ceed from opposites. And the general proposition itself is established by means of certain propositions which are particular, as that sleep comes from waking and vice versa, the greater from the less and vice versa. This is the form which he used to establish his own views. But, just as long ago in tragedy the chorus was the only actor, and afterwards, in order to give the chorus breathing space, Thespis devised a single actor, Aeschylus a second, Sophocles a third, and thus tragedy was completed, so too with philosophy: in early times it discoursed on one subject only, namely physics, then Socrates added the second subject, ethics, and Plato the third, dialectics, and so brought philosophy to perfection. Thrasylus says that he published his dialogues in tetralogies, like those of the tragic poets. Thus they contended with four plays at the Dionysia, the Lenaea, the Panathenaea and the festival of Chytri.¢ Of the four plays the last was a satiric drama; and the four together were called a tetralogy. Now, says Thrasylus, the genuine dialogues are fifty-six in all, if the Republic be divided into ten and the Laws into twelve. Favorinus, however, in the second book of his Miscellaneous History declares that nearly the whole of the Republic is to be found in a work of Protagoras entitled Controversies.? This gives nine tetralogies, if the Republic takes the place of one single work and the Laws of another. His first tetralogy has a common plan underlying it, for he wishes to describe what the life of the philosopher