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.
His successor, Democritus, adopted his theory of the origin of knowledge, and by applying it to the problem of the One and the Many, produced the most striking of ancient anticipations of modern science. He regarded the world as something made up of invisible particles, each absolutely similar to the other; these formed the essential unity which could be grasped only by the reason, but by their various combinations and arrangements they brought about the apparent multiplicity of objects which the senses perceived. Such was the foundation of the atomic theory of Democritus. He conceived the atom as a centre of force, and not as a particle having weight and material qualities. As, however, his hypothesis was purely a metaphysical one, it did not lead to any of the discoveries which have followed on the establishment of the modern scientific theory, which was arrived at in a different way, and has a different signification. Democritus also threw out in vague outline the idea of gravitation. But this was not science: it was guess-work; it afforded no ground on which the fabric of verified knowledge could be erected, and no sure method of obtaining this knowledge.
The World's Greatest Books (Philosophy and Economics), Vol. XIV, p. 45-48