n received theories of knowledge, he gives you no answer of his own.
He abruptly closes the discussion by naively remarking that, at any
rate, Theaetetus will learn that he does not understand the subject; and
the ground is now cleared for an original investigation.

[Footnote 518: "Republic," bk. v. ch. xx.]

[Footnote 519: Ibid., bk. v. ch. xxii.]

[Footnote 520: "Theaetetus," Sec. 10.]

This investigation is resumed in the "Republic." This greatest work of
Plato's was designed not only to exhibit a scheme of Polity, and present
a system of Ethics, but also, at least in its digressions, to propound a
system of Metaphysics more complete and solid than had yet appeared. The
discussion as to the _powers_ or _faculties_ by which we obtain
knowledge, the _method_ or _process_ by which real knowledge is
attained, and the ultimate _objects_ or _ontological grounds_ of all
real knowledge, commences at Sec. 18, book v., and extends to the end of
book vii.

That we may reach a comprehensive view of this "sublimest of sciences,"
we shall find it necessary to consider--

1st. _What are the powers or faculties by which we obtain knowledge, and
what are the limits and degrees of human knowledge?_

2d. _What is the method in which, or the processes and laws according to
which, the mind operates in obtaining knowledge?_

3d. _What are the ultimate results attained by this method? what are the
objective and ontological grounds of all real knowledge?_

The answer to the first question will give the PLATONIC PSYCHOLOGY; the
answer to the second will exhibit the PLATONIC DIALECTIC; the answer to
the last will reveal the PLATONIC ONTOLOGY.

I. PLATONIC PSYCHOLOGY.

Every successful inquiry as to the reality and validity of human
knowledge must commence by clearly determining, by rigid analysis, what
are the actual phenomena presented in consciousness, what are the powers
or faculties supposed by these phenomena, and what reliance are we to
place upon the testimony of these faculties? And, especially, if it be
asserted that there is a science of absolute Reality, of ultimate and
essential Being, then the most important and vital question is, By what
power do we cognize real Being? through what faculty do we obtain the
knowledge of that which absolutely _is_? If by sensation we only obtain
the knowledge of the fleeting and the transitory, "_the becoming_" how
do we attain to the knowledge of the unchangeable and permanent, "the