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Thomas S. Kuhn (1922–96) was the Laurence Rockefeller Professor Emeritus of linguistics and philosophy at the Massachusetts Institute of Technology. His books include The Essential Tension; Black-Body Theory and the Quantum Discontinuity, 1894–1912; and The Copernican Revolution. The University of Chicago Press, Chicago 60637 The University of Chicago Press, Ltd., London © 1962, 1970, 1996, 2012 by The University of Chicago All rights reserved. Published 2012. Printed in the United States of America 21 20 19 18 17 16 15 14 13 12 1 2 3 4 5 ISBN-13: 978-0-226-45811-3 (cloth) ISBN-13: 978-0-226-45812-0 (paper) ISBN-13: 978-0-226-45814-4 (e-book) ISBN-10: 0-226-45811-3 (cloth) ISBN-10: 0-226-45812-1 (paper) ISBN-10: 0-226-45814-8 (e-book) Library of Congress Cataloging-in-Publication Data Kuhn, Thomas S. The structure of scientific revolutions / Thomas S. Kuhn ; with an introductory essay by Ian Hacking.— Fourth edition. p. cm. Includes bibliographical references and index. ISBN-13: 978-0-226-45811-3 (cloth : alkaline paper) ISBN-10: 0-226-45811-3 (cloth : alkaline paper) ISBN-13: 978-0-226-45812-0 (paperback : alkaline paper) ISBN-10: 0-226-45812-1 (paperback : alkaline paper) [etc.] 1. Science—Philosophy. 2. Science—History. I. Hacking, Ian. II. Title. Q175.K95 2012 501—dc23 2011042476 This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). The Structure of Scientific REVOLUTIONS FOURTH EDITION THOMAS S. KUHN With an Introductory Essay by Ian Hacking The University of Chicago Press Chicago and London Contents Introductory Essay by Ian Hacking Preface I Introduction: A Role for History II The Route to Normal Science III The Nature of Normal Science IV Normal Science as Puzzle-solving V The Priority of Paradigms VI Anomaly and the Emergence of Scientific Discoveries VII Crisis and the Emergence of Scientific Theories VIII The Response to Crisis IX The Nature and Necessity of Scientific Revolutions X Revolutions as Changes of World View XI The Invisibility of Revolutions XII The Resolution of Revolutions XIII Progress through Revolutions Postscript—1969 Notes Index Introductory Essay IAN HACKING Great books are rare. This is one. Read it and you will see. Skip this introduction. Come back to it if you want to know how the book came into being half a century ago, what its impact was, and the disputes that raged around its theses. Come back if you want one experienced opinion of the status of the book today. These remarks introduce the book, not Kuhn and his life work. He usually referred to the book as Structure, and in conversation simply as “the book.” I follow his usage. The Essential Tension is a superb collection of philosophical (as opposed to historical) papers that he published immediately before or soon after Structure.1 It can be thought of as a series of commentaries and expansions, so it is excellent companion reading. Since this is an introduction to Structure, nothing beyond The Essential Tension will be discussed here. Note, however, that he often said in conversation that Black-Body and the Quantum Discontinuity, a study of the first quantum revolution launched by Max Planck at the end of the nineteenth century, is an exact example of what Structure is all about.2 Just because Structure is a great book, it can be read in endless ways and put to many uses. Hence this introduction is only one among many possible ones. The book launched a fleet of books about Kuhn’s life and work. An excellent short introduction to the work of Thomas Samuel Kuhn (1922–96), with a different slant from this one, is to be found in the online Stanford Encyclopedia of Philosophy.3 For Kuhn’s final reminiscences of his life and thoughts, see the interview conducted in 1995 by Aristides Baltas, Kostas Gavroglu, and Vassiliki Kindi.4 The book about his work that he most admired was Paul Hoyningen- Huene’s Reconstructing Scientific Revolutions.5 For a list of all Kuhn’s publications, see James Conant and John Haugeland’s The Road since Structure.6 One thing is not said often enough: like all great books, this is a work of passion and a passionate desire to get things right. This is plain even from its modest first sentence on page 1: “History, if viewed as a repository for more than anecdote or chronology, could produce a decisive transformation in the image of science by which we are now possessed.”7 Thomas Kuhn was out to change our understanding of the sciences—that is, of the activities that have enabled our species, for better or worse, to dominate the planet. He succeeded. 1962 The present edition commemorates the fiftieth anniversary of Structure. Nineteen sixty-two was a long time ago. The sciences themselves have radically changed. The queen of the sciences, then, was physics. Kuhn had been trained as a physicist. Few people knew much physics, but everybody knew that physics was where the action was. A cold war was in progress, so everyone knew about the Bomb. American schoolchildren had to practice cowering under their desks. At least once a year towns sounded an air raid siren, at which everyone had to take shelter. Those who protested against a nuclear weapon, by ostentatiously not taking shelter, could be arrested, and some were. Bob Dylan first performed “A Hard Rain’s A-Gonna Fall” in September 1962; everyone assumed it was about nuclear fallout. In October 1962 there was the Cuban Missile Crisis, the closest the world has come, after 1945, to nuclear war. Physics and its threat were on everyone’s mind. The Cold War is long over, and physics is no longer where the action is. Another event of 1962 was the awarding of Nobel prizes to Francis Crick and James Watson for the molecular biology of DNA and to Max Perutz and John Kendrew for the molecular biology of hemoglobin. That was the harbinger of change. Today, biotechnology rules. Kuhn took physical science and its history as his model. You will have to decide, after reading his book, about the extent to which what he said about the physical sciences holds true in the teeming, present world of biotechnology. Add in information science. Add in what the computer has done to the practice of science. Even experiment is not what it was, for it has been modified and to a certain extent replaced by computer simulation. And everyone knows that the computer has changed communication. In 1962 scientific results were announced at meetings, in special seminars, in preprints, and then in articles published in specialist journals. Today the primary mode of publication is in an electronic archive. There is yet another fundamental difference between 2012 and 1962. It affects the heart of the book, fundamental physics. In 1962 there were competing cosmologies: steady state and big bang, two completely different pictures of the universe and its origin. After 1965 and the almost fortuitous discovery of universal background radiation, there is only the big bang, full of outstanding problems pursued as normal science. In 1962 high-energy physics seemed to be an endless collection of more and more particles. What is called the standard model brought order out of chaos. It is unbelievably accurate in its predictions, even if we have no idea how to fit it together with gravity. Perhaps there will not be another revolution in fundamental physics, although for sure there will be surprises galore. Thus The Structure of Scientific Revolutions may be—I do not say is—more relevant to a past epoch in the history of science than it is to the sciences as they are practiced today. But is the book history or philosophy? In 1968 Kuhn began a lecture insisting, “I stand before you as a practicing historian of science. . . . I am a member of the American Historical, not the American Philosophical, Association.”8 But as he reorganized his own past, he increasingly presented himself as always having had primarily philosophical interests.9 Although Structure had an immense immediate impact on the community of historians of science, its more enduring effects have probably been upon philosophy of science and, indeed, on public culture. That is the perspective from which this introduction is written. Structure Structure and revolution are rightly put up front in the book’s title. Kuhn thought not only that there are scientific revolutions but also that they have a structure. He laid out this structure with great care, attaching a useful name to each node in the structure. He had a gift for aphorism; and his names have acquired an unusual status, for although they were once arcane, some of them are now part of colloquial English. Here is the sequence: (1) normal science (§§II–IV—he called these sections, not chapters, for he thought of Structure as more of a book outline than a book); (2) puzzle-solving (§IV); (3) paradigm (§V), a word which, when he used it, was rather uncommon, but which after Kuhn has become banal (not to mention paradigm shift!); (4) anomaly (§VI); (5) crisis (§§VII–VIII); and (6) revolution (§IX), establishing a new paradigm. That is the structure of scientific revolutions: normal science with a paradigm and a dedication to solving puzzles; followed by serious anomalies, which lead to a crisis; and finally resolution of the crisis by a new paradigm. Another famous word does not occur in the section titles: incommensurability. This is the idea that, in the course of a revolution and paradigm shift, the new ideas and assertions cannot be strictly compared to the old ones. Even if the same words are in use, their very meaning has changed. That in turn led to the idea that a new theory was not chosen to replace an old one, because it was true but more because of a change in world view (§X). The book ends with the disconcerting thought that progress in science is not a simple line leading to the truth. It is more progress away from less adequate conceptions of, and interactions with, the world (§XIII). Let us look at each idea in turn. Obviously the structure is all too neat. History, the historian protests, just is not like that. But it was precisely Kuhn’s instinct as a physicist that led him to find a simple and insightful all-purpose structure. It was a picture of science that the general reader could pick up. It had the merit of being to some extent testable. Historians of the sciences could look and see the extent to which momentous changes in their fields of expertise did in fact conform to Kuhn’s structure. Unfortunately it was also abused by the wave of skeptical intellectuals who called the very idea of truth in question. Kuhn had no such intention. He was a fact lover and a truth seeker. Revolution We think first of revolution in political terms: the American Revolution, the French Revolution, the Russian Revolution. Everything is overthrown; a new world order begins. The first thinker to extend this notion of revolution to the sciences may have been Immanuel Kant. He saw two great intellectual revolutions. They are not mentioned in the first edition (1781) of his greatest masterpiece The Critique of Pure Reason (another rare great book, but not a page turner like Structure!). In the preface to the second edition (1787), he speaks in almost purple prose of two revolutionary events.10 One was the transition in mathematical practice in which techniques familiar in Babylonia and Egypt were transformed in Greece to proofs from postulates. The second was the emergence of the experimental method and the laboratory, a series of events that he identified as beginning with Galileo. He repeats the word revolution several times in just two long paragraphs. Notice that although we think of Kant as the purest of scholars, he was living in turbulent times. Everyone knew that something profound was afoot all over Europe, and indeed the French Revolution was only two years away. It was Kant who set in place the idea of a scientific revolution.11 As a philosopher I find it amusing, and certainly forgivable, that honest Kant himself confesses, in a footnote, that he is not in a position to pay attention to the minutiae of historical details.12 Kuhn’s first book concerned with science and its history was not Structure but The Copernican Revolution.13 The idea of scientific revolution was already very much in circulation. After World War II there was a great deal of writing about the scientific revolution of the seventeenth century. Francis Bacon was its prophet, Galileo its lighthouse, and Newton its sun. A first point to notice—one that is not immediately obvious on a first skimming of Structure—is that Kuhn was not talking about the scientific revolution. That was quite a different kind of event from the revolutions whose structure Kuhn postulated.14 Indeed shortly before he published Structure, he had proposed that there was a “second scientific revolution.”15 It took place during the early years of the nineteenth century; whole new fields were mathematized. Heat, light, electricity, and magnetism acquired paradigms, and suddenly a whole mass of unsorted phenomena began to make sense. This coincided with—went hand-in-hand with—what we call the industrial revolution. It was arguably the beginning of the modern technoscientific world in which we live. But, no more than the first scientific revolution, did this second revolution exhibit the “structure” of Structure. A second point to notice is that the generation preceding Kuhn, the one that wrote so extensively on the scientific revolution of the seventeenth century, had grown up in a world of radical revolution in physics. Einstein’s special (1905) and then general (1916) theory of relativity were more shattering events than we can well conceive. Relativity had, at the beginning, far more repercussions in the humanities and arts than genuine testable consequences in physics. Yes, there was the famous expedition of Sir Arthur Eddington to test an astronomical prediction of the theory, but it was only later that relativity became integral to many branches of physics. Then there was the quantum revolution, also a two-stage affair, with Max Planck’s introduction of quanta around 1900 and then the full quantum theory of 1926–27, complete with Heisenberg’s uncertainty principle. Combined, relativity and quantum physics overthrew not only old science but basic metaphysics. Kant had taught that absolute Newtonian space and the principle of uniform causality are a priori principles of thought, necessary conditions on how human beings comprehend the world in which they live. Physics proved him totally mistaken. Cause and effect were mere appearance, and indeterminacy was at the root of reality. Revolution was the order of the scientific day. Before Kuhn, Karl Popper (1902–94) was the most influential philosopher of science—I mean the most widely read, and to some extent believed, by