In his view, any one single thing could not be all things, and in his famous saying, "The infinite is the origin of all things," he introduced into metaphysics an abstract conception in place of the inadequate concrete principles of Thales and his disciples. Pythagoras was a contemporary of Anaximander, and, like him, one of the great founders of mathematics. He held that the only permanent reality in the cosmos was the principle of order and harmony, which prevented the universe from becoming a blank, unintelligible chaos; and he expressed this idea in his mystic doctrine: "Numbers are the cause of the material existence of things." The movement which he spread by means of a vast, secret confraternity ended, however, in a barren symbolism, and it is impossible to trace what relation his strange theories of the transmigration of souls and the music of the spheres have to his general system of thought. Far more influence on the progress of speculation was exercised by Xenophanes of Colophon. Driven by the Persian invasion of 546 B.C. to earn his living as a wandering minstrel, he developed the ideas of Anaximander, and founded the school of great philosophic poets, to which Parmenides, Empedocles and Lucretius belong. He is the grand monotheist, and he has published his doctrines in his verses: There is one God alone, the greatest of spirits and mortals, Neither in body to mankind resembling, neither in ideas. Shelley's line: "The One remains, the Many change and pass," sums up the teaching of the line of thinkers which culminated in Plato. In their view, knowledge derived from the senses was fallacious because it touched only the diverse and changing appearances of things; absolute knowledge of the one abiding spiritual reality could, they held, only be obtained by the exercise of spiritual faculty of reason, which, unlike the animal power of sense, is the same in all men. One of the philosophers of this school, Zeno of Elea, was the inventor of the dialectic method of logic, which Socrates and Plato used with so tremendous an effect. Anaxagoras, however, attempted to reconcile the evidence of the sense with the dictates of the reason. He was the first philosopher to settle in Athens, and Pericles, Euripides, and Socrates were among his pupils. He was extraordinarily modern in many of his ideas. He held that the matter of knowledge was derived through the senses, but that reason regulated and verified it, and he carried this dualism into his conception of the universe, which he represented as a manifestation of a Divine intelligence, acting through invariable laws, but in no way confused with the matter acted on.