em a principle of absolute not-being which we shall
consider when we come to deal with his Physics. Objects of sense
participate both in the Ideas and in this not-being. They are,
therefore, half way between Being and not-being. They are half real.
Ideas, again, are universal; things of sense are always particular and
individual. The Idea is one, the sense-object is always {192} a
multiplicity. Ideas are outside space and time, things of sense are
both temporal and spatial. The Idea is eternal and immutable;
sense-objects are changeable and in perpetual flux.

As regards the last point, Plato adopts the view of Heracleitus that
there is an absolute Becoming, and he identifies it with the world of
sense, which contains nothing stable and permanent, but is a constant
flow. The Idea always is, and never becomes; the thing of sense always
becomes, and never is. It is for this reason that, in the opinion of
Plato, no knowledge of the world of sense is possible, for one can
have no knowledge of that which changes from moment to moment.
Knowledge is only possible if its subject stands fixed before the
mind, is permanent and changeless. The only knowledge, then, is
knowledge of the Ideas.

This may seem, at first sight, a very singular doctrine. That there
can be no knowledge of sense-objects would, it might seem to us
moderns, involve the denial that modern physical science, with all its
exactitude and accumulated knowledge, is knowledge at all. And surely,
though all earthly things arise and pass away, many of them last long
enough to admit of knowledge. Surely the mountains are sufficiently
permanent to allow us to know something of them. They have relative,
though not absolute, permanence. This criticism is partly justified.
Plato did underestimate the value of physical knowledge. But for the
most part, the criticism is a misunderstanding. By the world of sense
Plato means bare sensation with no rational element in it. Now
physical science has not such crude sensation for its object. Its
objects are rationalized sensations. {193} If, in Plato's manner, we
think only of pure sensation, then it is true that it is nothing but a
constant flux without stability; and knowledge of it is impossible.
The mountains are comparatively permanent. But our sensation of the
mountains is perpetually changing. Every change of light, every cloud
that passes over the sun, changes the colours and the shades. Every
time we move from one situat