Since everything that is moved functions as a sort of instrument of the first mover, if there was no first mover, then whatever things are in motion would be simply instruments. Of course, if an infinite series of movers and things moved were possible, with no first mover, then the whole infinity of movers and things moved would be instruments. Now, it is ridiculous, even to unlearned people, to suppose that instruments are moved but not by any principal agent. For, this would be like supposing that the construction of a box or bed could be accomplished by putting a saw or a hatchet to work without any carpenter to use them. Therefore - there must be a first mover existing above all - and this we call God.
From Demonstration of God's Existence
-St. Thomas Aquinas