George Smith

Thomas S. Kuhn's singular voice was stilled by cancer on June 17, 1996, some 49 years after his initial encounters with past science had drawn him into a career in the history and philosophy of science. One of the most widely-read and influential academics of the 20th century, Kuhn was educated at Harvard University, where he received an S.B. in Physics in 1943 and a Ph.D. in the subject in 1949. He remained there until 1956, first as a Junior Fellow in the Society of Fellows from 1948 to 1951, when he in effect retrained himself as a historian of science, and then as an Assistant Professor of General Education and History of Science. He joined the faculty of the University of California, Berkeley, in 1956, becoming Professor of History of Science in 1961. From 1964 to 1979 he was on the faculty of the History of Science Program of Princeton University, and from 1972 to 1979 also a member of the Institute for Advanced Study. He moved to the Department of Linguistics and Philosophy of Massachusetts Institute of Technology in 1979, where he became Professor Emeritus in 1991. He was President of the History of Science Society in 1968—70 and of the Philosophy of Science Association in 1988–90. He received the George Sarton Medal of the History of Science Society in 1982 and the John Desmond Bernal Award of the Society for the Social Studies of Science in 1983.

Incommensurability between successive scientific theories—the impossibility of empirical evidence dictating the choice between them—was Thomas Kuhn's most controversial proposal. Toward defending it, he directed much effort over his last 30 years into formulating precise conditions under which two theories would be undeniably incommensurable with one another. His first step, in the late 1960s, was to argue that incommensurability must result when two theories involve incompatible taxonomies. The problem he then struggled with, never obtaining a solution that he found entirely satisfactory, was how to extend this initial line of thought to sciences like physics in which taxonomy is not so transparently dominant as it is, for example, in chemistry. This paper reconsiders incommensurability in the light of examples in which evidence historically did and did not carry over continuously from old laws and theories to new ones. The transition from ray to wave optics early in the nineteenth century, we argue, is especially informative in this regard. The evidence for the theory of polarization within ray optics did not carry over to wave optics, so that this transition can be regarded as a prototypical case of discontinuity of evidence, and hence of incommensurability in the way Kuhn wanted. Yet the evidence for classic geometric optics did carry over to wave optics, notwithstanding the fundamental conceptual readjustment that Fresnel's wave theory required.