In the article, "Whether God Exists," Saint Thomas Aquinas addresses the issue of the existence of God, or lack thereof. He proves the existence of God in five ways.
Aquinas" first way of proving the existence of God is the argument from motion. In this argument he points out that everything in motion, (motion meaning "the reduction of something from a state of potentiality into a state of actuality"), cannot move itself. It only has the potentiality of being moved by something else already in motion. "If that by which is put in motion be itself put in motion, then this also must need to be put in motion by another, and that by another again." However, this regress cannot be infinite because if it were, there would be no first mover. A first mover is essential in this process for the simple fact that if there were none, nothing would be moved, and therefore nothing would exist. This first mover, Aquinas says, is what everyone understands to be God.
The second and third ways Aquinas proves the existence of God are quite similar. The second way supposes, "In the world of sense we find there is an order of efficient causation." This order makes it impossible for anything to be the efficient cause of itself. Nothing exists prior to itself in order for it to cause itself. Therefore, everything in existence is an effect of a prior cause. This, too, cannot infinitely regress, as there must be a "First Efficient Cause" for anything to be in effect. The third way states, "not all beings are merely possible, but there must exist something the existence of which is necessary." Every "necessary" thing has to have its necessity caused by another, because its purpose is the necessity of that which caused it. Aquinas says there must be a being that causes in others their necessity; God.
In the fourth way of proving there is a God, he turns to the gradation of things. When, for instance, we compare something, we say it is more or less than something else.