 An examination of Feyerabend’s use and development of the idea of incommensurability of scientific theories reveals just how widespread it was prior to 1962. It also reveals the veiled common core of Feyerabend’s philosophy of science, which otherwise appears to be a series of critical papers on unrelated topics (Oberheim 2005, 2006). That common core is the use of the idea of incommensurability to challenge many different forms of conceptual conservativism in science and philosophy. Conceptual conservativism unjustly favors existing ideas over potential improvements offered by incommensurable alternatives. Feyerabend used the notion of incommensurability to attack the following forms of conceptual conservativism: Heisenberg’s idea of closed theories (1951), attempts to distinguish philosophical accounts of ontology from physics (1954), the foundational role of the stability thesis in logical positivism (1951, 1958a), Bohr’s principle of complementarity (1958b, 1961, 1962, 1965b, 1970a), and the logical empiricists’ models of explanation and reduction (1962, 1963a). He also used the idea of incommensurability to attack forms of conceptual conservativism implicit in the models of theory testing promoted by classical empiricists, the logical positivists, the logical empiricists, Popper, and Kuhn (1958a, 1962, 1963a, 1965b, 1970a), Kantian transcendental necessities (1962, 1965b, 1965c), traditional philosophical conundrums like the problem of the existence of the external world, the problem of the existence of other minds, and especially the mind-body problem (1962, 1965b, 1967, 1969a), certain aspects of Wittgenstein’s later philosophy, as well as the linguistic philosophical movement that takes the utility of everyday concepts as given and merely attempts to analyze them, without changing their meanings (or ‘arguments from synonymy’) (1962, 1963a, 1965b, 1969b, 1970a), Popper’s conception of verisimilitude (1970b), and scientific realist characterizations of scientific knowledge and scientific advance (1970b). Feyerabend argued that all of these views cannot accommodate the fact that the history of science reveals revolutionary developments in which incommensurable alternatives replaced existing ideas in the course of progress.