It is necessary that whatever is moved, should be moved by something: for if it has not in itself the principle of motion, it is manifest that it is moved by something else. But if it has this principle in itself; let A B be assumed as that which is moved, not because some part of this is moved. In the first place, therefore, to apprehend that AB is moved by itself, because the whole is moved, and this by nothing external, is just as if some one, because D E moves E F, and E F is itself moved, should apprehend that D E F is moved by itself, because he does not perceive which of these is moved by the other, i.e. whether D E is moved by E F, or E F by D E. Farther still, that which is moved by itself, will never cease to be moved, because something else which is moved stands still. It is necessary, therefore, if any thing ceases to be moved because something else stands still, that this thing should be moved by something else. But this having become manifest, it is necessary that whatever is moved should be moved by something else: for since it has been assumed that A B is moved, it will be divisible. Let it, therefore, be divided into A C and B C. Hence it is necessary, when BC is at rest, that AB also should be at rest: for if not, let it be supposed to be moved. B C therefore being at rest, A C will be moved; and hence A B will not be moved by itself. It was; however, supposed to be first moved by itself. It is evident, therefore, that C B being at rest, B A also will be at rest, and will then cease to be moved. But if any thing stands still and ceases to be moved, because something else stands still, this will be moved by something else. It is evident, therefore, that every thing which is moved, is moved by something: for every thing which is moved is divisible; and the part being at rest, the whole also will be at rests. But since every thing that is moved, is moved by something, it is also necessary that whatever is moved in place should be moved by another. Hence that which moves, will be moved by another, since it is itself also moved; and again, this will be moved by something else.
