The immediate starting-point of Plato's philosophical speculation was the Socratic teaching. In his attempt to define the conditions of knowledge so as to refute sophistic skepticism, Socrates had taught that the only true knowledge is a knowledge by means of concepts. The concept, he said, represents all the reality of a thing. As used by Socrates, this was merely a principle of knowledge. Plato took it up as a principle of Being. If the concept represents all the reality of things, the reality must be something in the ideal order, not necessarily in the things themselves, but rather above them, in a world by itself (Chaput, C. p.2). For the concept,therefore, Plato substitutes the Idea. He completes the work of Socrates by teaching that the objectively real Ideas are the foundation and justification of scientific knowledge. At the same time he has in mind a problem which claimed much attention from pre-Socratic thinkers, the problem of change. The Platonic theory of Ideas is an attempt to solve this crucial question by a metaphysical compromise. The Eleatics, Plato said, are right in maintaining that reality does not change; for the ideas are immutable. Still, there is, as contended, change in the world of our experience, or, as Plato terms it, the world of phenomena. Plato, then, supposes a world of Ideas apart from the world of our experience, and immeasurably superior to it. He imagines that all human souls dwelt at one time in that higher world. When, therefore, we behold in the shadow-world around us a phenomenon or appearance of anything, the mind is moved to a remembrance of the Idea (of that same phenomenal thing) which it formerly contemplated. In its delight it wonders at the contrast, and by wonder is led to recall as perfectly as possible the intuition it enjoyed in a previous existence. This is the task of philosophy. Philosophy, therefore, consists in the effort to rise from the knowledge of phenomena, o...
