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4. Formal Logic / F. Set Theory ST / 3. Types of Set / a. Types of set

18958	In type theory, 'x ∈ y' is well defined only if x and y are of the appropriate type [Putnam]
	Full Idea: In the theory of types, 'x ∈ y' is well defined only if x and y are of the appropriate type, where individuals count as the zero type, sets of individuals as type one, sets of sets of individuals as type two.
	From: Hilary Putnam (Philosophy of Logic [1971], Ch.6)