I have already mentioned that Aquinas summarized five proofs for the existence of God. All of these proofs are deductive and begin with some obvious empirical fact about the world. The first way argues that God exists because he is the explanation for change in the world. It is a simple sounding argument that proves the existence of God from the fact that at least one thing changes in the world.
The argument starts with the fact that at least one thing changes in the world. Any form of change is allowed. Change can be a change of location, a change in size, a change in color, chemical properties, taste or anything else at all. So let’ s call this thing your can of cola. Things that are being changed are always being changed by something else. Let’ s call this something else Bob. Either Bob changes the can of cola without being changed or he does not. In Bob’ s case, he moves his fingers. So he does change when crushing the can of pop. So there must be something that causes Bob’ s hands to crush the can of soda. Remember that the cause we are looking for is vertical. We are asking what it causing Bob’ s hand to crush the soda at the same time that the soda is being crushed.
We could say that Bob’ s hand is crushing the soda because his muscles are contracting. But contraction is a kind of change and we would need to ask why the muscles are contracting. Now we cannot keep asking for these causes forever. There must be some cause that causes everything to happen but is not caused to happen by anything else. Imagine that Bob’ s muscles did not contract. If they did not contract, then his hand would not close and the cola can would not be crushed. So the soda can is only being crushed because Bob’ s muscles are contracting. Bob’ s hand is crushing the soda can only because his muscles are contracting. So Bob’ s hand is not the ultimate cause of the soda can being crushed. That is because if the ultimate cause did not exist, then Bob’ s hand would not close and the soda can would not be crushed. If there were an infinite series of these causes, then all of them would be the same kind of thing as ‘ Bob’ s hand is closing’. Without an ultimate cause, there would be no change at all in the world.
This ultimate cause is not changed by causing something to happen. This ultimate cause is not changed by causing the soda to be crushed. This ultimate cause is what Aquinas calls God. Later on, in other arguments, Aquinas will prove that there is only one ultimate cause, that this one cause is immaterial, all-knowing, perfectly good,all-powerful and necessary. He does not prove this with this kind of argument right here.