 The simplest theory of this kind, which we call TST (Théorie Simple des Types, from the French, following Forster and others) is obtained as follows. We admit sorts of object indexed by the natural numbers (this is purely a typographical convenience; no actual reference to natural numbers is involved). Type 0 is inhabited by “individuals” with no specified structure. Type 1 is inhabited by sets of type 0 objects, and in general type \(n + 1\) is inhabited by sets of type \(n\) objects.