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.

This poetic description of the origin of the world is to be found in
the "Timaeus." We have seen that the Ideas are absolute Being, and
that things of sense are half real and half unreal. They are partly
real because they participate in Being. They are partly unreal because
they participate in not-being. There must be, therefore, a principle
of absolute not-being. This, in Plato's opinion, is matter. Things of
sense are copies of the Ideas fashioned out of, or stamped upon,
matter. But Plato does not understand by matter what we, in modern
times, understand by it. Matter, in our sense, is always some
particular kind of matter. It is brass, or wood, or iron, or stone. It
is matter which has determinate character and quality. But the
possession of specific character means that it is matter with the copy
of Ideas already stamped upon it. Since iron exists in great
quantities in the world, and there is a common element in all the
various pieces of iron, by virtue of which all are classed together,
there must be a concept of iron. There is, therefore, an Idea of iron
in the world of Ideas. And the iron which we find in the earth must be
matter which is already formed into a copy of this Idea. It
participates in the Idea of iron. The same remarks apply to any othe