Therefore, since plainly the motion which many movable bodies undergo in common is without effect and, with regard to their mutual position toward one another, it is as if it did not exist, for there is no change among them; and since it only affects the relative position of such bodies as do not share in the movement, for in this case the mutual relation is changed; since we have divided the universe into two parts, of which one must be movable and the other immovable; then for all purposes this movement will be of the same effect whether it is ascribed to the earth alone or to all the rest of the universe. For the working of such a motion is on nothing but the relative position in which the earth and the heavenly bodies stand to one another, and aside from this relative position nothing changes. If now it is indifferent for accomplishing this result whether the earth alone moves and the whole universe rests, or the earth rests and the whole universe is subject to one common movement, who can believe that Nature--who by common agreement does not employ great means when she can obtain the same result by smaller ones--would have undertaken to set in motion an immeasurable number of mighty bodies, and that with incredible velocity, to accomplish what could be obtained by the moderate motion of one single body around the center? SIMPL.: I do not agree that that mighty movement would be as if it did not happen in regard to the sun, the moon, the innumerable host of fixed stars. Do you call it nothing that the sun goes from one meridian to another, rises from one horizon, sinks under another, brings now day, now night; that the moon goes through similar changes and likewise the other planets, as well as the fixed stars? SALV.: All the changes mentioned by you are such only with respect to the earth. To demonstrate this, only imagine yourself away from the earth; there is then no rising or setting of the sun, no horizons, no meridians, no day, no night; in a word, by the movement mentioned no change in the relation of the moon to the sun or to any other star is evoked. All these changes have reference to the earth; they are supposed only because the sun is first visible in China, then Egypt, Greece, France, Spain, America, and so on, and so also for the moon and the other heavenly bodies. The same process would occur in the same way, if, without disturbing so vast a part of the universe, the earth alone should be revolved.