It is even more certain that this sense was well known at Athens, at least in certain circles, not long after the middle of the fifth century. To all appearance, this was the work of Socrates (470-399 B.C.). Whatever view may be taken of the philosophy of Socrates or of its relation to that expounded in Plato's earlier dialogues (a point which need not be discussed here), it is at least not open to question that he was personally intimate with the leading Pythagoreans who had taken refuge at Thebes and at Phlius in the Peloponnesus when their society came to be regarded as a danger to the state at Croton and elsewhere in southern Italy. That happened about the middle of the fifth century, and Socrates must have made the acquaintance of these men not long after. At that time it would be quite natural for them to visit Athens; but, after the beginning of the Peloponnesian War (431 B. C.), all intercourse with them must have ceased. They were resident in enemy states, and Socrates was fighting for his country. With the exception of the brief interval of the Peace of Nicias (421 B. C.), he can have seen nothing of them for years. Nevertheless it is clear that they did not forget him; for we must accept Plato's statement in the _Phaedo_ that many of the most distinguished philosophers of the time came to Athens to be with Socrates when he was put to death, and that those of them who could not come were eager to hear a full account of what happened. It is highly significant that, even before this, two young disciples of the Pythagorean Philolaus, Simmias and Cebes, had come from Thebes and attached themselves to Socrates. For that we have the evidence of Xenophon as well as of Plato, and Xenophon's statement is of real value here; for it was just during these few years that he himself associated with Socrates, though he saw him for the last time a year or two before his trial and death. Whatever other inferences may be drawn from these facts, they are sufficient to prove that Socrates had become acquainted with some of the leading philosophers of the Greek world before he was forty, and to make it highly probable that it was he who introduced the word 'philosophy' in its Pythagorean sense to the Athenians.