Aquinas observes that there are some things in the world which change. Something which changes cannot be the very thing that causes its own change. In fact, anything that changes is changed by something else (e.g. it is the heat of fire that causes the wood to become hot, not the wood itself). This something else may be also changing, and if this is the case, then it must be changed by yet another thing. If this went on forever , there would be no first cause of the change, and if there weren't no first cause of the change, there would not be subsequent causes. Hence, this chain of causes cannot go on forever. Therefore, we are forced eventually to come to a first cause of change not itself being changed by anything. And this first cause of change is God. This is how Aquinas uses the idea of change to show that God exists.