On this scene Socrates appeared, plain, blunt, critical. His teaching was in effect an appeal to men to reflect: to turn their attention away from the world which they were supposed to be explaining to the contemplation of their own Minds by which the explanation was furnished. γνῶθι σεαυτόν was his motto. All explanations of the Universe or of Experience were, as he showed, vain unless the Cognitive Faculty by which they were constructed were operating truly. In particular, the process of Rational Discourse implied the use of concrete general terms, which were recognised to be the essential instruments of Cognition. Socrates therefore devoted his attention specially to a critical examination of these general terms and also of the abstract terms which were the familiar instruments of Discourse. The Greeks of that day were endowed with a singular clearness of intellectual vision. They readily recognised that Knowledge was an intellectual process; they appreciated the activity of Thought or Rational Discourse as essential to its formation. They quite understood that Knowledge is not of the nature of a photograph--a resemblant pictorial reproduction of the data furnished by sensation. Only very casually and occasionally do we ever attempt to supply ourselves with a resemblant reproduction of our sensations. Obviously such a reproduction would only be of value memorially and could tell us nothing new. These early Greeks realised this, and they appear to have realised also pretty clearly that it would be impossible by means of such pictorial impressions to establish any community of Knowledge. It is of the essence of Knowledge that it is something which can be communicated to, and which is the common possession of, several individuals. That can never be true of sensation. We can never tell whether our sensations are the same as those of other people--never at any rate by means of sensations themselves; never unless and until such sensations have been inter-related by some other instrument. A mere photographic reproduction of sensation is thus quite useless as a means of Knowledge. In some way or other general terms supply the common bond. The recognition of this fact was one of the great results of the Socratic discussion. This explains the immense importance which Socrates naturally attached to the criticism of general and abstract terms. * * * * * The work of Socrates in this direction was immediately taken up and carried much further by Plato. Plato maintained that these general and abstract terms were in truth the names of ideas (εἲδη) with which the mind is naturally furnished, and further that these ideas corresponded to and typified the eternal forms of things--the essential constituents of the real world. Knowledge was possible because there were such eternal forms or ideal elements--the archetypes--of which the εἴδη were the counterparts and representations.