Second definition given by Theætêtus — That Cognition consists in right or true opinion.
We have thus, in enquiring — What is Knowledge or Cognition? advanced so far as to discover — That it does not consist in sensible perception, but in some variety of that purely mental process which is called opining, believing, judging, conceiving, &c. And here Theætêtus, being called upon for a second definition, answers — That Knowledge consists in right or true opinion. All opinion is not knowledge, because opinion is often false.[104]
[104] Plato, Theæt. p. 187 B. It is scarcely possible to translate δοξάζειν always by the same English word.
Objection by Sokrates — This definition assumes that there are false opinions. But how can false opinions be possible? How can we conceive Non-Ens: or confound together two distinct realities?.
Sokr. — But you are here assuming that there are false opinions? How is this possible? How can any man judge or opine falsely? What mental condition is it which bears that name? I confess that I cannot tell: though I have often thought of the matter myself, and debated it with others.[105] Every thing comes under the head either of what a man knows, or of what he does not know. If he conceives, it must be either the known, or the unknown. He cannot mistake either one known thing for another known thing: or a known thing for an unknown: or an unknown for a known: or one unknown for another unknown. But to form a false opinion, he must err in one or other of these four ways. It is therefore impossible that he can form a false opinion.[106]
[105] Plato, Theæt. p. 187 C.
[106] Plato, Theæt. p. 188.
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.
[107] Plato, Theæt. p. 188-189.
[108] Plato, Theæt. p. 190.