“It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is moved is moved by another, for nothing can be moved except it is in potentiality to that towards which it is moved; whereas a thing moves in as much as it is in act.
For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.
Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.
Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different 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 moved must be moved by another. If that by which it is moved be itself moved, then this also must need be moved 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 in as much as they are moved by the first mover; as the staff moves only because it is moved by the hand. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.”
– St Thomas Aquinas, Summa Theologica, 1, 2, 3, in The Path from Science to God, by Roger Nesbitt (Faith Pamphlets)