ion 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 fleeting 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
doctrine of concepts, and proceeded to identify the Eleatic Being 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 because 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," "imitations," 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
"participation." 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 explanation of
beautiful objects, and so of all other Ideas. The Ideas are thus both
transcendent and i