 In his (1958a), Feyerabend criticized two conceptions of meaning. He argued against the logical positivist idea that the meaning of an observational term is determined by immediate experiences, as well as against the Wittgensteinian idea that the meaning of an observational term is determined by its use. Instead, Feyerabend argued that the meaning of a term, even an observation term, is determined by its theoretical context, or more precisely, by the theoretical principles that govern its correct use according to our best theories. For example, according to Feyerabend, the correct meaning of the term ‘temperature’ is not determined by its everyday use, but by the principles of statistical thermodynamics. On the basis of such a conception, Feyerabend developed ‘thesis I’, according to which the interpretation of an observation language is determined by the theories that we use to explain what we observe, and it changes as soon as those theories change (1958a, 163). Feyerabend proceeded to argue that when older theories are replaced, the meanings of the observational terms used to test those theories change. Just as in (1962), the result is incommensurability: The idea that successive scientific theories are conceptually incompatible and consequently logically disjoint due to differences in the meanings of the observation statements used to test them. Feyerabend’s ‘thesis I’ is thus an early version of his incommensurability thesis. In his (1958a), with this notion of incommensurability, Feyerabend challenged an implicit conceptual conservativism in logical positivism: The assumptions that theoretical terms derive their meaning solely through their connection with experience and that experience itself is a stable (or unchanging) foundation on which theoretical meaning can be based. Instead of such a bottom-up version of the relation of experience and theoretical knowledge, according to which experience determines the meanings of our theoretical terms, Feyerabend argued for a top-down version, according to which our best theories determine the meanings that we attach to our experiences. According to Feyerabend, experience cannot be taken for granted as some kind of fixed, neutral basis for comparing theories. Rather, it takes on its particular character in light of the theories we bring to it. 