PLATONIC THEORY OF IDEAS The theory of Ideas is not to be conceived as an absolutely new philosophical standpoint introduced for the first time by Plato B.c.). The form and elaboration which the theory re ceived in the Platonic Dialogues were unquestionably of so origi nal and unique a character as at once to give to it a position of quite exceptional significance in the history of Greek thought ; but the root conceptions from which it emanated are readily discov erable in prior speculation, and the problems of which it was offered as the solution had already been formulated by earlier inquirers. To Plato the reality of Ideas doubtless seemed, as Zeller observes, to be the direct and inevitable outcome of the Socratic doctrine of concepts. So far as rival views of the nature of knowledge were concerned, that which identified it with had been effectively handled by Socrates. It was through criticism of the other two definitions expounded in the Theaetetus that Plato proceeded to his own effort at construction. As regards the third of these, it was not difficult to show that it was exposed to objections of a formidable kind. If the complex which is as serted to be knowable is simply the sum of its parts, it is im possible to understand how it can be knowable while they are not ; while if the "complex" is an indivisible unity which, though it sprang out of a conjunction of elements, is wholly different from them, it would be in the same predicament as they, and conse quently not an object of knowledge. Moreover, what precisely could be meant by the term Xeryos in this context? The only in telligible meaning would be that it is a statement of that charac teristic of a thing which distinguishes it from other things, its differentia (SLact)opit, otackopOrns). If, however, we merely add to the true belief we already have a true belief about the differentia we get no further ; while if we are supposed in some way to know the differentia, we shall be stranded with the circular definition that "knowledge is true belief together with knowledge of the differentia." It was, then, the second definition, that knowledge is true be lief or opinion, with which Plato had chiefly to reckon. Convinced by Socrates that there is such a thing as a body of knowable truth, which is valid for every thinking mind, he was confronted by the doctrine that the only knowledge we possess is that which is furnished by the ordinary, unsystematised experience of everyday life, for that is what was meant by the term 64a, the union of perception and thought through which certainly a specific kind of information is obtainable.
The stress of the argument in the Theaetetus is directed to showing that, on the basis of this view, no satisfactory explana tion can be given of the possibility of error. The obvious explana tion that suggests itself is that false belief ensues when we com bine elements together in thought after a manner which does not represent reality. But that presupposes that we have some inde pendent mode of access to the real, which it is the express purpose of the theory to dispute. Again, if we say that a false belief is a belief to which no object corresponds, we should be assuming that the "unreal" or "what is not" (TO pi 6v) can stand in a simi lar relation to our faculties of apprehension as the "real" or "what is,"—an assumption to which no intelligible meaning can be attached. Or, once more, it may be argued that false belief arises from confusing one thing with another thing (6XXoboVa). Yet if both the things are known, how can one be mistaken for the other? Or if both are not known, how can either one that is known be mistaken for one that is not known, or one unknown thing be mistaken for another unknown thing? Furthermore, it is evident that a person may entertain a "true belief" without knowledge. If, for example, by skilful advocacy an accused in dividual be rightly convicted of dishonesty, the judges have no doubt a "true belief" about him, but they cannot be said to know that he is guilty of the crime, because for such knowledge they would need actually to have seen the act committed. Ac cordingly, knowledge and belief must be held to be radically dis tinct. The one can only be acquired by learning or being taught, the other may be engendered by mere persuasion. Knowledge im plies that grounds or reasons can be given for what is asserted; belief, even when true, is but the convinced state of the individual believer, and its correspondence with the truth of things is acci dental to its nature, a correspondence of which the thinker can only become aware on grounds external to the belief itself.