it is said that once he was led out of his house by an old woman for the purpose of observing the stars, and he fell into a ditch and bewailed himself, on which the old woman said to him: "Do you, O Thales, who cannot see what is under your feet, think that you shall understand what is in heaven?"[18] Pythagoras of Samos lived later in the sixth century than Thales. He was a great traveller and seems to have visited not only the mainland of Greece, but also Egypt and Crete, where he had {380} many rare experiences going into the innermost parts of temples where as a rule no strangers were admitted. He also went to Italy where he founded a school, and gathered about three hundred pupils round him. Though it was not believed by the world at large until nearly two thousand years later, Pythagoras taught that the world was round, and, as far as is known, he was the first thinker who made this discovery. It was Pythagoras who laid the foundations for later mathematical knowledge, especially in geometry and arithmetic, and who taught that there was a science of numbers apart from their use as a practical means of calculation. In the fifth century B.C., Athens had become an Empire and the "school of Hellas," and the centre of Greek learning was found there and no longer in Ionia. The story of Socrates has already been told.[19] This great teacher did not write anything himself because he believed that it was a greater thing for a man to live well than to write well, and that his particular way of teaching and constant intercourse with his fellow-men was the best way of teaching those truths in which he believed. The account of his life and teaching, however, was written down and given to the world by his pupil Plato, who carried on his master's work. Plato was about twenty-eight years old when Socrates was put to death, and for twelve years after that time he travelled. Then he returned to Athens, bought a house and garden (unlike Socrates he was well-off), and spent the {381} next forty years of his life teaching in the Academy. Plato was an idealist, and in addition to his writings about Socrates, he has left us the _Republic_, the picture of what he thought an ideal state should be, and some other works in which he discusses at great length what things it is most worth while that men should pursue in life, and why they should pursue them. He taught that goodness was worth being sought after for its own sake and not for any material reward that comes from pursuing it. In all his teaching he emphasizes the fact that the greatest things in life and those which are eternal are not always the things that can be seen, and that the soul of man does not live on material things but on wisdom, beauty, truth and love. The importance of Plato in this teaching was that he was the forerunner of the great Christian writers who believed with St. Paul that "the things which are seen are temporal, but the things which