 In his earlier paper “Categories”, Ryle describes the test for category differences as follows: “Two proposition-factors are of different categories or types, if there are sentence-frames such that when the expressions for those factors are imported as alternative complements to the same gap-signs, the resultant sentences are significant in the one case and absurd in the other” (1938 [1971], 181) – in other words, two expressions (or rather: what they signify) differ in category if there are contexts in which substituting one expression for the other results in absurdity. This test, of course, provides no way of establishing that two expressions are of the same category (but only that they are not), since there is an infinite number of sentence-frames, and one may always yet be found that does not permit the substitution to be made without absurdity. It also leaves open and merely intuitive the notion of ‘absurdity’ itself; in fact, Ryle concludes his paper “Categories” with the question “But what are the tests of absurdity?” (1938 [1971], 184). Ryle’s approach was further developed, in a more formal fashion, by Fred Sommers (1959, 1971).