Abstract

'Incommensurability' has, in the last forty years, gained wide currency in the literature of philosophy. Kuhn and Feyerabend used the term in the early 1960's to describe an issue in the philosophy of science. They suggested that, when scientific theories are introduced that are significantly different from their predecessors, it may happen that the meanings of key terms differ significantly, and to the extent that scientists may be unable to fully comprehend the new theory until they experience a form of radical conversion, in which they come to fully grasp the new theory. Key contributions have been made to this literature by Bishop, Devitt, Margolis, Sankey, and Scheffler. ;The notion of 'incommensurability' has also been widely used in discussions of ethics. Finnis has used the concept to effect a taxonomy of human values, arguing that there are seven such values and that incommensurability is the feature that contradistinguishes them. Key contributions have been made to this literature by Goodman, Griffin, Pannier, Raz, and Williams. ;In this work I discuss the literature of incommensurability in both the philosophy of science and ethics. I argue that there are at least two quite different notions of incommensurability: one is a well defined notion originally used by mathematicians; the other, less well defined, means, based on the etymology of the word, lacking a common measure. I argue, further, that the discussions of Kuhn and Feyerabend are based on the mathematical notion, while the discussions in ethics are based on the etymological notion. ;Further complicating the discussions in both ethics and the philosophy of science, is the connection between the notions of commensurability and comparability. I argue that the notions describe, differently, equivalent sets. ;Ultimately Kuhn's notion fails because he cannot resolve his arguments for both inexpressibility in a common language and for comparability of competing theories. Finnis's notion fails because incommensurability does not, in fact, uniquely describe the taxonomy for which he argues