  An example which we know Tarski had in mind is the sign for the membership relation. We find him saying: “sometimes it seems to me convenient to include mathematical terms, like the ∈-relation, in the class of logical ones, and sometimes I prefer to restrict myself to terms of ‘elementary logic’. Is any problem involved here?” (Tarski 1987, p. 29). We may take the sign for membership as a logical sign in some formalizations of the theory of types (which can in turn be used to formalize theories with arbitrary universes of individuals). But when doing set theory for its own sake, the appropriate thing is not to take membership as a logical notion, and not to assume principles about membership as forming part of the “logic” of the theory, but as postulates thereof. Tarski’s final remarks in the paper on logical consequence again put forward the view that the notion of logical constant may be of a relative character. In different contexts different terms may be taken as logical and therefore so may vary the extension of the relation of logical consequence. He says that “the fluctuation in the common usage of the concept of consequence would—in part at least—be quite naturally reflected in such a compulsory situation” of relativity (Tarski 1983c, p. 420).