It is a difficult task to comment on Saint Thomas Aquinas’s Summa Theologica briefly; it has meant and can mean many things to many people. Partly this is because of its length; it runs to many volumes. It is also because of the scope of the questions considered; they range from abstract and technical philosophy to minute points of Christian dogmatics. The situation is complicated because of Thomas’s style. Such works were common in his day, and his is only one of many that were written in this general form. The work consists entirely of questions, each in the form of an article in which the views Thomas considers important are summarized and then answered. Objections to the topic question are listed, often including specific quotations, and then an equal number of replies are given, based on a middle section (“I answer that”), which usually contains Thomas’s own position. However, this, in turn, is sometimes based on some crucial quotation from a philosopher or theologian.

Out of this complexity and quantity many have attempted to derive Thomistic “systems,” and both the commentators and the group of modern Thomists form a complex question in themselves. Thomas was considered to be near heresy in his own day, and his views were unpopular in some quarters. From the position of being not an especially favored teacher in a very fruitful and exciting era, he has come to be regarded as perhaps the greatest figure in the Catholic philosophy and theology of the day. His stature is due as much to the dogmatization and expansion of his thought that took place (for example, by Cardinal Cajetan and John of Saint Thomas) as it is to the position Thomas had in his own day. Without this further development, his writing might have been important, but perhaps it would have been simply one among a number of significant medieval works. The Encyclical Letter of Pope Leo XIII, “On the Restoration of Christian Philosophy,”...

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Theology and Philosophy

The first question, consisting of ten articles, presents Thomas’s definition of the nature and the extent of sacred doctrine or theology, and it opens by asking whether humanity requires anything more than philosophy. Thomas’s contention that the Scriptures are inspired by God and are not a part of philosophy indicates the usefulness of knowledge other than philosophy. Scriptural knowledge is necessary for humanity’s salvation, for Scripture offers the promise of salvation and pure philosophic knowledge does not. Philosophy is built up by human reason; however, certain truths necessary for humanity’s salvation but that exceed human reason have been made known by God through divine revelation. Such knowledge is not agreed to be reason; it is by nature accepted only on faith.

Now the question arises: Can such revealed knowledge be considered as a science (a body of systematic knowledge) along with philosophy? Of course, such a sacred science treats of God primarily and does not give equal consideration to creatures. This means that it is actually a speculative undertaking and is only secondarily a practical concern. Yet it is the most noble science, because of the importance of the questions it considers, and in that sense all other forms of scientific knowledge are theology’s handmaidens. Wisdom is knowledge of divine things, and in that sense theology has chief claim to the title of “wisdom.” Its principles are immediately revealed by God,...

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The Existence of God

As compared with other classical theologians, Thomas believed in a fairly straightforward approach to questions about God. However, Thomas did admit the necessity of the familiar “negative method” because where God is concerned, what he is not is clearer to us than what he is. The proposition “God exists” is not self-evident to us, although it may be in itself. The contradictory of the proposition “God is” can be conceived.

In this case, Thomas seems to oppose Saint Anselm’s ontological argument, although the opposition is not quite as straightforward as it seems. Thomas denies that people can know God’s essence directly, even though such vision would reveal that God’s essence and existence are identical and thus support Anselm’s contention. However, the ontological argument, he reasons, is built upon a kind of direct access to the divine that human reason does not have.

The existence of God, then, needs to be demonstrated from those of his effects that are known to us. Thomas readily admits that some will prefer to account for all natural phenomena by referring everything to one principle, which is nature itself. In opposition, he asserts that God’s existence can be proved in five ways: first, the argument from motion; second, the argument from the nature of efficient cause; third, the argument from possibility and necessity; fourth, the argument from the gradations of perfection to be found in things; and fifth, the argument from the order of the world. Without attempting individual analyses of these arguments, several things can be noted about them as a group. First, all are based on the principle that reason needs a final stopping point in any chain of explanation. Second, such a point of final rest cannot be itself within the series to be accounted for but must be outside it and different in kind. Third, in each case, it is a principle that is arrived at, not God himself, but these principles (for example, a first efficient cause) are shown to be essential parts of the nature of God. God’s existence is agreed to by showing reason’s need for one of his attributes in the attempt to explain natural phenomena.

Divine Nature and Its Attributes

It is probably true that Thomas’s five proofs have been given a disproportionate amount of attention, for following them, Thomas goes into elaborate detail in a discussion of the divine nature and its primary attributes. Simplicity, goodness, infinity, and perfection are taken up, and then the other chief attributes are discussed before Thomas passes on to the analysis of the three persons of the Trinitarian conception of God. Taken together, these passages form one of the most elaborate and complete discussions of God’s nature by a major theologian, and it is here that much of the disagreement about Thomas’s philosophy centers, rather than in the more formal and brief five proofs.

In spite of Thomas’s use of Aristotelian terms, he indicates his affinity with the Neoplatonic tradition by placing the consideration of “simplicity” first. This is the divine attribute most highly prized and most stressed by Neoplatonists, and Thomas concurs in their emphasis. God’s simplicity is first protected by denying absolutely that he is a body in any sense because what is corporeal is by nature subject to division and contains potentiality, the opposite of God’s required simplicity and full actuality. Nor is God within any genus, nor is he a subject as other individuals are. The First Cause rules all things without commingling with them.

God’s primary perfection is his actuality because Thomas accepts the doctrine that a thing is perfect in proportion to its state of actuality. All created perfections preexist in God also because he is the source of all things. As such a source of the multitude of things in this world, things diverse and in themselves opposed to each other preexist in God as one, without injury to his simplicity. This is no simple kind of simplicity that Thomas ascribes to his God as a perfection. God is also called good, although goodness is defined primarily in terms of full actuality, as both perfection and simplicity were. Everything is good insofar as it has being, and because God is being in a supremely actual sense, he is supremely good. An object can be spoken of as evil only insofar as it lacks being. Because God lacks being in no way, there is absolutely no evil in his nature but only good.

When Thomas comes to infinity, he is up against a particularly difficult divine attribute. By his time, infinity had become a traditional perfection to be ascribed to God, but Aristotle had gone to great lengths to deny even the possibility of an actual infinite. Concerning this point, Thomas makes one of his most significant alterations in the Aristotelian concepts that he employs. Aristotle had...

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God’s Knowledge

In the first thirteen questions, Thomas considers God as humans approach him. He then considers the world as it is viewed from the standpoint of the divine nature. Even the attributes of perfection that Thomas discussed, although they truly characterize God’s nature, are not separate when viewed from the divine perspective. The question is how God understands both himself and the world, and the first thing that must be established is that there actually is knowledge in God. This might seem obvious, but the Neoplatonic tradition denies knowing to its highest principle as implying separation and need. Thomas admits a mode of knowing into the divine nature, but he denies that God knows as creatures do. God understands everything...

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The Soul and Will

Thomas devotes considerable time to a consideration of the nature and function of angels. Part of his reason for doing so, of course, is undoubtedly their constant presence in the biblical record. Part of his interest comes from the necessity of having intermediary beings between God and humans. Having assigned to God a nature so different from humanity’s nature, beings who stand somewhere in between are now easy to conceive. When Thomas comes to describe human nature, he follows much of the traditional Aristotelian psychology, which he finds more amenable to Christianity than certain Platonistic theories. Angels are not corporeal; humans are composed of a spiritual and a corporeal substance. The soul has no matter, but it is...

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Human Nature

The human soul is not eternal; it was created. It is produced immediately by God, not by any lesser beings (as is suggested by Plato, for instance). Soul and body are produced simultaneously because they belong together as one organism. Humans were made in God’s image, but this in no way implies that there must be equality between Creator and created. Some natures may be more like God than others, according to their disposition and the direction of their activities. All humans are directed to some end. Just as their end is worthy of blame or praise, their deeds are also worthy of blame or praise. There is, however, one last fixed end for all humans, and people must, of necessity, desire all that they desire for the sake of that...

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Summa Theologica

Human reason, according to Aquinas, is capable of proving or clarifying a number of Christian beliefs which are based on faith. Reason can prove that God exists, for instance, as well as discern certain of God’s characteristics. Reason can do this without the help of faith.

Reason is limited, however, because it must rely upon knowledge gained through the senses. By making inferences from the material world, reason can discover that God is the basis of all reality, that God is the source of all truth, and that God is the cause of everything good. Revelation provides knowledge of things that cannot be derived from the senses. It is through revelation that human beings learn of the Trinity, the Incarnation, and the...

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Summa Theologica


Thomas Aquinas intended the Summa Theologica to provide instruction to students in Catholic theological schools who not only studied the Old and New Testaments of the Bible but also participated in oral disputations concerning controversial theological questions. The Summa is a written, if condensed, version of these theological debates.

The work was also intended to reform the teaching of sacred doctrine, which for Aquinas involved not only the exposition of those religious tenets known through revelation—such as the nature of salvation—but also the aspects of the Christian faith that are accessible to reason—such as the question of God’s existence. Reducing the...

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Sources for Further Study

Bradley, Denis J. M. Aquinas on the Twofold Human Good: Reason and Human Happiness in Aquinas’s Moral Science. Washington, D.C.: Catholic University of America Press, 1997. Bradley argues that Saint Thomas Aquinas was a theologian first and philosopher second. He contends that to avoid misinterpretation, Aquinas’s writings should be approached from a theological, rather than a philosophical, approach.

Chenu, M. D. A Guide to the Study of Thomas Aquinas. Chicago: University of Chicago Press, 1964. An in-depth study. Part 1 discusses Aquinas’ literary forms and his procedures of...

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