 As Farquharson noted (SI, 78 n.), Cook Wilson did not define his key notion of ‘apprehension’. (This is related with Cook Wilson’s claim discussed in section 5 that knowledge is undefinable.) The notion appears to be at the same time close to Aristotle’s ‘noesis’ and to Russell’s ‘acquaintance’. Cook Wilson obviously took his lead from a tradition beginning with Posterior Analytics B 19 and his comments are reminiscent of Thomas Reid, who argues in his Essays on the Intellectual Powers of Man (Bk. II, chap. v) that perception involves some conception of the object and the conviction of its existence, this conviction being immediate, non-inferential and not open to doubt. Cook Wilson was not exactly faithful to Aristotle and Reid, however, since he argued that apprehensions can be both perceptual and non-perceptual (SI, 79), and that some are obtained by inference while some are not, the latter being the material of inference (SI, 84–85). Furthermore, he argued that perceptual apprehensions should not be confused with sensations, as the mere having of a sensation is not yet to know what the sensation is, an idea that has echoes in Austin (Austin 1979, 91–94 &amp; 122 n.2). For this, one needs an “originative act of consciousness” that goes beyond mere passivity and compares the sensation in order to apprehend its definite character. As Cook Wilson put it: “we are really comparing but we do not recognize that we are” (SI, 46).