The corresponding argument from the capacity of our human imagination to think always of a greater than any given magnitude, _i.e._ its inability to rest short of the infinite, is expanded elsewhere. Our imaginative faculty is the _umbra_ or shadow of nature; its power, therefore, of adding quantity to quantity, _ad infinitum_, must have something in nature to which it corresponds; nature does not give a faculty for which there is no satisfaction. There is then in truth an infinite universe, such as our imagination demands. Bruno notices the objection that on this theory anything whatever might be said about the universe, _e.g._ that it is infinite man, since one can imagine a human form filling the universe; and he replies, "it _is_ infinite man, or infinite ass, or infinite tree, each and all, since in the infinite all particular things are one and the same."[299] [Sidenote: Aristotle.] [Sidenote: 1. The _primum mobile_.] [Sidenote: 2. The elements.] [Sidenote: 3. The whole and its parts.] The arguments we have traced are:--(1) What appears to be a limit to our senses always proves to be imaginary, when we are able to test it, therefore we may infer that it is imaginary in other cases; (2) the very notion of space, implying that it has neither form nor place, means that it is infinite, limitless; (3) we cannot imagine a portion of space than which there is not another greater, and so _ad infinitum:_ but reality cannot fall short of thought, therefore space is infinite. The arguments of Aristotle against the infinity of the world are taken up in detail in the second book of the _De Immenso_. As the controversy, however important at the time, has lost much of its interest for us, we need only give a brief sketch of its main lines. The first argument was drawn from the assumption of an ultimate sphere or _primum mobile_ which moved about the earth as a centre.[300] It was clear that if the universe were infinite the radii of this sphere would be infinitely prolonged, and therefore the termini of any two given radii at an infinite distance one from another. The motion of the sphere would thus be inconceivable, for it would require infinite time in which to pass from one point to another. The answer of Bruno was that the universe as a whole was not moveable at all, nor had it any centre; only its parts were moved and each of these had its own relative and finite centre. The apparent motion of the sphere was due to the real movement of the earth about its axis. A similar answer was given to the argument from the movements of bodies according to their elements. As to us on the earth, the earth appears to be the centre of the universe, so to the inhabitants of the moon, the moon will appear to be such. Matter rising from the earth to the moon would appear to the inhabitants of the latter to fall. These distinctions were relative to the finite worlds, but might not be referred to the whole universe.