Socrates defines the philosopher as the lover of knowledge. Knowledge is then distinguished from ignorance and, lastly, from opinion; it emerges as the faculty enabling the philosopher to see his way to true, undifferentiated being, to absolute beauty and the immutable, to the ideal. Opinion, on the other hand, is the domain of the manifest and manifold, of correlatives and opposites, such as light and heavy, soft and hard, etc. And so the philosopher seeks, by definition, knowledge of true being above all else.
Plato's epistemology is basically divided into three categories: non-being, manifest, and being. The sphere of non-being, or the nothing, belongs to the ignorant man; manifestation to the opinionated man; and being, of course, to the philosopher. The philosopher's mind, according to Plato, inhabits the highest and noblest sphere, the home of the forms, and ceaselessly aspires for truth and light.