These Egyptian systems survived in some places the true one, as they were thought to overcome the chief difficulties of the Ptolemaic without interfering with the stability of the earth, and they were known as the _common system_, _i.e._ containing the elements of both. Such were the astronomical systems in vogue before the time of Copernicus--all of them based upon the principle of the earth being the immovable centre of the universe. We must now turn to trace the history of the introduction of that system which has completely thrown over all these former ones, and which every one knows now to be the true one--the Copernican. No revolution is accomplished, whether in science or politics, without having been long in preparation. The theory of the motion of the earth had been conceived, discussed, and even taught many ages before the birth of Copernicus. And the best proof of this is the acknowledgment of Copernicus himself in his great work _De Revolutionibus Orbium Caelestium_, in which he laid down the principles of his system. We will quote the passage in which it is contained. "I have been at the trouble," he writes, "to read over all the works of philosophers that I could procure, to see if I could find in them any different opinion to that which is now taught in the schools respecting the motions of the celestial spheres. And I saw first in Cicero that Maetas had put forth the opinion that the earth moves. (Maetam sensisse terram moveri.) Afterwards I found in Plutarch that others had entertained the same idea." Here Copernicus quotes the original as far as it relates to the system of Philolaus, to the effect "that the earth turns round the region of fire (ethereal region), and runs through the zodiac like the sun and the moon." The principal Pythagoreans, such as Archytas of Tarentum, Heraclides of Pontium, taught also the same doctrine, saying that "the earth is not immovable in the centre of the universe, but revolves in a circle, and is far from occupying the chief place among the celestial bodies." Pythagoras learnt this doctrine, it is said, from the Egyptians, who in their hieroglyphics represented the symbol of the sun by the stercoral beetle, because this insect forms a ball with the excrement of the oxen, and lying down on its back, turns it round and round with its legs. Timaeus of Locris was more precise than the other Pythagoreans in calling "the five planets the organs of time, on account of their revolutions," adding that we must conclude that the earth is not immovable in one place, but that it turns, on the contrary, about itself, and travels also through space. Plutarch records that Plato, who had always taught that the sun turned round the earth, had changed his opinion towards the end of his life, regretting that he had not placed the sun in the centre of the universe, which was the only place, he then thought, that was suitable for that star.