respects. For what is actually hot cannot simultaneously be
potentially hot; but it is simultaneously potentially cold. It is
therefore impossible that in the same respect and in the same way a
thing should be both mover and moved, i.e. that it should move itself.
Therefore, whatever is in motion must be put in motion by another. If
that by which it is put in motion be itself put in motion, then this
also must needs be put in motion by another, and that by another
again. But this cannot go on to infinity, because then there would be
no first mover, and, consequently, no other mover; seeing that
subsequent movers move only inasmuch as they are put in motion by the
first mover; as the staff moves only because it is put in motion by
the hand. Therefore it is necessary to arrive at a first mover, put in
motion by no other; and this everyone understands to be God.

The second way is from the nature of the efficient cause. In the world
of sense we find there is an order of efficient causes. There is no
case known (neither is it, indeed, possible) in which a thing is found
to be the efficient cause of itself; for so it would be prior to
itself, which is impossible. Now in efficient causes it is not
possible to go on to infinity, because in all efficient causes
following in order, the first is the cause of the intermediate cause,
and the intermediate is the cause of the ultimate cause, whether the
intermediate cause be several, or only one. Now to take away the cause
is to take away the effect. Therefore, if there be no first cause
among efficient causes, there will be no ultimate, nor any
intermediate cause. But if in efficient causes it is possible to go on
to infinity, there will be no first efficient cause, neither will
there be an ultimate effect, nor any intermediate efficient causes;
all of which is plainly false. Therefore it is necessary to admit a
first efficient cause, to which everyone gives the name of God.