 In 1962, Feyerabend illustrated the incommensurability of scientific theories with a comparison of six pairs of central concepts drawn from three episodes of fundamental theory change in the course of scientific advance: (1) The dynamical characterization of impetus in the impetus theory of motion and the concept of force in the conceptual apparatus of Newtonian mechanics, (2 &amp; 3) the concepts of temperature and entropy in the transition from phenomenological thermodynamics to kinetic theory, and (4, 5 &amp; 6) the concepts mass, length and time in the transition from Newtonian to relativistic mechanics. Feyerabend subsequently added the example of geometrical optics and wave optics (1965b, 227). In 1962, Feyerabend also claimed that incommensurability occurs between traditional, fundamental theories in philosophy, and consequently arises in the discussions of the mind-body problem, the problem of the reality of the external world, and the problem of other minds (1962, 31, 90). Feyerabend argued that such age-old philosophical problems have not been solved because the disputants resist the kind of meaning change necessary for their dissolution. Concepts are incommensurable, according to Feyerabend, because they belong to mutually exclusive theoretical perspectives. These perspectives are mutually exclusive because they give incompatible accounts of the nature of reality. Feyerabend argued that because the meanings of their terms are provided by the theories to which they belong, when there is theoretical change, there are meaning changes (1962, 68, 94). Feyerabend argued that these changes in meaning affect our theoretical and observational terms, as well as our conception of the nature of reality. When this occurs, there is incommensurability; or as Feyerabend later characterized his view: “a theory is incommensurable with another if its ontological consequences are incompatible with the ontological consequences of the latter” (1981a, xi). The idea is intended to capture conceptual incompatibility due to changes of meaning that occur in theoretical transitions that affect our ontological beliefs. Two fundamental theories are incommensurable because the meanings of their terms are determined by the theoretical principles that govern their use, and these principles are qualitatively incompatible (1962, 58).