…then all things that are in motion must be moved by something.
Chapter 5
Now
this may come about in either of two ways. Either the movent is not itself responsible
for the motion, which is to be referred to something else which moves the movent,
or the movent is itself responsible for the motion. Further, in the latter case,
either the movent immediately precedes the last thing in the series, or there
may be one or more intermediate links: e.g. the stick moves the stone and is
moved by the hand, which again is moved by the man: in the man, however, we
have reached a movent that is not so in virtue of being moved by something else.
Now we say that the thing is moved both by the last and by the first movent
in the series, but more strictly by the first, since the first movent moves
the last, whereas the last does not move the first, and the first will move
the thing without the last, but the last will not move it without the first:
e.g. the stick will not move anything unless it is itself moved by the man.
If then everything that is in motion must be moved by something, and the movent
must either itself be moved by something else or not, and in the former case
there must be some first movent that is not itself moved by anything else, while
in the case of the immediate movent being of this kind there is no need of an
intermediate movent that is also moved (for it is impossible that there should
be an infinite series of movents, each of which is itself moved by something
else, since in an infinite series there is no first term)-if then everything
that is in motion is moved by something, and the first movent is moved but not
by anything else, it much be moved by itself.
This same
argument may also be stated in another way as follows. Every movent moves something
and moves it with something, either with itself or with something else: e.g.
a man moves a thing either himself or with a stick, and a thing is knocked down
either by the wind itself or by a stone propelled by the wind. But it is impossible
for that with which a thing is moved to move it without being moved by that
which imparts motion by its own agency: on the other hand, if a thing imparts
motion by its own agency, it is not necessary that there should be anything
else with which it imparts motion, whereas if there is a different thing with
which it imparts motion, there must be something that imparts motion not with
something else but with itself, or else there will be an infinite series. If,
then, anything is a movent while being itself moved, the series must stop somewhere
and not be infinite. Thus, if the stick moves something in virtue of being moved
by the hand, the hand moves the stick: and if something else moves with the
hand, the hand also is moved by something different from itself. So
when motion by means of an instrument is at each stage caused by something different
from the instrument, this must always be preceded by something else which imparts
motion with itself. Therefore, if this last movent is in motion and there is
nothing else that moves it, it must move itself. So this reasoning also shows
that when a thing is moved, if it is not moved immediately by something that
moves itself, the series brings us at some time or other to a movent of this
kind.