Sophocles a third, and so they made tragedy complete. So in the same manner, philosophical discourse was originally uniform, concerning itself solely about natural philosophy; then Socrates added to it a second character, the ethical: and Plato a third, the dialectic: and so he brought philosophy to perfection.
XXXV. But Thrasybulus says that he published his dialogues as the dramatic poets published their tetralogies. For, they contended with four plays, (and at four festivals, the Dionysiac, the Lenæan, the Panathenæan, and the Chytri), one of which was a satiric drama, and the whole four plays were called a tetralogy. Now, people say, the whole of his genuine dialogues amount to fifty-six; the treatise on the Republic being divided into ten books, (which Phavorinus, in the second book of his Universal History, says may be found almost entire in the Contradictions of Protagoras), and that on Laws into twelve. And there are nine tetralogies, if we consider the Republic as occupying the place of one book, and the Laws of another. He arranges, therefore, the first tetralogy of these dialogues which have a common subject, wishing to show what sort of life that of the philosopher may have been. And he uses two titles for each separate book, taking one from the name of the principal speaker, and the other from the subject.
This tetralogy then, which is the first, is commenced by the Euthyphron, or what is Holy; and that dialogue is a tentative one. The second is the Defence of Socrates, a moral one. The third is the Criton, or What is to be done, a moral one. The fourth is the Phædo, or the Dialogue on the Soul, a moral one.
The second tetralogy is that of which the first piece is the Cratylus, or the correctness of names, a logical one. The Theætetus, or Knowledge, a tentative one. The Sophist, or a dialogue on the Existent, a logical one. The Statesman, or a dialogue of Monarchy, a logical one.