ively, or instinctively, that
something is true, though we cannot give any definite grounds for our
belief. The belief may be quite correct, but it is not, according to
Plato, knowledge. It is only right opinion. To possess knowledge, one
must not only know that a thing is so, but why it is so. One must know
the reasons. Knowledge must be full and complete understanding,
rational comprehension, and not mere instinctive belief. {182} It must
be grounded on reason, and not on faith. Right opinion may be produced
by persuasion and sophistry, by the arts of the orator and
rhetorician. Knowledge can only be produced by reason. Right opinion
may equally be removed by the false arts of rhetoric, and is therefore
unstable and uncertain. But true knowledge cannot be thus shaken. He
who truly knows and understands cannot be robbed of his knowledge by
the glamour of words. Opinion, lastly, may be true or false. Knowledge
can only be true.

These false theories being refuted, we can now pass to the positive
side of the theory of knowledge. If knowledge is neither perception
nor opinion, what is it? Plato adopts, without alteration, the
Socratic doctrine that all knowledge is knowledge through concepts.
This, as I explained in the lecture on Socrates, gets rid of the
objectionable results of the Sophistic identification of knowledge
with perception. A concept, being the same thing as a definition, is
something fixed and permanent, not liable to mutation according to the
subjective impressions of the individual. It gives us objective truth.
This also agrees with Plato's view of opinion. Knowledge is not
opinion, founded on instinct or intuition. Knowledge is founded on
reason. This is the same as saying that it is founded upon concepts,
since reason is the faculty of concepts.

But if Plato, in answering the question, "What is knowledge?" follows
implicitly the teaching of Socrates, he yet builds upon this teaching
a new and wholly un-Socratic metaphysic of his own. The Socratic
theory of knowledge he now converts into a theory of the nature of
reality. This is the subject-matter of Dialectic.

{183}

3. Dialectic, or the Theory of Ideas.

The concept had been for Socrates merely a rule of thought.
Definitions, like guide-rails, keep thought upon the straight path; we
compare any act with the definition of virtue in order to ascertain
whether it is virtuous. But what was for Socrates merely regulative of
thought, Plato now