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.

Finally, we observe in nature that inanimate and nonintelligent objects act toward the best possible purpose, even though these objects are not aware of doing so. It is clear that these objects do not achieve their purpose by sheer chance but rather according to a plan. Any inanimate or nonintelligent object that acts toward a purpose, though, must be guided by a being that possesses knowledge and intelligence, just as an arrow is directed by an archer. Therefore, there must be some intelligent being that directs all natural things toward their purpose. We call this being God.

Having presented these proofs for the existence of God, Aquinas goes on to discuss God in terms of his simplicity, perfection, goodness, infinity, knowledge, and other attributes. This discussion leads into a protracted consideration of questions pertaining to the Creation, the nature of angels, demons, and the work done on the individual six days of the Creation, which culminated with the creation of man.


The existence of God is the necessary foundation of any theology. Before discussing any other topics, Aquinas needs to establish the crucial fact that God exists, since, without certainty of God’s existence, the conclusions of the rest of the Summa would be in doubt or even in vain. To this end, he advances five arguments intended to prove the existence of God. Arguments 1, 2, and 5 are based on observation of the natural world, whereas Arguments 3 and 4 are based on rational speculation. In Arguments 1, 2, 4, and 5, Aquinas concludes that only the existence of God can provide a sufficient explanation for the questions raised. In Argument 3, he concludes that God must necessarily exist for his own sake. Thus, Arguments 1, 2, 4, and 5 conclude that God exists because the world requires him as an explanation, and Argument 3 concludes that God could not not exist.

Argument 1 considers and attempts to account for the presence of change in the world. Aquinas draws his argument from Aristotelian physics, which was known as “natural philosophy” in Aquinas’s day and which studied motion and change in the physical world. Just as everything that exists in the world is generated by something before it, so too must motion be passed from one object to another. Rigidly applying this principle, though, we find ourselves confronted with an infinitely regressive series and thus with the need for a first unmoved mover to set the entire series into motion. Aquinas is saying that an infinitely regressive series is impossible, and from the impossibility of such a series, he concludes that the first unmoved mover can be only God.

Argument 2 marks a transition from argumentation based on physics to argumentation based on metaphysics and considers the existence of the world as a whole. In this argument, Aquinas relies on the “principle of efficient causation,” a cardinal assumption of physics which states that every effect must have a cause. Aquinas reasons by analogy that, just as no object in the world comes into being from nothing or by itself but every object is caused, so too must the world as a whole come into being through a cause, namely, through God.

Argument 3 carries the premise of Argument 2 into the realm of metaphysics and rational speculation about being itself. Aquinas first defines possible beings as those that can either exist or not exist, thereby implying that necessary beings are those that necessarily must, and thus do, exist. All objects in the world are possible beings and thus can either exist or not exist. Aquinas reasons that, since these objects can, in principle, either exist or not exist at any time, then they did in fact not exist at some time. Yet, Aquinas continues, if they did not exist at some time, then we are at a loss to explain the obvious existence of the world now, since all that exists requires a cause for its existence. Aquinas concludes that there must be an absolutely necessary being, that is, one that (1) must necessarily exist and (2) thus owes its existence to no other being.

Argument 4 is unique among the five Arguments in that it considers not the physical or metaphysical but the qualitative. By a leap of abstraction, Aquinas, adopting Aristotle, concludes that there must be something in relation to which all individual qualities, such as good, true, beautiful, and noble, are measured and from which those qualities derive their existence. For example, the existence of something good implies the existence of something best that not only serves as the ultimate benchmark against which the good thing is measured but also even causes the good thing to exist. The idea that ultimate qualities are responsible for the existence of lesser instances of qualities is strongly reminiscent of Plato’s idea that Forms (i.e., essences) are the real and true originals of which lesser beings (i.e., existences) are pale and inferior copies. Nevertheless, Aquinas, following Aristotle, invests these ultimate qualities with being—in other words, with existence.

Argument 5 appeals to our wonder in the face of the apparent purposive activity of the animate and inanimate worlds alike. The world, functioning with such smoothness, efficiency, detail, and aim, simply cannot be the product of chance but must be the product of a sort of grand architect, that is, of God. Aquinas is drawing two rather bold conclusions here: there is a designer and that the designer is God.

There are strong conceptual ties between and among the first three Arguments. Arguments 1 and 2 are similar in that both maintain that there cannot be a series of causes stretching back infinitely. The two Arguments are different, though, in that Argument 1 considers the cause of motion in individual objects in the world, whereas Argument 2 considers the cause of the entire world itself. Argument 1 takes the existence of the world for granted and seeks to account for observable change in the physical world. Argument 2, on the other hand, does draw on observation of the world but attempts to account for the existence of the world. Argument 3 considers the concept of being itself and casts its gaze toward theoretical, nonobservable states of the world far beyond our possible experience. Thus, the first three Arguments attempt to force one to accept the proposition that only the existence of God can account for (1) change in the physical world, (2) the existence of the physical world, and (3) existence itself.

Having established that God exists, Aquinas is free to consider God’s nature and works.