Whenever we begin to think about the formation of the universe we get at once into the realms of speculation, and the only value of our thoughts rests in their probability. In everything unknown we must first form an idea—that is, speculate; then, by partial gatherings of facts, or by positive reasoning, we may theorize. Ultimately, by the accumulation of evidence, we may prove that which, in the first place, we only imagined. When first men observed the sun, they regarded the earth as a flat plain, over which the sun passed in his heavenly course, and below which, at eve, he retired to rest. It was not until many ages had elapsed that the world came to be regarded as round, and even then it was long before the sun was considered as a fixed centre of the planetary system revolving round him.

By no nation of ancient times has astronomy been more advanced than the Greeks. Not that the Greeks ever worked out much to a proved result, but they were an imaginative people, and they invented notions. If one theory or speculation was disproved, they invented another; and, hit or miss, they always seemed to have fresh ideas in reserve. In some things astronomical, as in many other things that the world believes in, we may be heretics, and we admit we do not adhere to all the cosmical, physical, geological, and spiritual tenets of the popular faiths. We may not entirely believe in the perfect stability of the universe; we may doubt the eternal endurance of the sun's bright rays; and we may not quite acquiesce in the unchangeable permanence of tne planetary orbits: in short, we do not believe in the permanence of anything whatever in creation. All ever has been change, and changeful all things ever will be. Diversity and change are visible in the first created things of which any relics have been left us.