“Auch hieraus erwächst eine sehr entscheidende, nur ebenfalls nicht ausdrücklich gezogene, Folgerung, dass die reine Erkenntniss gar nicht auf demselben Gebiet liegen könne mit dem Irrthum — und es in Beziehung auf sie kein Wahr und Falsch gebe, sondern nur ein Haben oder Nicht Haben.” (Schleiermacher, Einleit. zum Theæt. p. 176.)
Steinhart (in his Einleit. zum Theæt. p. 94) contests this opinion of Schleiermacher (though he seems to give the same opinion himself, p. 92). He thinks that Plato does not recognise so very marked a separation between Knowledge and Opinion: that he considers Knowledge as the last term of a series of mental processes, developed gradually according to constant laws, and ascending from Sensible Perception through Opinion to Knowledge: that the purpose of the Theætêtus is to illustrate this theory.
Ueberweg, on the contrary, defends the opinion of Schleiermacher and maintains that Steinhart is mistaken (Aechtheit und Zeit. Platon. Schriften, p. 279).
Passages may be produced from Plato’s writings to support both these views: that of Schleiermacher, as well as that of Steinhart. In Timæus, p. 51 E, the like infallibility is postulated for Νοῦς (which there represents ἐπιστήμη) as contrasted with δόξα. But I think that Steinhart ascribes to the Theætêtus more than can fairly be discovered in it. That dialogue is purely negative. It declares that ἐπιστήμη is not αἴσθησις. It then attempts to go a step farther towards the affirmative, by declaring also that ἐπιστήμη is a mental process of computation, respecting the impressions of αἴσθησις — that it is τὸ συλλογίζεσθαι, which is equivalent to τὸ δοξάζειν: compare Phædrus, 249 B. But this affirmative attempt breaks down: for Sokrates cannot explain what τὸ δοξάζειν is, nor how τὸ δοξάζειν ψευδῆ is possible; in fact he says (p. 200 B) that this cannot be explained until we know what ἐπιστήμη is. The entire result of the dialogue is negative, as the closing words proclaim emphatically. On this point many of the commentators agree — Ast, Socher, Stallbaum, Ueberweg, Zeller, &c.
Whether it be true, as Schleiermacher, with several others, thinks (Einl. pp. 184-185), that Plato intends to attack Aristippus in the first part of the dialogue, and Antisthenes in the latter part, we have no means of determining.
This at least is the meaning which Plato assigns to the two words corresponding to Cognition and to Opinion, in the present dialogue, and often elsewhere. But he also frequently employs the word Cognition in a lower and more general signification, not restricted, as it is here, to the highest philosophical reach, with infallibility — but comprehending much of what is here treated only as opinion. Thus, for example, he often alludes to the various professional men as possessing Cognition, each in his respective department: the general, the physician, the gymnast, the steersman, the husbandman, &c.[102] But he certainly does not mean, that each of them has attained what he calls real essence and philosophical truths — or that any of them are infallible.
[102] Compare Plato, Sophistes, pp. 232 E, 233 A.
Plato did not recognise Verification from experience, or from facts of sense, as either necessary or possible.
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]
[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 συλλογισμὸς under the same respect to observed facts. Aristotle might probably have done so; but this is uncertain.