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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.
Gist of Idea
In type theory, 'x ∈ y' is well defined only if x and y are of the appropriate type
Source
Hilary Putnam (Philosophy of Logic [1971], Ch.6)
Book Ref
Putnam,Hilary: 'Philosophy of Logic' [Routledge 1972], p.48