One farther remark must be made on Plato's doctrine. His remark--That Cognition consists not in the affections of sense, but in computation or reasoning respecting those affections, (_i. e._ abstraction, generalisation, &c.)--is both true and important. But he has not added, nor would he have admitted, that if we are to decide whether our computation is true and right, or false and erroneous--our surest way is to recur to the simple facts of sense. Theory must be verified by observation; wherever that cannot be done, the best guarantee is wanting. The facts themselves are not cognition: yet they are the test by which all computations, pretending to be cognitions, must be tried.[103] [Footnote 103: See the remarks on the necessity of Verification, as a guarantee for the Deductive Process, in Mr. John Stuart Mill's System of Logic, Book iii. ch. xi. s. 8. Newton puts aside his own computation or theory respecting gravity as the force which kept the moon in its orbit, because the facts reported by observers respecting the lunar motions were for some time not in harmony with it. Plato certainly would not have surrendered any [Greek: sullogismo\s] under the same respect to observed facts. Aristotle might probably have done so; but this is uncertain.] [Side-note: 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] [Footnote 104: Plato, Theæt. p. 187 B. It is scarcely possible to translate [Greek: doxa/zein] always by the same English word.] [Side-note: 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] [Footnote 105: Plato, Theæt. p. 187 C.] [Footnote 106: Plato, Theæt. p. 188.]