Guthrie: 2. The Archetypal World of Ideas.

2. The Archetypal World of Ideas.—In explaining what Plato meant by his World of Ideas, we must notice the fact that he accepts the identification of Being and Thought of Parmenides. As a consequence, his "intelligible world" is the world of true existence, and everything exists only inasmuch as it participates in this existence. An Idea is that which makes a horse a horse, and a tree a tree; in short it is a general notion, an universal, a species or genus, which abides unchanged amidst all the changes of the individuals to which it applies. Hence the world of Ideas is "in the supercelestial place," "En topo huperouranio," beyond all change, far beyond this world, separate from the objects participating in it. The Ideas are archetypes, "paradigms," "Paradeigmata," of every quality and every thing, many Ideas at times being present in one and the same thing, as "just" and "tall" in a "man." These Ideas are co-ordinate, being distinct entities, although they also rank hierarchically from the highest genus to the lowest species, as they are the existence, being, aim and end of everything subsumed under them. Yet they are passive thoughts, and are without energy; they are only objects of contemplation, far from the world.