ics of the individual.
Certain portions of the system, the doctrine of Eros, for example, do
not fall very naturally into any of these divisions. But, on the other
hand, though some dialogues are mixed as to their subject matter,
others, and those the most important, fall almost exclusively into one
or other division. For example, the "Timaeus," the "Phaedo," and the
"Phaedrus," are physical. The "Philebus," the "Gorgias," and the
"Republic," are ethical. The "Theaetetus," the "Sophist," and the
"Parmenides," are dialectical.

2. The Theory of Knowledge.

The theory of Ideas is itself based upon the theory of knowledge. What
is knowledge? What is truth? Plato opens the discussion by telling us
first what knowledge and truth are not. His object here is the
refutation of false theories. These must be disposed of to clear the
ground preparatory to positive exposition. The first such false theory
which he attacks is that knowledge {178} is perception. To refute this
is the main object of the "Theaetetus." His arguments may be
summarized as follows:--

(1) That knowledge is perception is the theory of Protagoras and the
Sophists, and we have seen to what results it leads. What it amounts
to is that what appears to each individual true is true for that
individual. But this is at any rate false in its application to our
judgment of future events. The frequent mistakes which men make about
the future show this. It may appear to me that I shall be Chief
Justice next year. But instead of that, I find myself, perhaps, in
prison. In general, what appears to each individual to be the truth
about the future frequently does not turn out so in the event.

(2) Perception yields contradictory impressions. The same object
appears large when near, small when removed to a distance. Compared
with some things it is light, with others heavy. In one light it is
white, in another green, and in the dark it has no colour at all.
Looked at from one angle this piece of paper seems square, from
another it appears to be a rhombus. Which of all these impressions is
true? To know which is true, we must be able to exercise a choice
among these varying impressions, to prefer one to another, to
discriminate, to accept this and reject that. But if knowledge is
perception, then we have no right to give one perception preference
over another. For all perceptions are knowledge. All are true.

(3) This doctrine renders all teaching, all discussion, pr