At Theaetetus 163d-164b Socrates objects to the thesis that knowledge is perception by pointing out that a man who has seen something can still remember it, and so has knowledge of it; but this is impossible, if knowledge is perception, since he is no longer perceiving it.To this Protagoras is made to reply with two sentences at 166b 1–4:.Cornford translates ‘ For instance, do you think you will find anyone to admit that one's present memory of a past impression is an impression of the same character as one had during the original experience, which is now over? It is nothing of the sort’.Cornford understands this as the suggestion that the memory and the original perception are of different things: ‘ All that the objection in fact established was that “ perception” must be stretched to include awareness of memory images’. So too Lee: ‘Protagoras’ “way out”… appears to be to say that what we now know is not properly X but rather (say) our memory trace of X - some present πά θ ο ς (Y) quite distinct from X (or, more exactly, from our earlier perception of X: Protagoras must thoroughly subjectivize the matter) and very different from that (perhaps along Humean lines of vividness and the like)’. (Relevant passages from Hume are given by Campbell in his note ad loc.) McDowel