 In 1965, Feyerabend attempted to clarify his notion, claiming that two theories are incommensurable when the meanings of their main descriptive terms depend on mutually inconsistent principles (1965c, 227; 1975, 269–270, 276). He also claimed that two theories are incommensurable if a new theory entails that all the concepts of the preceding theory have zero extension or if it introduces rules that change the system of classes itself (1965a, 268). On this account, in order for there to be incommensurability, the “situation must be rigged in such a way that the conditions of concept formation in one theory forbid the formation of the basic concepts of the other” (Feyerabend 1978, 68; cf. 1975, 269; 1981b, 154). Still later, he claimed that with the term “incommensurability” he always meant “deductive disjointedness, and nothing else” (1977, 365). These subsequent formulations were all intended to capture the same idea.