Thomas Samuel Kuhn (1922-1996) was an American historian and philosopher of science. He found that basic ideas about how nature should be studied were dogmatically accepted in normal science, increasingly questioned, and overthrown during scientific revolutions.

Born in Cincinnati, Ohio, in 1922, Thomas Kuhn was trained as a physicist but became an educator after receiving his Ph.D. in physics from Harvard in 1949. He taught as an assistant professor of the history of science at Harvard from 1952 to 1957, as a professor of the history of science at Berkeley (California) from 1958 to 1964, as a professor of the history of science at Princeton from 1964 to 1979, as a professor of philosophy and the history of science at Massachusetts Institute of Technology (MIT) from 1979 to 1983, and finally, Laurence Rockefeller professor of philosophy at MIT from 1983 to 1991. A member of many professional organizations, he was president of the History of Science Society from 1968 to 1970. He received the Howard T. Behrman award at Princeton in 1977 and the George Sarton medal from the History of Science Society in 1982.

Kuhn's scholarly achievements were many. He held positions as a Lowell lecturer in 1951, Guggenheim fellow from 1954 to 1955, fellow of the Center for Advanced Studies in Behavioral Science from 1958 to 1959, director of the Sources for the History of Quantum Physics Project from 1961 to 1964, director of the Social Science Research Council from 1964 to 1967, director of the program for history and philosophy of science at Princeton from 1967 to 1972, member of the Institute for Advanced Study at Princeton from 1972 to 1979, and member of the Assembly for Behavioral and Social Science in 1980.

Kuhn was best known for debunking the common belief that science develops by the accumulation of individual discoveries. In the summer of 1947 something happened that shattered the image of science he had received as a physicist. He was asked to interrupt his doctorate physics project to lecture on the origins of Newton's physics. Predecessors of Newton such as Galileo and Descartes were raised within the Aristotelian scientific tradition. Kuhn was shocked to find in Aristotle's physics precious little a Newtonian could agree with or even make sense of. He asked himself how Aristotle, so brilliant on other topics, could be so confused about motion and why his views on motion were taken so seriously by later generations. One hot summer day while reading Aristotle, Kuhn said he had a brainstorm. "I gazed abstractly out the window of my room. Suddenly the fragments in my head sorted themselves out in a new way, and fell into place together, my jaw dropped," as reported by his friend and admirer, Malcolm Gladwell, in the July 8th issue of The New Yorker. He realized that he had been misreading Aristotle by assuming a Newtonian point of view. Taught that science progresses cumulatively, he had sought to find what Aristotle contributed to Newton's mechanics. This effort was wrong-headed, because the two men had basically different ways of approaching the study of motion.

For example, Aristotle's interest in change in general led him to regard motion as a change of state, whereas Newton's interest in elementary particles, thought to be in continuous motion, led him to regard motion as a state. That continuous motion requires explanation by appeal to some force keeping it in motion was taken as obvious by Aristotle. But Newton thought that continued motion at a certain speed needed no explanation in terms of forces. Newton invoked the gravitational force to explain acceleration and advanced a law that an object in motion remains in motion unless acted upon by an external force.

This discovery turned Kuhn's interest from physics to the history of physics and eventually to the bearing of the history of science on philosophy of science. His working hypothesis that reading a historical text requires sensitivity to changes in meaning provided new insight into the work of such great physicists as Boyle, Lavoisier, Dalton, Boltzmann, and Plank. This hypothesis was a generalization of his finding that Aristotle and Newton worked on different research projects with different starting points which eventuated in different meanings for basic terms such as "motion" or "force." Most people probably think that science has exhibited a steady accumulation of knowledge. But Kuhn's study of the history of physics showed this belief to be false for the simple reason that different research traditions have different basic views that are in conflict. Scientists of historically successive traditions differ about what phenomena ought to be included in their studies, about the nature of the phenomena about what aspects of the phenomena do or do not need explanation, and even about what counts as a good explanation or a plausible hypothesis or a rigorous test of theory.

Especially striking to Kuhn was the fact that scientists rarely argued explicitly about these basic research decisions. Scientific theories were popularly viewed as based entirely on inferences from observational evidence. But no amount of experimental testing can dictate these decisions because they are logically prior to testing by their nature. What, if not observations, explains the consensus of a community of scientists within the same tradition at a given time? Kuhn boldly conjectured that they must share common commitments, not based on observation or logic alone, in which these matters are implicitly settled. Most scientific practice is a complex mopping-up operation, based on group commitments, which extends the implications of the most recent theoretical breakthrough. Here, at last, was the concept for which Kuhn had been searching: the concept of normal science taking for granted a paradigm, the locus of shared commitments.

In 1962 Kuhn published his landmark book on scientific revolutions, which was eventually translated into 16 languages and sold over a million copies. He coined the term "paradigm" to refer to accepted achievements such as Newton's Principia which contain examples of good scientific practice. These examples include law, theory, application, and instrumentation. They function as models for further work. The result is a coherent research tradition. In his postscript to the second edition, Kuhn pointed out the two senses of "paradigm" used in his book. In the narrow sense, it is one or more achievement wherein scientists find examples of the kind of work they wish to emulate, called "exemplars." In the broad sense it is the shared body of preconceptions controlling the expectations of scientists, called a "disciplinary matrix." Persistent use of exemplars as models gives rise to a disciplinary matrix that determines the problems selected for study and the sorts of answers acceptable to the scientific community.

Using the paradigm concept, Kuhn developed a theory of scientific change. A tradition is pre-scientific if it has no paradigm. A scientific tradition typically passes through a sequence of normal science-crisis-revolution-new normal science. Normal science is puzzle-solving governed by a paradigm accepted uncritically. Difficulties are brushed aside and blamed on the failure of the scientist to extend the paradigm properly. A crisis begins when scientists view these difficulties as stemming from their paradigm, not themselves. If the crisis is not resolved, a revolution sets in, but the old paradigm is not given up until it can be replaced by a new one. Then new normal science begins and the cycle is repeated. Just when to accept a new paradigm and when to stick to the old one is a matter not subject to proof, although good reasons can be adduced for both options. Scientific rationality is not found in rules of scientific method but in the collective judgment of the scientific community. We must give up the notion that science progresses cumulatively toward the truth about reality; after a revolution it merely replaces one way of seeing the world with another.

Kuhn's theory of scientific change was the most widely influential philosophy of science since that of his mentor, Sir Karl Popper. Kuhn's claims were much discussed by scientists, who generally accepted them; by sociologists, who took them to elucidate the subculture of scientists; by historians, who found cases of scientific change not fitting his model; and by philosophers, who generally abhorred Kuhn's historical relativism about knowledge but accepted the need for their theories of science to do justice to its history. Kuhn was often perturbed by those who sought to--in his view--apply his ideas to areas where it was inappropriate. "I'm much fonder of my critics than my fans," he often said, according to Gladwell's New Yorker article. Indeed, he even tried in later years to replace the term "paradigm"--which he felt was being overused--with "exemplar." Kuhn died June 17, 1996, at his home in Cambridge, Massachusetts. Notwithstanding the tendency of some to misapply his theories, history will show that Kuhn indeed transformed the image of science by making it exciting and emphasizing that it is a social process in addition to being a rational one.

Further Reading

• Kuhn's four books are The Copernican Revolution (1957), The Essential Tension (1959), The Structure of Scientific Revolutions (1962, second edition 1970), and Black-Body Theory and the Quantum Discontinuity 1894-1912 (1978). Clear discussions of his views in order of increasing sophistication are found in George Kneller's Science as a Human Endeavor (1978), Garry Gutting's Paradigms and Revolutions (1980), Harold Brown's Perception, Theory and Commitment (1977), and Ian Hacking's Scientific Revolutions (1981). "My Jaw Dropped," by Malcolm Gladwell in the July 8th issue of The New Yorker is a tribute by an admirer. His obituary, by Lawrence Van Gelder, is in the June 29th edition of The New York Times.