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 § 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 _Being_?" Have we a faculty of universal, necessary, and eternal principles? Have we a faculty, an interior eye which beholds "_the intelligible_," ideal, spiritual world, as the eye of sense beholds the visible or "_sensible world_?"[521]