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3. There is no difficulty in seeing that Plato's divisions of

knowledge are based, first, on the fundamental antithesis of

sensible and intellectual which pervades the whole pre-Socratic

philosophy; in which is implied also the opposition of the

permanent and transient, of the universal and particular. But the

age of philosophy in which he lived seemed to require a further

distinction;-numbers and figures were beginning to separate from

ideas. The world could no longer regard justice as a cube, and

was learning to see, though imperfectly, that the abstractions of

sense were distinct from the abstractions of mind. Between the

Eleatic being or essence and the shadows of phenomena, the

Pythagorean principle of number found a place, and was, as

Aristotle remarks, a conducting medium from one to the other.

Hence Plato is led to introduce a third term which had not

hitherto entered into the scheme of his philosophy.