Anaximenes of Miletus taught, like his predecessors, crude notions of the sun and stars, and speculated on the nature of the moon, but did nothing to advance his science on true grounds, except by the construction of sun-dials. The same may be said of Heraclitus, Xenophanes, Parmenides, and Anaxagoras: they were great men, but they gave to the world mere speculations, some of which are very puerile. They all held to the idea that the heavenly bodies revolved around the earth, and that the earth was a plain; but they explained eclipses, and supposed that the moon derived its light from the sun. Some of them knew the difference between the planets and the fixed stars. Anaxagoras scouted the notion that the sun was a god, and supposed it to be a mass of ignited stone,--for which he was called an atheist. Socrates, who belonged to another school, avoided all barren speculations concerning the universe, and confined himself to human actions and interests. He looked even upon geometry in a very practical way, valuing it only so far as it could be made serviceable to land-measuring. As for the stars and planets, he supposed it was impossible to arrive at a true knowledge of them, and regarded speculations upon them as useless. It must be admitted that the Greek astronomers, however barren were their general theories, laid the foundation of science. Pythagoras taught the obliquity of the ecliptic, probably learned in Egypt, and the identity of the morning and evening stars. It is supposed that he maintained that the sun was the centre of the universe, and that the earth revolved around it; but this he did not demonstrate, and his whole system was unscientific, assuming certain arbitrary principles, from which he reasoned deductively. "He assumed that fire is more worthy than earth; that the more worthy place must be given to the more worthy; that the extremity is more worthy than the intermediate parts,--and hence, as the centre is an extremity, the place of fire is at the centre of the universe, and that therefore the earth and other heavenly bodies move round the fiery centre." But this was no heliocentric system, since the sun moved, like the earth, in a circle around the central fire. This was merely the work of the imagination, utterly unscientific, though bold and original. Nor did this hypothesis gain credit, since it was the fixed opinion of philosophers that the earth was the centre of the universe, around which the sun, moon, and planets revolved. But the Pythagoreans were the first to teach that the motions of the sun, moon, and planets are circular and equable. Their idea that the celestial bodies emitted a sound, and were combined into a harmonious symphony, was exceedingly crude, however beautiful "The music of the spheres" belongs to poetry, as well as to the speculations of Plato.