I first read Thomas Kuhn’s famous book The Structure of Scientific Revolutions1 a quarter-century ago, soon after the publication of the second edition. I had known Kuhn only slightly when we had been together on the faculty at Berkeley in the early 1960s, but I came to like and admire him later, when he came to MIT. His book I found exciting.
Evidently others felt the same. Structure has had a wider influence than any other book on the history of science. Soon after Kuhn’s death in 1996, the sociologist Clifford Geertz remarked that Kuhn’s book had “opened the door to the eruption of the sociology of knowledge” into the study of the sciences. Kuhn’s ideas have been invoked again and again in the recent conflict over the relation of science and culture known as the science wars.
Structure describes the history of science as a cyclic process. There are periods of “normal science” that are characterized by what Kuhn sometimes called a “paradigm” and sometimes called a “common disciplinary matrix.” Whatever you call it, it describes a consensus view: in a period of normal science, scientists tend to agree about what phenomena are relevant and what constitutes an explanation of these phenomena, about what problems are worth solving and what is a solution of a problem. Near the end of a period of normal science a crisis occursâ€”experiments give results that don’t fit existing theories, or internal contradictions are discovered in these theories. There is alarm and confusion. Strange ideas fill the scientific literature. Eventually there is a revolution. Scientists become converted to a new way of looking at nature, resulting eventually in a new period of normal science. The “paradigm” has shifted.
To take an example given special attention in Structure, after the widespread acceptance of Newton’s physical theoriesâ€”the Newtonian paradigmâ€”in the eighteenth century, there began a period of normal science in the study of motion and gravitation. Scientists used Newtonian theory to make increasingly accurate calculations of planetary orbits, leading to spectacular successes like the prediction in 1846 of the existence and orbit of the planet Neptune before astronomers discovered it. By the end of the nineteenth century there was a crisis: a failure to understand the motion of light. This problem was solved through a paradigm shift, a revolutionary revision in the understanding of space and time carried out by Einstein in the decade between 1905 and 1915. Motion affects the flow of time; matter and energy can be converted into each other; and gravitation is a curvature in space-time. Einstein’s theory of relativity then became the new paradigm, and the study of motion and gravitation entered upon a new period of normal science.
Though one can question the extent to which Kuhn’s cyclic theory of scientific revolution fits what we know of the history of science, in itself this theory would not be very disturbing, nor would it have made Kuhn’s book famous. For many people, it is Kuhn’s reinvention of the word “paradigm” that has been either most useful or most objectionable. Of course, in ordinary English the word “paradigm” means some accomplishment that serves as a model for future work. This is the way that Kuhn had used this word in his earlier book2 on the scientific revolution associated with Copernicus, and one way that he continued occasionally to use it.
The first critic who took issue with Kuhn’s new use of the word “paradigm” in Structure was Harvard President James Bryant Conant. Kuhn had begun his career as a historian as Conant’s assistant in teaching an undergraduate course at Harvard, when Conant asked Kuhn to prepare case studies on the history of mechanics. After seeing a draft of Structure, Conant complained to Kuhn that “paradigm” was “a word you seem to have fallen in love with!” and “a magical verbal word to explain everything!” A few years later Margaret Masterman pointed out that Kuhn had used the word “paradigm” in over twenty different ways. But the quarrel over the word “paradigm” seems to me unimportant. Kuhn was right that there is more to a scientific consensus than just a set of explicit theories. We need a word for the complex of attitudes and traditions that go along with our theories in a period of normal science, and “paradigm” will do as well as any other.
What does bother me on rereading Structure and some of Kuhn’s later writings is his radically skeptical conclusions about what is accomplished in the work of science.3 And it is just these conclusions that have made Kuhn a hero to the philosophers, historians, sociologists, and cultural critics who question the objective character of scientific knowledge, and who prefer to describe scientific theories as social constructions, not so different from democracy or baseball.
Kuhn made the shift from one paradigm to another seem more like a religious conversion than an exercise of reason. He argued that our theories change so much in a paradigm shift that it is nearly impossible for scientists after a scientific revolution to see things as they had been seen under the previous paradigm. Kuhn compared the shift from one paradigm to another to a gestalt flip, like the optical illusion created by pictures in which what had seemed to be white rabbits against a black background suddenly appear as black goats against a white background. But for Kuhn the shift is more profound; he added that “the scientist does not preserve the gestalt subject’s freedom to switch back and forth between ways of seeing.”
Kuhn argued further that in scientific revolutions it is not only our scientific theories that change but the very standards by which scientific theories are judged, so that the paradigms that govern successive periods of normal science are incommensurable. He went on to reason that since a paradigm shift means complete abandonment of an earlier paradigm, and there is no common standard to judge scientific theories developed under different paradigms, there can be no sense in which theories developed after a scientific revolution can be said to add cumulatively to what was known before the revolution. Only within the context of a paradigm can we speak of one theory being true or false. Kuhn in Structure concluded, tentatively, “We may, to be more precise, have to relinquish the notion explicit or implicit that changes of paradigm carry scientists and those who learn from them closer and closer to the truth.” More recently, in his Rothschild Lecture at Harvard in 1992, Kuhn remarked that it is hard to imagine what can be meant by the phrase that a scientific theory takes us “closer to the truth.”
Kuhn did not deny that there is progress in science, but he denied that it is progress toward anything. He often used the metaphor of biological evolution: scientific progress for him was like evolution as described by Darwin, a process driven from behind, rather than pulled toward some fixed goal to which it grows ever closer. For him, the natural selection of scientific theories is driven by problem solving. When, during a period of normal science, it turns out that some problems can’t be solved using existing theories, then new ideas proliferate, and the ideas that survive are those that do best at solving these problems. But according to Kuhn, just as there was nothing inevitable about mammals appearing in the Cretaceous period and out-surviving the dinosaurs when a comet hit the earth, so also there’s nothing built into nature that made it inevitable that our science would evolve in the direction of Maxwell’s equations or general relativity. Kuhn recognizes that Maxwell’s and Einstein’s theories are better than those that preceded them, in the same way that mammals turned out to be better than dinosaurs at surviving the effects of comet impacts, but when new problems arise they will be replaced by new theories that are better at solving those problems, and so on, with no overall improvement.
All this is wormwood to scientists like myself, who think the task of science is to bring us closer and closer to objective truth. But Kuhn’s conclusions are delicious to those who take a more skeptical view of the pretensions of science. If scientific theories can only be judged within the context of a particular paradigm, then in this respect the scientific theories of any one paradigm are not privileged over other ways of looking at the world, such as shamanism or astrology or creationism. If the transition from one paradigm to another cannot be judged by any external standard, then perhaps it is culture rather than nature that dictates the content of scientific theories.
Kuhn himself was not always happy with those who invoked his work. In 1965 he complained that for the philosopher Paul Feyerabend to describe his arguments as a defense of irrationality in science seemed to him to be “not only absurd but vaguely obscene.” In a 1991 interview with John Horgan, Kuhn sadly recalled a student in the 1960s complimenting him, “Oh, thank you, Mr. Kuhn, for telling us about paradigms. Now that we know about them, we can get rid of them.” Kuhn was also uncomfortable with the so-called “strong program” in the sociology of science, which is “strong” in its uncompromisingly skeptical aim to show how political and social power and interests dominate the success or failure of scientific theories. This program is particularly associated with a group of philosophers and sociologists of science that at one time worked at the University of Edinburgh. About this, Kuhn remarked in 1991, “I am among those who have found the claims of the strong program absurd, an example of deconstruction gone mad.”
But even when we put aside the excesses of Kuhn’s admirers, the radical part of Kuhn’s theory of scientific revolutions is radical enough. And I think it is quite wrong.
It is not true that scientists are unable to “switch back and forth between ways of seeing,” and that after a scientific revolution they become incapable of understanding the science that went before it. One of the paradigm shifts to which Kuhn gives much attention in Structure is the replacement at the beginning of this century of Newtonian mechanics by the relativistic mechanics of Einstein. But in fact in educating new physicists the first thing that we teach them is still good old Newtonian mechanics, and they never forget how to think in Newtonian terms, even after they learn about Einstein’s theory of relativity. Kuhn himself as an instructor at Harvard must have taught Newtonian mechanics to undergraduates.
In defending his position, Kuhn argued that the words we use and the symbols in our equations mean different things before and after a scientific revolution; for instance, physicists meant different things by mass before and after the advent of relativity. It is true that there was a good deal of uncertainty about the concept of mass during the Einsteinian revolution. For a while there was talk of “longitudinal” and “transverse” masses, which were supposed to depend on a particle’s speed and to resist accelerations along the direction of motion and perpendicular to it. But this has all been resolved. No one today talks of longitudinal or transverse mass, and in fact the term “mass” today is most frequently understood as “rest mass,” an intrinsic property of a body that is not changed by motion, which is much the way that mass was understood before Einstein. Meanings can change, but generally they do so in the direction of an increased richness and precision of definition, so that we do not lose the ability to understand the theories of past periods of normal science.
Thomas S. Kuhn, The Structure of Scientific Revolutions (University of Chicago Press, 1962; second edition, 1970), quoted below as Structure. This essay is based in part on the author's 1997 Bohner Lecture at Rice University, delivered as part of its yearlong symposium on the work of Thomas Kuhn, and on a 1998 colloquium talk given at the Department of Physics at Harvard University.↩
Thomas S. Kuhn, The Copernican Revolution (Harvard University Press, 1957).↩
Kuhn was first trained as a physicist, and despite the presence of the wide-ranging word "scientific" in its title, The Structure of Scientific Revolutions is almost entirely concerned with physics and allied physical sciences like astronomy and chemistry. It is Kuhn's view of their history that I will be criticizing. I don't know enough about the history of the biological or behavioral sciences to judge whether anything I will say here also applies to them.↩
Thomas S. Kuhn, The Structure of Scientific Revolutions (University of Chicago Press, 1962; second edition, 1970), quoted below as Structure. This essay is based in part on the author’s 1997 Bohner Lecture at Rice University, delivered as part of its yearlong symposium on the work of Thomas Kuhn, and on a 1998 colloquium talk given at the Department of Physics at Harvard University.↩
Thomas S. Kuhn, The Copernican Revolution (Harvard University Press, 1957).↩
Kuhn was first trained as a physicist, and despite the presence of the wide-ranging word “scientific” in its title, The Structure of Scientific Revolutions is almost entirely concerned with physics and allied physical sciences like astronomy and chemistry. It is Kuhn’s view of their history that I will be criticizing. I don’t know enough about the history of the biological or behavioral sciences to judge whether anything I will say here also applies to them.↩