_Demonstration._—For let A be moved by B, and B by C, I say that this will some time or other stop, and that not everything which moves will be itself moved: for, if possible, let this take place. Motions, therefore, are either in a circle, or _ad infinitum_. But, if things moving and moved are infinite, there will be infinite multitude and magnitude: for everything which is moved is divisible, and moves from contact. Hence, that which consists from things moving and moved infinite in multitude, will be infinite in magnitude. But it is impossible that any body, whether composite or simple, can be infinite. But if motions are in a circle, some one of things moved at a certain time, will be the cause of perpetual motion, if all things move and are moved by each other in a circle. This, however, is impossible: for that which moves a perpetual motion is perpetual. Neither, therefore, is the motion of things moved, in a circle, nor _ad infinitum_. There is, therefore, that which moves immoveably, and which is perpetual. But from hence it is evident, that all things are not moved; for there is also something which is immoveable. Nor are all things at rest; for there are also things which are moved. Nor are some things always at rest, but others always moved; for there are also things which are sometimes at rest, and sometimes moved, such as are things which are moved from contraries into contraries. Nor are all things sometimes at rest, and sometimes moved; for there is that which is perpetually moved, and also that which is perpetually immoveable. THEOREM 12. Everything which is moved, is moved by something. _Demonstration._—Let A be that which is moved, I say that A is moved by something: for it is either moved according or contrary to nature. If, therefore, it is moved according to nature, that which moves is nature; but, if contrary to nature, that which employs violence moves; for every motion contrary to nature is violent. THEOREM 13. That which first moves a circular motion is impartible, or without parts.