Applying this doctrine to sense-apprehension, we should be entitled to say that, for example, a colour seen is neither "in" our eye nor "in" an external object, for, as a matter of fact what we call our eye and the external object are two sets of process, which when they come into contact give rise to the momentary appearance of the colour. Hence the colour will not appear the same to an animal as it appears to a man, nor even appear to a man twice the same. It is not likely that this elaboration of the theory was derived from the work of Protagoras ; it is probably Plato's version of what he had learned from Cratylus, the Hera cleitean, with whose doctrines Aristotle tells us he was in his youth familiar.
The second of the theories advanced is that knowledge is true belief or opinion (8o a),—judgment, that is to say, based on the impressions obtained by the individual in and through the process of perception. We form, on the ground of perception, empirical notions of the things we encounter in nature, and in so far as these notions correctly represent the things in question, they may be said to constitute what is meant by knowledge. Who exactly the thinkers are that Plato had here in mind it is difficult to say ; perhaps, indeed, none in particular. For the theory is more or less that of ordinary common-sense reflection, and may be re garded as having been that tacitly assumed by various contempo rary philosophers.
Finally, a third theory of considerable interest in the history of the subject is examined,—namely, that knowledge is true be lief accompanied by definition X6-yov). According to this view, the simple elements out of which things are composed are undefinable and therefore unknowable. They can be apprehended by sense and they can be named, but nothing can be predicated of them, because to attribute a predicate to them would be to vio late the hypothesis of their simplicity. The things compounded of them can, however, be defined, for if we give the names of their elements we obtain assertions (XO-yoL) which make the things in question knowable. The doctrine is, in fact, a doctrine of extreme nominalism. Truth consisted of identical propositions of the form A is A, or of what amount to the same thing, analytical propositions of the form A is X, when A is compounded of X and Y and Z—a doctrine which plays a conspicuous part in subsequent philosophical thinking. It would seem highly probable that Plato had here in mind the teaching of Antisthenes, a contemporary of Socrates, and founder of the Cynic School. We are informed by Aristotle that Antisthenes maintained that nothing could be pred icated of a thing except the expression peculiar to itself, one of one (6tKitor Xeryos, and this doctrine was afterwards elaborated by the Stoics, who inherited many of the tenets advo cated by the Cynic thinkers.
even formulate in terms which do not contradict the premises the doctrine that to know consists simply in being acquainted with the changing, transitory, limited impressions of the individual percipient.
As contrasted, then, with this Protagorean thesis, Socrates ad vanced the view that in general notions is to be found the truth of things. The two logical processes of induction (brarrucol Xeryot) and definition era 6A'EaOat Ka06Xov), the original employment of which Aristotle ascribes to Socrates, had for their aim the attainment of such general notions. By collecting, comparing and sifting a number of instances of things called by the same name, omitting what is peculiar to each and determining what is corn mon to all, there would necessarily be reached the thought of the essence of the things in question. In two fundamental respects, the Socratic position presents, then, the sharpest contrast to that just mentioned. On the one hand, the process of reasoning by means of which concepts are reached was a process which, unlike that of sense-perception, did not vary from individual to individ ual but was, when rightly carried out, similar in character in every individual mind; and on the other hand, the concepts thus ob tained were not the private property of any one thinking being but the common property of all thinking beings.
Thus, positively there is involved in the Socratic method the principle that knowledge, completeness of insight, is attainable only through means of concepts or notions that possess the char acteristics of generality and stability, of which the vague, fluctuat ing presentations of sense are devoid. And negatively there is in volved the further principle that in what is contrasted with the faculty of forming notions, the unreflective casual process of sense-apprehension, is to be found the ground of error or illusion.
In the Socratic teaching there must have lain from the outset the implicit thought of an ideal of human knowledge, from which the crude beliefs formed on the basis of individual perceptions widely diverge. What gives point to the Socratic confession of ignorance, what enables us to get to the real meaning of the maxim on which Socrates proceeded that self-knowledge is the only way to dispel apparent knowledge and to attain truth, is just the presence of this ideal of knowledge which animated his thought throughout. While in so far accepting the Sophistic dictum as it emphasised the intimate relation of the human mind to the world of truth and reality, Socrates brought into prominence the dis tinctive character of the human, as distinguished from the ani mal, mind on which that relation rested, and in virtue of which the human individual transcending his own individuality, participates in that which is common and abiding. Not as a sentient but as a thinking, rational being,-man is the measure of all things.