If indeed a man ascribed to any subject a predicate which was non-existent, this would be evidently a false opinion. But how can any one conceive the non-existent? He who conceives must conceive _something_: just as he who sees or touches, must see or touch _something_. He cannot see or touch the non-existent: for that would be to see or touch nothing: in other words, not to see or touch at all. In the same manner, to conceive the non-existent, or _nothing_, is impossible.[107] _Theæt._--Perhaps he conceives two realities, but confounds them together, mistaking the one for the other. _Sokr._--Impossible. If he conceives two distinct realities, he cannot suppose the one to be the other. Suppose him to conceive, just and unjust, a horse and an ox--he can never believe just to be unjust, or the ox to be the horse.[108] If, again, he conceives one of the two alone and singly, neither could he on that hypothesis suppose it to be the other: for that would imply that he conceived the other also. [Footnote 107: Plato, Theæt. p. 188-189.] [Footnote 108: Plato, Theæt. p. 190.] [Side-note: Waxen memorial tablet in the mind, on which past impressions are engraved. False opinion consists in wrongly identifying present sensations with past impressions.] Let us look again in another direction (continues Sokrates). We have been hasty in our concessions. Is it really impossible for a man to conceive, that a thing, which he knows, is another thing which he does not know? Let us see. Grant me the hypothesis (for the sake of illustration), that each man has in his mind a waxen tablet--the wax of one tablet being larger, firmer, cleaner, and better in every way, than that of another: the gift of Mnemosynê, for inscribing and registering our sensible perceptions and thoughts. Every man remembers and knows these, so long as the impressions of them remain upon his tablet: as soon as they are blotted out, he has forgotten them and no longer knows them.[109] Now false opinion may occur thus. A man having inscribed on his memorial tablet the impressions of two objects A and B, which he has seen before, may come to see one of these objects again; but he may by mistake identify the present sensation with the wrong past impression, or with that past impression to which it does not belong. Thus on seeing A, he may erroneously identify it with the past impression B, instead of A: or _vice versâ_.[110] False opinion will thus lie, not in the conjunction or identification of sensations with sensations--nor of thoughts (or past impressions) with thoughts--but in that of present sensations with past impressions or thoughts.[111] [Footnote 109: Plato, Theæt. p. 191 C. [Greek: kê/rinon e)kmogei=on].] [Footnote 110: Plato, Theæt. p. 193-194.] [Footnote 111: Plato, Theæt. p. 195 D.] [Side-note: Sokrates refutes this assumption. Dilemma. Either false opinion is impossible, or else a man may know what he does not know.]