nence. This criticism is partly justified. Plato did underes-
timate 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 sta-
bility; 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 situation
to another, the mountain appears a different shape. The
permanence of the mountain itself is due to the fact that
all these varying sensations are identified as sensations of
one and the same object. The idea of identity is involved
here, and it is, as it were, a thread upon which these fleet-
ing sensations are strung. But the idea of identity cannot
be obtained from the senses. It is introduced into things
by reason. Hence knowledge of this permanent mountain
is only possible through the exercise of reason. In Plato’s
language, all we can know of the mountain is the Ideas in
which it participates. To revert to a previous example, even
the knowledge “this paper is white” involves the activity of
intellect, and is impossible through sensation alone. Bare
sensation is a flow, of which no knowledge is possible.
Aristotle observes that Plato’s theory of Ideas has three
sources, the teachings of the Eleatics, of Heracleitus, and
of Socrates. From Heracleitus, Plato took the notion of a
sphere of Becoming, and it appears in his system as the
world of sense. From the Eleatics he took the idea of a
sphere of absolute Being. From Socrates he took the doc-
trine of concepts, and proceeded to identify the Eleatic Be-
ing with the Socratic concepts. This gives him his theory
of Ideas.{194}Sense objects, so far as they are knowable, that is so far
as they are more than bare sensations, are so only be-
cause the Idea resides in them. And this yields the clue
to Plato’s teaching regarding the relation of sense objects
to the Ideas. The Ideas are, in the first place the cause,
that is to say, the ground (not the mechanical cause) of
sense-objects. The Ideas are the absolute reality by which
individual things must be explained. The being of things
flows into them from the Ideas. They are “copies,” “imi-
tations,” of the Ideas. In so far as they resemble the Idea,
they are real; in so far as they differ from it, they are unreal.
In general, sense objects are, in Plato’s opinion, only very
dim, poor and imperfect copies of the Ideas. They are mere
shadows, and half-realities. Another expression frequently
used by Plato to express this relationship is that of “par-
ticipation.” Things participate in the Ideas. White objects
participate in the one whiteness, beautiful objects, in the
one beauty. In this way beauty itself is the cause or expla-
nation of beautiful objects, and so of all other Ideas. The
Ideas are thus both transcendent and immanent; immanent
in so far as they reside in the things of sense, transcendent
inasmuch as they have a reality of their own apart from
the objects of sense which participate in them. The Idea of