  From the consideration of the examples provided by the theory of types and first-order set theory, Tarski extracts a conclusion that he welcomed. This is the conclusion that, in a certain sense, the distinction between logical and non-logical constants is relative to a context of investigation. Recall Tarski’s remarks about the sign for membership in a letter of 1944. In the 1966 lecture Tarski says that membership is a logical notion in some formalizations of the theory of types. He says: