As the earth is one world, the moon another, so each has its own centre, each its own _up_ and _down_: nor can these differences be assigned absolutely to the whole and its parts together, but only relatively to the position and condition of the latter.[301] In his _third_ argument Aristotle sought to prove that infinite body in general was impossible.[302] If the whole is infinite its simple elements must be so also. These must be either of an infinite number of kinds, different from one another, or of a finite number of kinds, or all of the same kind. But the first of the alternatives is impossible on the _a priori_ ground that each element must have a special kind of movement corresponding to it, and the kinds of movement are actually few in number; the second and third, because the movement of the elements should then be infinite, whereas in the actual universe motion is limited both in centre and circumference. The arguments, however, do not apply to Bruno's theory of the universe. Motion is always from one definite point to another; we do not set out from Italy in order to go on _ad infinitum_, but to go to some definite point. He does not, as Epicurus did, regard all minima as in infinite motion downwards through the universe; there is no down, no centre, no up, all is simply and generally in flux. It is not the elements that are innumerable in kind, but the composite bodies, the stars, which are constituted by them; and of these the parts move about their natural body, as the parts of the earth towards the earth, and those of the moon toward the moon in their own regions; all motion is therefore limited,--each world has, as it were, _margins_ of its own. The idea that if any of the elements, as fire or water, were infinite, there would be infinite lightness or gravity, and hence that the universe would move as a whole upwards or downwards, is equally at fault. To the universe as a whole the terms heavy and light do not apply, but only to its parts, the finite and determinate bodies consisting of finite and determinate elements. These elements, whether they be taken as of one or more kinds, since they cannot move outside of the universe, must have finite movements. [Sidenote: 4. Action between the infinite and the finite.]