3. The philosophy of Plato is so multifarious and unsystematic; that it would be difficult, or rather impossible, to reduce its contents to any very exact classification. It may be sufficient at present to mention the ordinary scheme which divides it into the three branches, ethics, physics, and dialectics. These are the three sciences which are treated of in the writings of Plato. His ethics are a carrying out and enforcement of the ethical opinions of his great master Socrates. His physics are for the most part crude and fanciful, although marked here and there by very profound and luminous observations. The science of dialectic is supposed to belong more peculiarly to Plato, and his philosophy centres in it more essentially than in either of the other two departments; it therefore behoves us to inquire more particularly into the meaning or purport of the Platonic dialectic.
4. We ask, then, what is dialectic the science of? The answer is, that it is the science of ideas. Ideas, as all the world knows, play a most important part in the philosophy of Plato. He was indeed the first philosopher who treated expressly of these mysterious entities, endeavouring to explain their nature, to establish them as the true constituents of the universe, and to displace by their means the sensible phenomena from the hold which they have on the opinions of mankind generally as the only realities which exist. Ideas are the Alpha and Omega in the
