 A predicative propositional function is one that involves no quantifications over types higher than those of its arguments. Russell stratifies not only the universe of discourse (the various types, and objects of those types); he also stratifies the language. Suppose that a Russellian class (or predicative propositional function) β is first formed at a higher rank than is α. Then it is supposed to be meaningless (in the language of type theory) to say that β is a member of α, where this is construed in the official sense of supposedly attributing the attribute corresponding to α to the object β. (By contrast, in the language of set theory, it is meaningful—even if false—to say that β ∈ α.) Thus it is impossible, within Russell’s type theory, to deal with the would-be predicate or property of non-self-membership. For that requires that the predicate x ∈ x of self-membership be meaningful (and well-formed); which it is not. So, in his type theory, Russell blocked the kind of derivation of his own paradox to which Frege’s class theory fell victim. 