Socrates, the Athenian, brought philosophy, as Cicero says, from heaven to earth; that is to say, in place of one-sided speculation upon nature he pursued an equally one-sided study of ethics. In his practical, matter-of-fact way he availed himself of what Eleatic ontology had acquired in order to settle the fundamental ideas of morality and to demonstrate the possibility of scientific proof in face of the nihilistic fallacies of sophistry which despaired of both. So much we may accept as certain from received accounts. All the details of his teaching are wrapped in doubt, for we possess no historical account of it, but merely works of an apologetic character, in which liberal and justifiable advantage is taken of the prerogatives of fiction. Neither Plato nor Xenophon (the latter of whom did not take up his pen until after a superabundant crop of Socratic literature had come into being) can be accepted as historic evidence without further ado. Nevertheless both the disciples of Socrates and his opponents, Aristophanes and Spintharus (the father of Aristoxenus), bear witness to the extraordinary personality of the man. The rights of the individual were not recognised until the fifth century. The atomistic theory of Leucippus and Democritus sees the Eternal and Constant not in the All-One of Xenophanes and Parmenides, but in the individual. The philosophy of the Sophists breaks the bonds of authority, and in the motto “Man (the individual) is the measure of all things,” Protagoras sets up the charter of subjective inclination. This charter Socrates adopts, but he opposes to the liberty of the individual will the counteracting force of obedience to the dictates of the individual conscience. But conscience, as the German and Latin name for it alike imply, means knowledge. A man should therefore act upon his own judgment, but only in so far as his action is founded upon norms scientifically determined. Thus Socrates reads a deeper meaning into the admonition of the Delphic god, “Know thyself,” by recognising the independence of the will. Inasmuch as traditional usage and the law of the state are thus tacitly set aside (and on this point Aristophanes judged more correctly in his caricature than the apologists Plato and especially Xenophon will admit) Socrates is the preacher of a new private morality which traverses the public morality of classic antiquity. His death sentence is so far intelligible, though it remains an act of crude, reactionary violence. The greatness of soul, so far beyond the ordinary level of mankind, which, according to all accounts, the philosopher displayed at the near prospect of death, wrought upon a far wider circle than that of his disciples and contemporaries. His martyrdom set the seal upon the victory of the Ideal philosophy in Athens.