense of the probability calculus). Criticism of our conjectures is of decisive importance: by bringing out our mistakes it makes us understand the difficulties of the problem which we are trying to solve. This is how we become better acquainted with our problem, and able to propose more mature solutions: the very refutation of a theory--that is, of any serious tentative solution to our problem--is always a step forward that takes us nearer to the truth. And this is how we can learn from our mistakes. As we learn from our mistakes our knowledge grows, even though we may never know--that is, know for certain. Since our knowledge can grow, there can be no reason here for despair of reason. And since we can never know for certain, there can be no authority here for any claim to authority, for conceit over our knowledge, or for smugness. Those among our theories which turn out to be highly resistant to criticism, and which appear to us at a certain moment of time to be better approximations to truth than other known theories, may be described, together with the reports of their tests, as 'the science' of that time. Since none of them can be positively justified, it is essentially their critical and progressive character -the fact that we can argue about their claim to solve our problems better than their competitors--which constitutes the rationality of science. This, in a nutshell, is the fundamental thesis developed in this book and applied to many topics, ranging from problems of the philosophy and history -vii- of the physical sciences and of the social sciences to historical and political problems. I have relied upon my central thesis to give unity to the book, and upon the diversity of my topics to make acceptable the marginal overlapping of some of the chapters. I have revised, augmented, and re-written most of them, but I have refrained from changing the distinctive character of the lectures and broadcast addresses. It would have been easy to get rid of the tell-tale style of the lecturer, but I thought that my readers would rather make allowances for that style than feel that they had not been taken into the author's confidence. I have let a few repetitions stand so that every chapter of the book remains self-contained. As a hint to prospective reviewers I have also included a review--a severely critical one; it forms the last chapter of the book. I have excluded all those papers which presuppose acquaintance on the part of the reader with technicalities in the field of logic, probability theory, etc. But in the Addenda I have put together a few technical notes which may be useful to those who happen to be interested in these things. The Addenda and four of the chapters are published here for the first time. To avoid misunderstandings I wish to make it quite clear that I use the terms 'liberal', 'liberalism', etc., always in a sense in which they are still generally used in England (though perhaps not in America): by a liberal I do not mean a sympathizer with any one political party but simply a man who values individual freedom and who is alive to the dangers inherent in all forms of power and authority. ACKNOWLEDGEMENTS THE PLACE and date of the first publication of the papers here collected are mentioned in each case at the bottom of the first page of each chapter. I wish to thank the editors of the various periodicals for giving me permission to include these papers in the present book. I have been helped in various ways with the revision of the text, the reading of the proofs, and the preparation of the index, by Richard Gombrich, Lan Freed and Dr. Julius Freed, J. W. N. Watkins, Dr. William W. Bartley, Dr. Ian Jarvie, Bryan Magee, A. E. Musgrave, and S. C. Parikh. I am greatly indebted to all of them for their help. K. R. P. Berkeley, California Spring 1962 -viii- CONTENTS Preface vii INTRODUCTION On the Sources of Knowledge and of Ignorance 3 CONJECTURES 1 Science: Conjectures and Refutations 33 Appendix: Some Problems in the Philosophy of Science 59 2 The Nature of Philosophical Problems and their Roots in Science 66 3 Three Views Concerning Human Knowledge 97 1The Science of Galileo and Its New Betrayal 97 2The Issue at Stake 100 3The First View: Ultimate Explanation by Essences 103 4The Second View: Theories as Instruments 107 5Criticism of the Instrumentalist View 111 6The Third View: Conjectures, Truth, and Reality 114 4 Towards a Rational Theory of Tradition 120 5 Back to the Presocratics 136 Appendix: Historical Conjectures and Heraclitus on Change 153 6 A Note on Berkeley as Precursor of Mach and Einstein 166 7 Kantapos;s Critique and Cosmology 175 1Kant and the Enlightenment 176 2Kantapos;s Newtonian Cosmology 177 3The Critique and the Cosmological Problem 178 -ix- 4Space and Time 179 5Kant's Copernican Revolution 180 6The Doctrine of Autonomy 181 8 On the Status of Science and of Metaphysics 184 1Kant and the Logic of Experience 184 2The Problem of the Irrefutability of Philosophical Theories 193 9 Why are the Calculi of Logic and Arithmetic Applicable to Reality? 201 10 Truth, Rationality, and the Growth of Scientific Knowledge 215 1The Growth of Knowledge: Theories and Problems 215 2The Theory of Objective Truth: Correspondence to the Facts 223 3Truth and Content: Verisimilitude versus Probability 228 4Background Knowledge and Scientific Growth 238 5Three Requirements for the Growth of Knowledge 240 Appendix: A Presumably False yet Formally Highly Prob- able Non-Empirical Statement 248 REFUTATIONS 11 The Demarcation Between Science and Metaphysics 253 1Introduction 253 2My Own View of the Problem 255 3Carnap's First Theory of Meaninglessness 258 4Carnap and the Language of Science 264 5Testability and Meaning 273 6Probability and Induction 280 12 Language and the Body-Mind Problem 293 1Introduction 293 2Four Major Functions of Language 295 3A Group of Theses 295 4The Machine Argument 296 5The Causal Theory of Naming 297 6Interaction 298 7Conclusion 298 -x- 13 A Note on the Body-Mind Problem 299 14 Self-Reference and Meaning in Ordinary Language 304 15 What is Dialectic? 312 1Dialectic Explained 312 2Hegelian Dialectic 324 3Dialectic After Hegel 331 16 Prediction and Prophecy in the Social Sciences 336 17 Public Opinion and Liberal Principles 347 1The Myth of Public Opinion 347 2The Dangers of Public Opinion 349 3Liberal Principles: A Group of Theses 350 4The Liberal Theory of Free Discussion 352 5The Forms of Public Opinion 353 6Some Practical Problems: Censorship and Monopolies of Publicity 353 7A Short List of Political Illustrations 353 8Summary 354 18 Utopia and Violence 355 19 The History of Our Time: An Optimist's View 364 20 Humanism and Reason 377 ADDENDA Some Technical Notes 385 1Empirical Content 385 2Probability and the Severity of Tests 388 3Verisimilitude 391 4Numerical Examples 397 5Artificial vs. Formalized Languages 398 Index of Names 401 Index of Subjects 406 -xi- [This page intentionally left blank.] -xii- INTRODUCTION -1- True philosophers who are burning with love for truth and learning never see themselves . as wise men, brim-full of knowledge . For most of them would admit that even the very greatest number of things of which we know is only equal to, the very smallest fraction of things of which we are ignorant. Nor are these philosophers so addicted to any kind of tradition or doctrine that they suffer themselves to become their slaves, and thus lose their liberty. WILLIAM HARVEY -2- ON THE SOURCES OF KNOWLEDGE AND OF IGNORANCE It follows, therefore, that truth manifests itself . BENEDICTUS DE SPINOZA . it is impossible for us to think of any thing, which we have not antecedently felt, either by our external or internal senses. DAVID HUME THE TITLE of this lecture is likely, I fear, to offend some critical ears.