That which is immoveable is the leader of things moving and moved.
_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.