We shall have to distinguish between propositions that refer to some totality of propositions and propositions that do not. Those that refer to some totality of propositions can never be members of that totality. We may define first-order propositions as those referring to no totality of propositions; second-order propositions, as those referring to totalities of first-order propositions; and so on, ad infinitum.
Source: My Philosophical Development, chap. 7：1959．
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