Lecture Notes: Aquinas's Summa Theologica

UC Davis Philosophy 1

G. J. Mattey

Thomas Aquinas

Thomas Aquinas (1224-1274) is the best-known philosopher of the later medieval period, due to endorsement of his work by the Roman Catholic Church. In 1879, some 600 years after his death, Pople Leo XXIII declared him the "chief and master among all the scholastic doctors." He recommended that "thoughtfully chosen teachers apply themselves to introducing the doctrine of Thomas Aquinas into the minds of students and set its soundness and excellence clearly ahead of others" (Cited in Anthony Kenny, ed., The Oxford Illustrated History of Western Philosophy, p. 88.) Aquinas had studied the newly-available work of Aristotle in his native Italy (he was born in Roccasecca), before pursuing his further studies at the University of Paris. His most important work is the Summa Theologica, which he intended as a textbook. He wrote a number of additional books on various theological topics, as well as commentaries on Aristotle.

Aquinas's Contributions

Aquinas is best known for his "synthesis" of Aristotleian philosophy and Christian theology. As will be seen below, he exploited the rational arguments of the pagan he called "the Philosopher" in support of Christian doctrines. In this way, he challenged the older Augustinean synthesis of neo-Platonic philosophy and the newly-emerging doctrines of the Christian church. Of particular importance in this connection is the careful separation made by Aquinas between matters that are in the provence of reason and those which require divine revelation. This will be discussed in what follows. Aquinas tried to establish the existence of God through five rational arguments adapted from Aristotle, the so-called "Five Ways." Each of these arguments proceeded from effect to cause. Aquinas rejected Anselm's attempt to establish God's existence from pure thought.

The Recovery of Aristotle

With the decline of the Roman empire in the West, the philosophical works of the ancient philosophers became mostly inaccessible. The only available writing of Plato was part of his cosmological work Timaeus. In the 5th and 6th centuries, the philosopher Boethius had translated two of the logical works of Aristotle (Categories and On Interpretation) into Latin, so that Aristotle was known as a philosopher of logic. (Latin was the language of the scholars by this time.) In the 12th century, other of the logical writings began circulating widely, and finally the rest of the corpus. The texts had survived under the care of the Arabs, and a number of commentaries written by them were translated into Latin along with Aristotle's works. The two leading Arabic commentators were Avicenna (Ibn Sina, 980-1037) and Averroes (Ibn Rushd, 1126-1198).

Reason and Revelation

The works of the pagan philosophers must have been stunning to the Medievals, raising the question whether reason alone could discover the nature of all of reality, and whether divine revelation was needed at all. Aquinas argues that reason must be supplemented by revelation. He gives two reasons. First, reason is limited in its scope. It cannot penetrate into the mind of God and discern God's purposes for creation. But among these purposes are those of human existence. If humans are to live as God intends them to live, they must know God's will, and this can be discovered only through revelation. The second argument is related. Everyone needs to know how to live, but results of philosophical reflection are available only to a few. In Aquinas's day, those would have been literate scholars, who generally were either priests or monks.


Aquinas wished to bring theology to the level of a science, as Aristotle conceived a science. But whereas Aristotelian science begins with sense-perception and works up to the universal, theology begins with divine revelation and arranges it into a systematic unity. What unifies the science is not its subject-matter: not, for example, in the way that for Aristotle metaphysics is the science of being insofar as it is being. The only thing that unifies the subject-matter is the source of knowledge. (Organized in this way, there might also be a science of things given in perception, or a science of things known through reason.) The subject-matter of the science is heterogeneous in the sense that it is both theoretical and practical. That is, pertains both to what is known and to the regulation of action. This is in keeping with the fact that revelation is about matters such as the nature of God and God's creation, as well as prescriptions for the proper living of one's life. Aquinas held that theology is more certain than the lower sciences (say, physics) because its source is the infallible, while the sources of knowledge of other things, the senses or reasoning, are fallible. Philosophy is integrated into theology, but it always plays a secondary role. That is, it only clarifies the teachings of revelation. It is revealed that God exists, and Aristotle provides arguments that allow us to see more clearly why this is so.

The Practice of Theology

Although theology treats of God's creation, its primary subject-matter is God. Aquinas takes it for granted that God has revealed to human being information about what he is and how he is related to his creation. Like Aristotle, Aquinas believed that philosophy should rest on undemonstrated first principles. But Aquinas's first principles are articles of faith. Its basic arguments are from authority, the authority of God, who is infallible. Aquinas appealed to other authorities, such Aristotle and earlier Christian thinkers such as Augustine, but these appeals produced weaker arguments. Anyone who rejects the authority of divine revelation will reject many of Aquinas's arguments. He recognized this point and declared that there is no disputing with such people.

The Investigation of God

Theological science has three divisions, treating of two different kinds of beings and one that is a synthesis of the two. Thus its three main divisions are treatments of Here we will be concerned only with the treatment of God.

Within that threatment, there is another threefold division, concerning:

Our notes here will he confined to part of the treatment of the divine essence, which itself is divided again into three subjects:

Is God's Existence Self-Evident?

Aquinas will be presenting rational arguments for the existence of God, and so the question arises as to whether such arguments are needed at all. The existence of God might be thought to be self-evident. There is scripture which says that God has implanted knowledge of himself in us. Moreover, Anselm had argued that anyone who understands the meaning of 'God' is compelled to admit God's existence.

Aquinas replies by describing, in Aristotelian fashion, how it is that we know something self-evidently. This occurs only when we recognize that the predicate is included in the essence of the subject. For example, if the subject is man (or human being), it is self-evident that the predicate animal applies, because animal is part of the essence of man (i.e., part of rational animal). In the present case, the subject is God and the predicate is existence. So does existence belong to the essence of God? Aquinas asserts that it does, but that this is not enough to make the existence of God self-evident to us humans. It is self-evident only "in itself." So if we are to know that God exists, we must demonstrate this from other principles. The evidence for God's existence will be effects in the created world, which will be connected by reason to God as a cause.

The Ontological Argument

As we have seen, Anselm argued that if one thinks of God, one must admit that God exists. This is because God is correctly understood as something than which nothing greater can be thought. If one succeeds in thinking this thought, one must recognize that this level of greatness can be possessed only by something that exists.

Aquinas posed three objections to this argument. The first is that many people do not grasp the concept of God, so it is at least not self-evident to them through the thought of God that God exists. But even if we grant that everyone understands something than which nothing greater can be thought, the argument fails. Its only conclusion is that no one can think of such a being as not existing. This may only be a limitation of human thought.

Finally, Aquinas held that the argument poses no difficulty for the atheist. There is only a contradiction in holding that something than which nothing greater can be thought does not exist if it is given that such a being does exist. But the atheist will not admit that. There is no contradiction in thinking the non-existence of something greater than which nothing can be thought. The objection is a little clearer in Aquinas's earlier work Summa Contra Gentiles.

Hence, that than which a greater cannot be thought will likewise not have to exist save only in the intellect. From this it does not follow that there exists in reality something than which a greater cannot be thought. No difficulty, consequently, befalls anyone who posits that God does not exist. For that something greater can be thought than anything given in reality or in the intellect is a difficulty only to him who admits that there is something than which a greater cannot be thought in reality. (Book I, Chapter 11)
We might adapt this to Gaunilo's argument for the existence of the Lost Island. Such an island would be greater than anything given in reality (it is greater than all existing islands), and nothing is given in the intellect about the existence of such an island. So there is no problem in my thinking that there is no lost island, unless I am already committed to its existence.

Proving the Existence of God

The proper way to prove that God exists is through something given that is given in reality, the created world. Aquinas's "Five Ways" all appeal in one way or another to God as the cause of what is patently real in nature. Since this exists, its cause must exist prior to it, as that which is responsible for its existence. Arguments of this sort are limited in what they can demonstrate, Aquinas admits. What we observe in nature is finite, while the cause, God, is infinite. As David Hume put it some 500 years later, we can only infer enough about the cause to explain the effect.

The Argument From Motion

The first argument virtually duplicates Aristotle's own proof for the existence of his God, the unmoved mover. It is important to note that this does not mean that the pagan Aristotle conceived of God in the same way as did Aquinas. For Aristotle, being the mover of the universe is God's only relation to the universe, while for Aquinas, God's relation to the world is extremely complex.
  1. An object cannot both move and be moved in the same respect and the same way
  2. So, if an object is moved, there is a distinct mover
  3. The series of movers cannot go on infinitely
  4. So, there is a first mover which is not moved
  5. The unmoved mover is God
  6. So, God exists
The original argument from Aristotle's Physics runs as follows.
It is necessary, then, for whatever is moved to be moved by the agency of some mover, either by the agency of a mover that is in turn moved by the agency of something else moving it or by the agency of a mover that is not moved by the agency of something else moving it. And if it is moved by the agency of something else moving it, there must be some first mover that is not moved by the agency of something else; and if this is the character of the first mover, it is not necessary for there to be another mover. For it is impossible to have an infinite series of movers each of which initiates motion and is moved by the agency of something else; for there is no first term in an infinite series.

If, then, everything that is moved is moved by the agency of something, and if the first mover is moved, but not by the agency of something else, then it necessarily follows that it is moved by its own agency. (BooK VIII, Chapter 5, translated by Irwin and Fine in Aristotle: Introductory Readings)

One possible weak point in the argument is step 3, which both Aristotle and Aquinas accept without comment. Even if we cannot conceive of how an infinite chain of movers is possible, it seems that we cannot demonstrate that it is impossible, which was pointed out by Immanuel Kant in his Critique of Pure Reason (Antinomy of Pure Reason, Section IX).

The Argument from Causality

The second argument is quite similar to the first. Whereas the argument from motion focuses on a property of an object, that it is moving, the second appeals to the existence of the object itself. For an object to be caused to exist, there must be a separate, prior efficient cause or agent that brings it into existence.
  1. Nothing can be prior to itself
  2. So, nothing in nature can be the efficient cause of itself
  3. The series of efficient causes cannot go on infinitely
  4. So, there is a first efficient cause
  5. The first efficient cause is God
  6. So, God exists
The Kantian point made about step 3 of the first argument applies here as well. A further point of interest is that by step 1, God would have to be an uncaused cause. Other philosophers (most notably Spinoza) have held that this is an unintelligible notion, from which they conclude that God is the cause of himself, a causa sui. It is questionable whether either of these two conceptions is superior to that of an infinite series of causes.

The Argument from Contingency

The third argument is modal in character. That is, it appeals to what can and what cannot be the case, rather than what is and what is not the case. Things in nature are contingent, in that they might or might not exist. Aquinas claims that if there are contingent beings, there has to be a necessary being, one which cannot fail to exist.
  1. Everything in nature can not be
  2. What can not be sometimes is not
  3. If everything can not be, then once nothing existed
  4. What exists only comes from what exists already
  5. So, if everything can not be, then nothing would exist
  6. So, some being cannot not be
  7. A being that cannot not be is God
  8. So, God exists
This argument is less straightforward than the first two, and it is less plausible than either of them. Aquinas in step 2 makes a very questionable claim. Why must we think that a thing that might not exist actually does not exist at some time? The only reason would seem to be that to say that something can exist is to say that it does exist at a time, and to say that what can fail to exist does not exist at a time. This was the interpretation of modality given by the ancient philosopher Diodorus, and it has come to be known as the Diodorian interpretation of modality.

Even if we grant that modality should be understood in terms of time, it seems that the third step involves a straightforward mistake. Suppose each thing is such that it can fail to exist. Then by step 2, at some time that thing does not exist. But it is easily conceivable that at the time that that object does not exist, some other object does exist, and when it does not exist some other object does, etc. Aquinas seems to have understood the idea that everything cannot be as meaning that all things taken together cannot be, which would mean that at one time, nothing existed.

A final question about this argument is why things have to come into existence from a being that cannot not be. Perhaps the universe spontaneously came to be from nothing. Why is this unintelligible as a hypothesis?

The Argument from Gradation

The fourth argument is the most questionable of all. It seeks to move from degrees of perfection of things in the world to a perfect being as the origin of all perfection.
  1. All things in nature come in degrees
  2. If something comes in degrees, it must be comparable to a maximum
  3. The maximum in a genus is the cause of all that falls into the genus
  4. So, there must be a maximum of goodness and all perfections
  5. The maximum of perfections is God
  6. So, God exists
The second step is very suspicious. To see this, consider something like speed. We can compare speeds without comparing a given speed to a maximum speed. Or consider beauty. Is there a maximum of beauty to which all beautiful things must approach in order to be beautiful? This argument is rather Platonistic in its tone, and it supports the existence of the Platonic Forms as well as the existence of God. For Plato, the forms are both perfect exemplars of sensory objects and causes of their being the kind of things they are.

The Argument from Governance

The final argument is teleological in a way that conflicts with Aristotle's views. For Aristotle, there are ends which exist without being the products of deliberation. Aquinas denies this and holds that ends must be conceived in terms of purposes. Since many natural objects which have ends lack the rationality which would allow them to have purposes, the purpose must come from elsewhere.
  1. Natural bodies act for an end
  2. Acting for an end depends on a purpose
  3. A purpose depends on knowledge
  4. Many beings that act for an end lack knowledge
  5. So, those beings are directed by a being that has knowledge
  6. God is the being who directs all natural things
  7. So, God exists
There are numerous possible objections to this argument. One is that we can explain the actions of plants, animals, etc., without attributing an end to them at all. It may be that they act purely mechanically, from efficient causes. Descartes was notorious for holding this view in the seventeenth century. Another objection is that there might be beings lesser than God who direct the activities of animals.

The Problem of Evil

Aquinas tried to confront the existence of evil in a world created by an infinitely good God. This has come to be known as "the problem of evil." The argument is quite simple.
  1. If God exists, then goodness is infinite, and there is no room for evil in the world
  2. There is evil
  3. So, God does not exist
Augustine had confronted the argument by in effect denying the second step, claiming that as a privation, evil is unreal. Aquinas accepts the reality of evil and so meets the argument in a different way, which he attributes to Augustine. God allows evil for the production of good, according to Aquinas, and so the existence of evil is compatible with God's goodness. One might question why it would be necessary for evil to exist in order for there to be maximal goodness in the world. Why does not God's infinite goodness, in and of itself, bring about the greatest possible goodness in the world?
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