21. Division of Plato's Works and Philosophy. -- The works of Plato comprise thirty-five dialogues, fifteen letters, and a collection of definitions bearing chiefly on Ethics. As he had no clear conception of an exact division of the various philosophical branches, it is hard to group his works, embracing as they do the most widely different questions. Aristotle distinguishes, in the philosophy of his master, dialectics, ethics, and physics. This classification is not explicitly found in Plato, but it corresponds with his thought. We will therefore adopt it, adding to it a few principles of esthetics.^{1}

The Idea is the corner-stone of Plato's philosophy; dialectic studies the Idea in itself; physics, ethics, and esthetics consider its applications to nature, to human conduct and to works of art.^{2}

{1} Strictly speaking, Plato has no system of Formal Logic; this science is one of Aristotle's greatest achievements. Yet we find in Plato some notions on logic. Notably, he has dealt ex professo with the dialectic method, with its twofold process, induction and deduction (synagôgê diairesis). By the dialectic method, we learn how to free ourselves from exterior things, in order to rise to the contemplation of the Idea. Plato's preference is for deduction. His induction is the development of the Socratic induction; it leads up to and ends in definition.

{2} If we take account of the chief topics treated in the most important of his works, we may classify as Physics the Timaeus and the Phaedo; as Ethics, the Republic, the Laws, the Politics, Philebus, and Gorgias; as Dialectic, the Theaetetus, Sophist and Parmenides.