The Ptolemaic hypothesis of the universe was then in fashion. It was supposed that the earth was the centre of celestial motions, that the sun, the moon, and the stars revolved around the world which we inhabit. Not that the Pythagorean hypothesis was totally forgotten. There were those who believed that the sun, not the earth, is the centre of the great circle in which the heavenly bodies perform their evolutions; but the Ptolemaic hypothesis had the ascendency beyond all doubt; and with this hypothesis Copernicus could not rest satisfied. It appeared to him beset with insuperable difficulties. True enough, the rotation of the heavens around the earth seemed to be what the human eye beheld, as anyone watched sunrise and sunset. But what the senses thus presented, reason, in its ponderings, was led to contradict. For the notion of a huge mechanism like the celestial sphere, spinning round the terraqueous globe as its pivot looked unreasonable. To explain it in any way on mathematical principles needed a most complicated array of cycles and epicycles. Symmetry and simplicity were wanting in the theory. _A priori_ objections started up against it. If the senses pointed to the earth as a centre, reason pointed to a centre elsewhere. Copernicus studied the works of ancient philosophers on the question. He examined mathematical traditions and criticised the opinions of learned professors. He found accounts of those who had asserted the motion of the earth. "Though," he says, "it appeared an absurd opinion, yet, since I knew that in former times liberty had been permitted to others to figure as they pleased certain circles for the purpose of demonstrating the phenomena of the stars, I considered that to me also it might be easily allowed to try whether, by a supposition of the earth's motion, a better explanation might be found of the revolution of the celestial orbs. Having assumed," he goes on to say, "the motions of the earth, by laborious and long observation I at length found that if the motions of the other planets be compared with the revolution of the earth, not only these phenomena follow from the suppositions, but also that the several orbs and the whole system are so connected in order and magnitude, that no one part can be transposed without disturbing the rest and introducing confusion into the whole universe." What Copernicus was in search of was some simple and symmetrical theory of the appearances of the heavens which would relieve him of the complexity and confusion attendant on the Ptolemaic system so popular in the schools. He started from an _a priori_ point of reasoning--the only one thought of in his day--but he came to certain conclusions which _a posteriori_ examination in after times abundantly confirmed. [Illustration: Copernicus.]