Yes! cried Philo, interrupting him, we have other earths. Is not the moon another earth, which we see to turn round its centre? Is not Venus another earth, where we observe the same phenomenon? Are not the revolutions of the sun also a confirmation, from analogy, of the same theory? All the planets, are they not earths, which revolve about the sun? Are not the satellites moons, which move round Jupiter and Saturn, and along with these primary planets round the sun? These analogies and resemblances, with others which I have not mentioned, are the sole proofs of the Copernican system; and to you it belongs to consider, whether you have any analogies of the same kind to support your theory. In reality, Cleanthes, continued he, the modern system of astronomy is now so much received by all inquirers, and has become so essential a part even of our earliest education, that we are not commonly very scrupulous in examining the reasons upon which it is founded. It is now become a matter of mere curiosity to study the first writers on that subject, who had the full force of prejudice to encounter, and were obliged to turn their arguments on every side in order to render them popular and convincing. But if we peruse Galilæo's famous Dialogues concerning the system of the world, we shall find, that that great genius, one of the sublimest that ever existed, first bent all his endeavours to prove, that there was no foundation for the distinction commonly made between elementary and celestial substances. The schools, proceeding from the illusions of sense, had carried this distinction very far; and had established the latter substances to be ingenerable, incorruptible, unalterable, impassible; and had assigned all the opposite qualities to the former. But Galilæo, beginning with the moon, proved its similarity in every particular to the earth; its convex figure, its natural darkness when not illuminated, its density, its distinction into solid and liquid, the variations of its phases, the mutual illuminations of the earth and moon, their mutual eclipses, the inequalities of the lunar surface, &c. After many instances of this kind, with regard to all the planets, men plainly saw that these bodies became proper objects of experience; and that the similarity of their nature enabled us to extend the same arguments and phenomena from one to the other.