but there are also many complete works which profess to have been written by the authors of these speculations. Confessedly the most important of these Greek writings are Xenophon’s Memorabilia of Socrates, and the works of Plato, and Aristotle. But from the beginning of Grecian philosophy in Thales and Pythagoras to its culmination in Socrates, Plato, and Aristotle, was not quite two hundred years, while its whole history covers a period of six centuries and a half before, and five centuries after