Question 1 of part 1 of the Summa considers the nature and extent of “sacred doctrine,” or theology. Aquinas concludes that, although theology does not require philosophy to promote knowledge of God, philosophy nevertheless can be of service to the aims of theology.
Question 2 of part 1 concerns the existence of God and is subdivided into three Articles. In the First Article, Aquinas maintains that the proposition “God exists” is self-evident in itself, but not to us, and thus requires demonstration. The Second Article concludes that such a demonstration is indeed possible, despite objections to the contrary. The famous Third Article addresses the question of whether God exists, and in this Article, Aquinas offers his Five Ways as proofs for the existence of God.
First, we observe that some things in the world are in motion. Whatever is in motion is put into motion by another object that is in motion. This other object, in turn, was put into motion by still another object preceding it, and so forth. This series cannot go on backward to infinity, though, since there would otherwise be no first mover and thus no subsequent movement. Therefore, we must conclude that there is a first unmoved mover, which we understand to be God.
Second, we observe that everything has an efficient cause and that nothing is or can be the cause of itself. It is impossible, though, that the series of causes should extend back to infinity because every cause is dependent on a prior cause and the ultimate cause is thus dependent on a previous cause. So if there is no first cause, there will be no intermediate causes and no final cause. But the absence of such causes clearly does not square with our observation, and so there must therefore be a first efficient cause, which everyone calls God.
Third, we observe in nature things that are possible to be and not to be, as they come into existence and pass out of existence. Such things could not always exist, though, because something that could possibly not exist at some time actually does not exist at some time. Thus, if it is possible for everything not to exist, then, at some time, nothing did exist. But if nothing ever did exist, then nothing would exist even now, since everything that exists requires for its existence something that already existed. Yet it is absurd to claim that nothing exists even now. Therefore, not all beings are merely possible, but there must be something the existence of which is necessary. Now, every necessary thing has its necessity caused by something else or it does not. Since it is impossible for there to exist an infinite series of causes of necessary things, we must conclude that there is something that is necessary in itself. People speak of this thing as God.
Fourth, beings in the world have characteristics to varying degrees. Some are more or less good, true, noble, and so forth. Such gradations are all measured in relation to a maximum, however. Thus, there must be something best, truest, noblest, and so on. Now, as Aristotle teaches, things that are greatest in truth are also greatest in being. Therefore, there must be something that is the cause of being, goodness, and every other perfection that we find in beings in the world. We call this maximum cause God.