Dialectic is the theory of reality, physics the theory of existence, dialectic of that which lies behind things as their ground, physics of the things which are thus grounded. That is to say, physics is concerned with phenomena and appearances, things which exist in space and time, as opposed to the timeless and non-spatial Ideas. Things of this kind are both corporeal and incorporeal. Physics falls therefore into two parts, the doctrine of the outward corporeality, the world, with its incorporeal essence, the World-Soul, and the doctrine of the incorporeal soul of man. _(a) The Doctrine of the World_. If, in the dialectic, Plato has given an account of the nature of the first principle and ground of all things, the problem now arises of explaining how the actual universe of things arises out of that ground, how it is derived from the first principle. In other words, the Ideas being the absolute reality, how does the world of sense, and, in general, the existent universe, arise out of the Ideas? Faced with this problem, the system of Plato broke down. The things of sense are, we are told, "copies" or "imitations" of the Ideas. {208} They "participate" in the Ideas. So far, so good. But why should there be any copies of the Ideas? Why should the Ideas give rise to copies of themselves, and how is the production of these copies effected? To these questions Plato has no answer, and he therefore has recourse to the use of myths. Poetic description here takes the place of scientific explanation.