Single Idea 18958

[catalogued under 4. Formal Logic / F. Set Theory ST / 3. Types of Set / a. Types of set]

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 Reference

Putnam,Hilary: 'Philosophy of Logic' [Routledge 1972], p.48