Thomas Aquinas, St. (c. 1224–1274)

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(c. 12241274)

St. Thomas Aquinas, the Catholic theologian and philosopher, was born at Roccasecca, Italy, the youngest son of Landolfo and Teodora of Aquino. At about the age of five he began his elementary studies under the Benedictine monks at nearby Montecassino. He went on to study liberal arts at the University of Naples. It is probable that Thomas became a master in arts at Naples before entering the Order of Preachers (Dominicans) in 1244. He studied in the Dominican courses in philosophy and theology, first at Paris and, from 1248 on, under Albert the Great at Cologne. In 1252 he was sent to the University of Paris for advanced study in theology; he lectured there as a bachelor in theology until 1256, when he was awarded the magistrate (doctorate) in theology. Accepted after some opposition from other professors as a fully accredited member of the theology faculty in 1257, Thomas continued to teach at Paris until 1259.

Thomas Aquinas then spent almost ten years at various Dominican monasteries in the vicinity of Rome, lecturing on theology and philosophy (including an extensive study of the major works of Aristotle) and performing various consultative and administrative functions in his order. In the fall of 1268 Aquinas returned for his second professorate in theology at the University of Paris. He engaged in three distinct controversies: against a group of conservative theologians who were critical of his philosophic innovations; against certain radical advocates of Aristotelianism or Latin Averroism; and against some critics of the Dominicans and Franciscans and their right to teach at the university. Many of Aquinas's literary works were in process or completed at this time. It is thought that he was provided with secretarial help in this task, partly in view of the fact that his own handwriting was practically illegible. Called back to Italy in 1272, Aquinas taught for a little more than a year at the University of Naples and preached a notable series of vernacular sermons there. Illness forced him to discontinue his teaching and writing toward the end of 1273. Early in 1274 he set out for Lyons, France, to attend a church council. His failing health interrupted the trip at a point not far from his birthplace, and he died at Fossanova in March of that year.

The writings of Thomas Aquinas were produced during his twenty years (12521273) as an active teacher. All in Latin, they consist of several large theological treatises, plus recorded disputations on theological and philosophical problems (the "Disputed Questions" and "Quodlibetal Questions"), commentaries on several books of the Bible, commentaries on twelve treatises of Aristotle, and commentaries on Boethius, the pseudo-Dionysius, and the anonymous Liber de Causis. There are also about forty miscellaneous notes, letters, sermons, and short treatises on philosophical and religious subjects. Although Aquinas's philosophic views may be found in almost all his writings (thus the "Exposition of the Book of Job" reads like a discussion among philosophers), certain treatises are of more obvious interest to philosophers. These are listed in detail at the end of this entry.

General Philosophical Position

In the main, Aquinas's philosophy is a rethinking of Aristotelianism, with significant influences from Stoicism, Neoplatonism, Augustinism, and Boethianism. It also reflects some of the thinking of the Greek commentators on Aristotle and of Cicero, Avicenna, Averroes, Solomon ben Judah ibn Gabirol, and Maimonides. This may suggest that we are dealing with an eclectic philosophy, but actually Aquinas reworked the speculative and practical philosophies of his predecessors into a coherent view of the subject that shows the stamp of his own intelligence and, of course, the influence of his religious commitment.

One of the broad characteristics of Aquinas's work in philosophy is a temperamental tendency to seek a middle way on questions that have been given a wide range of answers. This spirit of moderation is nowhere better illustrated than in his solution to the problem of universals. For centuries philosophers had debated whether genera and species are realities in themselves (Plato, Boethius, William of Champeaux) or mere mental constructs (Roscelin, Peter Abelard). What made this odd discussion important was the conviction (certainly shared by Aquinas) that these universals (such as humanity, justice, whiteness, dogness) are the primary objects of human understanding. Most thinkers in the Middle Ages felt that if something is to be explained, it must be treated in universal terms. Therefore, the problem of universals was not simply an academic question.

Aquinas's position on this problem is now called moderate realism. He denied that universals are existing realities (and frequently criticized Plato for having suggested that there is a world of intelligible Forms), but he also insisted that men's universal concepts and judgments have some sort of foundation in extramental things. This basis for the universality, say of humanity, would consist in the real similarity found among all individual men. It was not that Aquinas attributed an actual, existent universal nature to all individual men: that would be an extreme realism. Rather, only individuals exist; but the individuals of a given species or class resemble each other, and that is the basis for thinking of them as universally representative of a common nature.

Thomas's spirit of compromise as a philosopher was balanced by another tendency, that toward innovation. His original Latin biographers all stress this feature of his work. Thomas introduced new ways of reasoning about problems and new sources of information, and he handled his teaching in a new way. In this sense Thomas Aquinas was not typical of the thirteenth century and was perhaps in advance of his contemporaries.

Faith and Rational Knowledge

As Aquinas saw it, faith (fides ) falls midway between opinion and scientific knowledge (scientia ); it is more than opinion because it involves a firm assent to its object; and it is less than knowledge because it lacks vision. Both are intellectual acts and habits of assent: in the case of faith a person is not sufficiently moved by the object to accept it as true, so, by an act of will, he inclines himself to believe. Knowledge implies assent motivated by a personal seeing of the object without any direct influence from will. Where objects of belief have to do with divine matters that exceed man's natural cognitive capacity, the disposition to believe such articles of religious faith is regarded as a special gift from God. Reason (ratio ) is another type of intellectual activity: Simple understanding and reasoning differ only in the manner in which the intellect works. Through intellection (understanding) one knows simply by seeing what something means, while through reason one moves discursively from one item of knowledge to another. (These functions of believing and knowing are treated in many places by Aquinas: Summa Contra Gentiles III, 147; In Boethii de Trinitate, Ques. II and III; Summa Theologiae I, Ques. 7984.)

Aquinas thought that philosophy entailed reasoning from prior knowledge, or present experience, to new knowledge (the way of discovery) and the rational verification of judgments by tracing them back to more simply known principles (the way of reduction). Where the basic principles are grasped by man's natural understanding of his sensory experiences, the reasoning processes are those of natural science and philosophy. If one starts to reason from judgments accepted on religious faith, then one is thinking as a theologian. Questions V and VI of In Boethii de Trinitate develop Aquinas's methodology of the philosophical sciences: philosophy of nature, mathematics, and metaphysics. He distinguished speculative or theoretical reasoning from the practical: The purpose of speculation is simply to know; the end of practical reasoning is to know how to act. He described two kinds of theology: The philosophical "theology," metaphysics, which treats divine matters as principles for the explanation of all things, and the theology taught in Scripture, which "studies divine things for their own sakes" (In Boethii de Trinitate V, 4 c).

Thus philosophy, for Aquinas, was a natural type of knowledge open to all men who wish to understand the meaning of their ordinary experiences. The "philosophers" whom he habitually cited were the classic Greek, Latin, Islamic, and Jewish sages. Christian teachers mentioned by Aquinas were the "saints" (Augustine, John of Damascus, Gregory, Ambrose, Dionysius, Isidore, and Benedict); they were never called Christian philosophers. The word theology was rarely used by Aquinas. In the first question of his Summa Theologiae he formally calls his subject sacred doctrine (sacra doctrina ) and says that its principles, unlike those of philosophy, are various items of religious faith.

Thus, Thomas Aquinas was by profession a theologian, or better, a teacher of sacred doctrine who also studied and wrote about philosophy. He obviously used a good deal of pagan and non-Christian philosophy in all his writings. His own understanding of these philosophies was influenced by his personal faithas almost any man's judgment is influenced by his stand for or against the claim of religious faithin this sense Thomism is a "Christian philosophy." Aquinas did not ground his philosophical thinking on principles of religious belief, however, for this would have destroyed his distinction between philosophy and sacred doctrine, as presented in the opening chapters of the first book of Summa Contra Gentiles. One of the clearest efforts to maintain the autonomy of philosophy is found in Aquinas's De Aeternitate Mundi (about 1270), in which he insists that, as far as philosophical considerations go, the universe might be eternal. As a Christian, he believed that it is not eternal.

Among interpreters of Aquinas there has been much debate whether his commentaries on Aristotle deal with his personal thinking. It is generally agreed even by non-Thomists (W. D. Ross, A. E. Taylor) that these expositions are helpful to the reader who wishes to understand Aristotle. It is not so clear whether the mind of Aquinas is easily discernible in them. One group of Thomists (Étienne Gilson, Joseph Owens, A. C. Pegis) stresses the more obviously personal writings (such as the two Summa 's) as bases for the interpretation of his thought; another school of interpretation (J. M. Ramírez, Charles De Koninck, J. A. Oesterle) uses the Aristotelian commentaries as the main sources for Aquinas's philosophic thought.

theory of knowledge

The Thomistic theory of knowledge is realistic. (This theory is presented in Summa Theologiae I, 7985; Quaestiones Disputatae de Veritate I, II; In Libros Posteriorum Analyticorum I, 5; II, 20.) Men obtain their knowledge of reality from the initial data of sense experience. Apart from supernatural experiences that some mystics may have, Thomas limited human cognition to sense perception and the intellectual understanding of it. Sense organs are stimulated by the colored, audible, odorous, gustatory, and tactile qualities of extramental bodies; and sensation is the vital response through man's five external sense powers to such stimulation. Aquinas assumed that one is cognitively aware of red flowers, noisy animals, cold air, and so on. Internal sensation (common, imaginative, memorative, and cogitative functions) works to perceive, retain, associate, and judge the various impressions (phantasms) through which things are directly known. Man's higher cognitive functions, those of understanding, judging, and reasoning, have as their objects the universal meanings that arise out of sense experience. Thus, one sees and remembers an individual apple on the level of sensationbut he judges it to be healthful because it contains vitamins, or for any other general reason, on the level of intellectual knowledge. Universals (health, humanity, redness) are not taken as existing realities but are viewed as intelligibilities (rationes ) with a basis in what is common to existents. As a moderate realist, Aquinas would resent being classified as a Platonist; yet he would defend the importance of our knowledge of the general and common characteristics of things.

Although human cognition begins with the knowing of bodily things, man can form some intellectual notions and judgments concerning immaterial beings: souls, angels, and God. Aquinas taught that man does this by negating certain aspects of bodies (for instance, a spirit does not occupy space) and by using analogy. When the notion of power is attributed to God, its meaning is transferred from an initially physical concept to the analogous perfection of that which can accomplish results in the immaterial order. Thomas did not think that men, during earthly life, can know the nature of God in any adequate, positive way.

Discursive reasoning was taken as an intellectual process moving from or toward first principles in logical processes of demonstration (the ways of discovery and reduction, described above). In one way, sense experience is the first principle (starting point) for all of man's natural knowledge. This is one aspect of Aquinas's empiricism. Following Aristotle's Posterior Analytics, Thomas taught that many sensations combine to form a unified memory, and many memories constitute sense experience. From this manifold of experience, by a sort of sensory induction, there arises within human awareness a beginning (principium ) of understanding. Such first principles are not demonstrated (they naturally emerge from sense cognition), but they become the roots for consequent intellectual reasoning. A doctor who tries a variety of remedies to treat headaches eventually notices that one drug works well in almost all casesat some point he grasps the universal "Drug A is a general remedy for headache." From this principle he proceeds rationally to order his practice. If he becomes a teacher of medicine, he uses such a theoretical principle to instruct others. This is the basis of the life of reason.

Philosophy and the Physical World

In his exposition of the Liber de Causis (Lect. 1), Aquinas described a sequence of philosophic studies: logic, mathematics, natural philosophy (physics), moral philosophy, and, finally, metaphysics. The first kind of reality examined in this course would be that of the physical world. (At the start of the next century, John Duns Scotus criticized Thomas for attempting to base his metaphysics and his approaches to God on physics.) Interpreters still debate whether Aquinas himself felt that this was the order to be followed in learning philosophy, or whether he was merely reporting one way that the "philosophers" had taught it. In any case, the philosophical study of bodies, of mobile being in the Aristotelian sense, was important to Aquinas. One group of his writings (De Principiis Naturae, parts of Book II of the Summa Contra Gentiles, the treatise De Aeternitate Mundi ) offers a quite personal treatment of this world of bodies. Another set of writings (the commentaries on Aristotle's Physics and De Generatione et Corruptione ) shows how indebted Aquinas was to Aristotle in his theory of physical reality.

matter and form

The philosophy of nature (phusis ) was understood as the study of a special kind of beings, those subject to several kinds of change. Physical beings have primary matter as one component and, depending on their species or kind, substantial form as their other integral principle. Neither matter nor form is a thing by itself; matter and form are simply the determinable and determining factors within any existing physical substance. Like Aristotle, Aquinas took it that there are many species of bodily substances: all the different kinds of inanimate material (wood, gold, water, etc.) and all the species of plants and animals. Within each such species there is one specifying principle (the substantial form of wood, potato plant, or dog), and the many individual members of each species are differentiated by the fact that the matter constituting dog A could not also constitute dog B (so viewed, matter is said to be quantified, or marked by quantity).


Being mobile, physical beings are subject to four kinds of change (motus ): of place (locomotion), of size (quantitative change), of color, shape, and so on (qualitative change), and of species of substance (generation and corruption, substantial change). Basically, prime matter is that which remains constant and provides continuity during a change from one substance to another. When a pig eats an apple, that part of the apple really assimilated by the pig becomes the very substance of the pig; some factor in the apple, the prime matter, must continue on into the pig. All four types of change are explained in terms of the classic theory of four causes. The final cause is the answer to the question "why" something exists or occurs; the agent or efficient cause is the maker or producer of the change; the material cause is that out of which the change comes; and the formal cause is the specifying factor in any event or existent. So used, "cause" has the broad meaning of raison d'être.

space and time

Certain other points in Aquinas's philosophy of nature further illustrate the influence of Aristotle. Place, for instance, is defined as the "immobile limit of the containing body" (In IV Physicorum 6). Moreover, each primary type of body (the four elements still are earth, air, fire, and water) is thought to have its own "proper" place. Thus, the place for fire is "up" and that for earth is "down." Some sort of absolute, or box, theory of space may be presupposed; yet in the same passage Aquinas's discussion of the place of a boat in a flowing river indicates a more sophisticated understanding of spatial relativity. Time is defined, as in Aristotle, as the measure of motion in regard to "before" and "after." Eternity is a type of duration differing from time in two ways: The eternal has neither beginning nor termination, and the eternal has no succession of instants but exists entirely at once (tota simul ).

encouragement of science

Doubtless Aquinas's philosophy of the physical world was limited and even distorted by certain views and factual errors derived from Aristotle and from thirteenth-century science. Apart from the mistaken hypothesis that each element has its proper place in the universe, Thomas also used the Eudoxian astronomy, which placed the earth at the center of a system of from 49 to 53 concentric spheres. (Besides the Commentary on De Caelo II, 10, and the Commentary on Meteorologia II, 10; see Summa Contra Gentiles I, 20, and Summa Theologiae I, 68, 4 c.) At times Thomas showed an open mind on such questions and an ability to rise above the limitations of his period. His Commentary on Aristotle's Metaphysics (Lect. 1 on Book III and Lect. 9 on Book XII) provides a key instance. Pointing out that astronomers differ widely on the number and motions of the planets, Aquinas recommended that one study all the reports and theories of such scientists, even though these scientific explanations are not the last word on the matter and are obviously open to future revision. He further compared the study of physical science to the work of a judge in a court of law. One should listen to, and try to evaluate, all important testimony before attempting to formulate one's own judgment on the problems of contemporary science. This is Aquinas at his best, hardly a philosophical dogmatist.

Human Functions and Man's Nature

Anthropology, or psychology, in the classical sense of the study of man's psyche, forms an important part of Aquinas's philosophy. His view of man owed much to the Aristotelian treatise On the Soul, to the Christian Platonism of Augustine and John of Damascus, and to the Bible. This part of Aquinas's thought will be found in Scriptum in IV Libros Sententiarum (Commentary on the Sentences ) I, Dists. 1627; Summa Contra Gentiles II, 5890; Quaestio Disputata de Anima ; the Libros de Anima ; and Summa Theologiae, I, 7590.

Aquinas's usual way of working out his theory of human nature was first to examine certain activities in which man engages, then to reason to the kinds of operative powers needed to explain such actions, and finally to conclude to the sort of substantial nature that could be the subject of such powers. He described the biological activities of man as those of growth, assimilation of food, and sexual reproduction. A higher set of activities included sensory perception, emotive responses to what is perceived, and locomotion: These activities man shares with brute animals. A third group of activities comprises the cognitive functions of understanding, judging, and reasoning, as well as the corresponding appetitive functions of affective inclination toward or away from the objects of understanding. To these various functions Aquinas assigned generic powers (operative potencies) of growth, reproduction, sensory cognition and appetition, physical locomotion, and intellectual cognition and appetition (will).

Reexamining these functional powers in detail, Aquinas distinguished five special sense powers for the cognition of physical individuals: sight, hearing, smell, taste, and touch. These functions and powers are called external because their proper objects are outside the mental awareness of the perceiver: This is essential to epistemological realism. Following these are four kinds of internal sensory activities: the perceptual grasping of a whole object (sensus communis ), the simple retention of sensed images (imagination), the association of retained images with past time (sense memory), and concrete discrimination or judgment concerning individual things (cogitative sense, particular reason). Still on the level of sensory experience, Aquinas (here influenced by John of Damascus) described two kinds of appetition (emotion): A simple tendency toward or away from what is sensed as good or evil (this affective power is called the concupiscible appetite), and a more complicated sensory inclination to meet bodily threats, obstacles, and dangers by attacking or avoiding them or by putting up with them (this affective power is called irascible appetite). Eleven distinct kinds of sensory passions (emotions) are attributed to these two sensory appetites: love, desire, delight, hate, aversion, and sorrow to the concupiscible; fear, daring, hope, despair, and anger to the irascible. Much of this psychological analysis is quite sophisticated, employing data from Greek, Roman, and early Christian thought and also using the physiological and psychological treatises of Islamic and Jewish scholars. It also forms the basis of the analysis of human conduct in Thomistic ethics.

On the higher level of distinctively human experience, Aquinas found various other activities and powers. These are described in his commentary on Book III of Aristotle's De Anima, in the Summa Contra Gentiles (II, 5978), and in Questions 8485 of the Summa Theologiae. The general capacity to understand (intellectus ) covers simple apprehension, judging, and reasoning. The objects of intellection are universal aspects (rationes ) of reality. Since universal objects do not exist in nature, Aquinas described one intellectual action as the abstraction of universal meanings (intentiones ) from the individual presentations of sense experience. This abstractive power is called agent intellect (intellectus agens ). A second cognitive function on this level is the grasping (comprehensio ) of these abstracted meanings in the very act of cognition; this activity is assigned to a different power, the possible intellect (intellectus possibilis ). Thus, there are two quite different "intellects" in Thomistic psychology: One abstracts, the other knows. No special power is required for intellectual memory; the retention of understandings is explained by habit formation in the possible intellect.


Affective responses to the universal objects of understanding are functions of intellectual appetition. Considered quite different from sensory appetition, this is the area of volition, and the special power involved is the will (voluntas ). Aquinas distinguished two kinds of volitional functions. First, there are those basic and natural tendencies of approval and affective approach to an object that is judged good or desirable without qualification. In regard to justice, peace, or a perfectly good being, for instance, Aquinas felt that a person's will would be naturally and necessarily attracted to such objects. This natural movement of the will is not free. Second, there are volitional movements toward or away from intellectually known objects that are judged as partly desirable or as partly undesirable. Such movements of will are directed by intellectual judgments evaluating the objects. In this case volition is said to be "deliberated" (specified by intellectual considerations) and free. It is in the act of decision (arbitrium ) that man is free. Aquinas did not talk about "free will"; the term libera voluntas is found only twice in all his works, and then in a nontechnical usage; rather, he spoke of free choice or decision (liberum arbitrium ). Man, by virtue of his intellectual powers, is free in some of his actions.


Although Aquinas sometimes spoke as if these various "powers" of man were agents, he formally stressed the view that it is the whole man who is the human agent. A human being is an animated body in which the psychic principle (anima ) is distinctive of the species and determines that the material is human. In other words, man's soul is his substantial form. Some of man's activities are obviously very like those of brutes, but the intellectual and volitional functions transcend materiality by virtue of their universal and abstracted character. Aquinas took as an indication of the immateriality of the human soul the fact that it can understand universal meanings and make free decisions. The soul is a real part of man and, being both immaterial and real, it is spiritual. From certain other features of man's higher activities, especially from the unity of conscious experience, Aquinas concluded to the simplicity and integration of man's soul: It is not divisible into parts. This, in turn, led him to the conclusion that the soul is incapable of corruption (disintegration into parts) and thus is immortal.

Since Thomas thought the soul incapable of being partitioned, he could not explain the coming into being of new human souls by biological process. He was thus forced to the view that each rational soul is originated by divine creation from nothing. Human parents are not the total cause of their offspring; they share the work of procreation with God. This view explains why Aquinas put so much stress on the dignity and sanctity of human reproduction, which he regarded as more than a biological function. When he claimed, in his ethics, that the begetting and raising of children is the primary purpose of married life, he was not thinking of simple sexual activity but of a human participation in God's creative function. This does not mean that man is the highest of God's creatures; Aquinas speculated that there are other kinds of purely intellectual beings with activities, powers, and natures superior to those of men. These are angels. Thomas Aquinas is called the Angelic Doctor in Catholic tradition because of his great interest in these purely spiritual but finite beings. They would constitute the highest realm of the universe.

Metaphysics and Real Being

Aquinas devoted much thought to the question "What does it mean to be?" Many Thomists think that his greatest philosophical ability was shown in the area of metaphysics. His general theory of reality incorporates much of the metaphysics of Aristotle, and some interpreters have seen Thomistic metaphysics as but a baptized Aristotelianism. Recent Thomistic scholarship has selected two non-Aristotelian metaphysical teachings for new emphasis: the theory of participation and the general influence of Platonic metaphysics (L. B. Geiger, Cornelio Fabro, R. J. Henle), and the primacy of esse, the fundamental act of being (Gilson, Jacques Maritain, G. P. Klubertanz). Because esse, which simply means "to be," is sometimes translated as "existence," this second point of emphasis is called by some writers the existentialism of Thomistic metaphysics. It has little, however, to do with present-day existentialism. A major treatment of metaphysical problems is to be found in Aquinas's long Commentary on Aristotle's Metaphysics, but here again the problem is to decide how much is Thomistic. Some very competent scholars (Pegis, Gilson) regard this work as a restatement of Aristotelianism; others (De Koninck, Herman Reith) consider the Commentary to be a key exposition of Aquinas's own metaphysics. It is admitted by all that there are some explanations in it that are not found in Aristotle.

Metaphysics, for Aquinas, was the effort to understand reality in general, to find an ultimate explanation of the manifold of experience in terms of the highest causes. His predecessors had variously described the subject matter of this study as existing immaterial substances, as the most universal and common aspects of being, as the first causes of all things, and as the divine being in itself. Commenting on these opinions in the prologue to his Commentary on the Metaphysics, Aquinas remarked: "Although this science considers these items, it does not think of each of them as its subject; its subject is simply being in general." In this sense, he called the study of being "first philosophy."


It is distinctive of Aquinas's thought to maintain that all existing realities, from God down to the least perfect thing, are beingsand that "being" has in this usage an analogical and not a univocal meaning. In a famous passage (In I Sententiarum 19, 5, 2, ad 1) Aquinas describes three sorts of analogy: one in which a given perfection is present in one item but only attributed to another; one in which one perfection exists in a somewhat different way in two or more items; and one in which some sort of remote resemblance or community is implied between two items which have no identity either in existence or in signification. "In this last way," Aquinas adds "truth and goodness, and all things of this kind, are predicated analogously of God and creatures." In later works the notion of proportionality is introduced to develop the concept of the analogy of being. Vision in the eye is a good of the body in somewhat the same way that vision in the intellect is a good of the soul. Similarly, the act of being in a stone is proportional to the act of being in a man, as the nature of a stone is proportional to the nature of man. Whereas some interpreters feel that the analogy of proportionality is the central type of analogy of being, others insist that Aquinas used several kinds of analogy in his metaphysics.

being and essence

One early but certainly personal presentation of the metaphysics of Aquinas is to be found in the brief treatise De Ente et Essentia, which was strongly influenced by Avicenna. His usage of basic terms of analysis, such as being (ens ), essence (essentia ), nature, quiddity, substance, accident, form, matter, genus, species, difference, immaterial substance (substantia separata ), potency, and act, is clearly but rather statically defined in this opusculum. Additional precisions, particularly on the meaning of element, principle, cause, and esse, are to be found in the companion treatise, De Principiis Naturae. A more dynamic approach to being and its operations is offered in the Quaestiones Disputatae de Potentia Dei and in Part I of the Summa Theologiae.

Fundamental in the metaphysical thinking of Aquinas is the difference between what a being is and the fact that it is. The first is a question of essence; the second is the act of being, esse. Essences are many (various kinds of thingsstones, cows, air, men) and are known through simple understanding, without any necessity of adverting to their existence or nonexistence. For a thing to be is entirely another matter; the fact that something exists is noted in human experience by an act of judgment. Many essences of things are material, but there is nothing about esse that requires it to be limited to materiality. This proposition (to be is not necessarily to be material) is the "judgment of separation" (In Boethii de Trinitate V, 3). Many Thomists now regard it as a fundamental point of departure for Aquinas's metaphysical thinking.

There are also certain most general features of real beings that transcend all division into genera and species; these are convertible with metaphysical being. In other words, they are coextensive and really identical with being. Such transcendentals are thing (res ), something (aliquid ), one, true, good, and (according to some interpreters) beautiful. The more important of these transcendentals suggest that every being is internally undivided but externally distinct from all else (unum ), that every being has some intelligible meaning (verum ), and that every being is in some way desirable (bonum ). The theory of transcendentals is much more expanded and stressed in later scholasticism than in Aquinas's own writings. He barely touches upon it in Questions I and XXI of De Veritate and in the discussion of God's attributes in Summa Theologiae (I, Ques. 6, 11, 16).

potency and act

Potency and act are important principles in Aquinas's metaphysical explanation of the existence and operation of things. In De Potentia Dei (I, 1) Aquinas pointed out that the name "act" first designated any activity or operation that occurs. Corresponding to this sort of operational act is a dual meaning of potency (or power). Consider the activity of sawing wood: The passive potency of wood to be cut is required (water, for instance, cannot be sawed); also required is the active potency of the sawyer to do the cutting. In addition, in the same text, Aquinas says that the notion of "act" is transferred to cover the existence of a being. Essential potency, the metaphysical capacity to exist, would correspond to this act of being (esse ). In this way the theory of act and potency was applied to all levels of being. At the highest level, God was described as Pure Act in the existential order, but this did not prevent Aquinas from attributing to God an active potency for operating.


Still another dimension of metaphysical reality, for Aquinas, was that of finality. He thought of all activities as directed toward some end or purpose, a basic assumption in Aristotle. But Aquinas developed this tendential, vector characteristic of being and applied it to the inclination of possible beings to become actual. The finality of being, in Thomism, is that dynamic and ongoing inclination to be realized in their appropriate perfections that is characteristic of all realities and capacities for action. In this sense the finality of being is an intrinsic perfectionism in the development of all beings. Aquinas also held that all finite beings and events are tending toward God as Final Cause. This is metaphysical finality in the sense of order to an external end. This theme runs through Book III of Summa Contra Gentiles.

Philosophy and God

The consideration of the existence and nature of God was approached by Aquinas both from the starting point of supernatural revelation (the Scriptures), which is the way of the theologian, and from the starting point of man's ordinary experience of finite beings and their operations, which is the way of the philosopher: "The philosophers, who follow the order of natural cognition, place the knowledge of creatures before the divine science; that is, the philosophy of nature comes before metaphysics. On the other hand, the contrary procedure is followed among the theologians, so that the consideration of the Creator precedes the consideration of creatures" (In Boethii de Trinitate, Prologue). In the same work (II, 3 c) we are told that the first use of philosophy in sacred doctrine is "to demonstrate items that are preambles to faith, such as those things that are proved about God by natural processes of reasoning: that God exists, that God is one," and so on.

Aquinas recognized two types of demonstration, one moving from cause to effects and the other from effects back to their cause. The arguments that he selected to establish that God exists use the second procedure and are technically called quia arguments. In other words, these proofs start with some observed facts of experience (all Aquinas's arguments to God's existence are a posteriori) and conclude to the ultimate cause of these facts. Well aware of his debt to his predecessors, Aquinas outlined three arguments for the existence of God in De Potentia Dei (III, 5 c). The first shows that, since the act of being is common to many existents, there must be one universal cause of all (Plato's argument, Aquinas noted); the second argument starts from the fact that all beings in our experience are imperfect, not self-moved, and not the source of their actual being, and the reasoning concludes to the existence of a "mover completely immobile and most perfect" (Aristotle's argument); the third argument simply reasons from the composite nature of finite beings to the necessary existence of a primary being in which essence and the act of existing are identical (Avicenna's proof). Aquinas felt that these two pagan philosophers and an Islamic thinker had successfully established the conclusion "that there is a universal cause of real beings by which all other things are brought forth into actual being."

the "five ways."

The most famous of the arguments are the "Five Ways" (Quinque Viae ) of reasoning to the conclusion that God exists (Summa Theologiae I, 2, 3, c). All these ways employ the principle of causality and start from empirical knowledge of the physical world. They are not entirely original with Aquinas, depending not only on Plato, Aristotle, and Avicenna but also on Augustine and especially on Moses Maimonides. The First Way begins with the point that things in the world are always changing or moving and concludes to the existence of one, first, moving Cause. The Second Way argues from the observation of efficient production of things in the universe to the need of an existing, first, efficient Cause. The Third Way reasons from the contingent character of things in the world (none of them has to be) to the existence of a totally different kind of being, a necessary one (which has to be). The Fourth Way argues from the gradations of goodness, truth, and nobility in the things of man's experience to the existence of a being that is most true, most good, and most noble. The Fifth Way starts from the orderly character of mundane events, argues that all things are directed toward one end (the principle of finality), and concludes that this universal order points to the existence of an intelligent Orderer of all things. At the end of his statement of each "way," Thomas simply said, "and this is what all men call God," or words to that effect. Obviously, he presupposed a common meaning of the word God in the dictionary or nominal sense. There is disagreement among interpreters as to whether the "ways" are five distinct proofs or merely five formulations of one basic argument. Most Thomists now favor the second view.

Aquinas favored the argument from physical motion (prima autem et manifestior via est ). The Summa Contra Gentiles (I, 13) offers an extended version of this first argument and frankly indicates its relation to the ideas in the last books of Aristotle's Physics. The other four ways are but briefly suggested in the Summa Contra Gentiles. In another, much neglected, work (Compendium Theologiae I, 3) the first way is stated clearly and concisely. Before attempting to establish in detail the various attributes of God, such as divine unity, one should consider whether he exists. Now, all things that are moved must be moved by other things; furthermore, things of an inferior nature are moved by superior beings. (Aquinas's examples are chosen from thirteenth-century physics and astronomy, in which the four basic elements were thought to be under dynamic influence of the stars, and lower celestial bodies were considered to be moved about by those at a greater distance from Earth. How much of the force of this argument may depend on outmoded science is a matter of debate in present-day Thomism.)

Aquinas next argues that the process in which A moves B, B moves C, and so on cannot be self-explanatory. His way of saying this is "This process cannot go on to infinity." He concludes that the only possible explanation of the series of physical motions observed in the universe requires the acceptance of the existence of a different sort of "mover"a being that is not moved by another, in other words, a first mover. This would have to be a real being, of course, and of a quite different nature from bodily things. He eventually suggests that this "first mover existing above all else" is what Christians call God.

In the same passage from the Compendium, two other facets of the argument from motion are introduced. First, Aquinas claims that all causes observed as acting in the physical universe are instrumental in character and must be used, as it were, by a primary agent. This primary agent is again another name for God. To suppose that the universe is self-explanatory is, to Aquinas, like thinking that a bed could be constructed by putting the tools and material together, "without any carpenter to use them." This is an important case of the conception of God as a divine craftsman. In the second place, this text suggests briefly that an infinite series of moved movers is an impossibility; the length of the series has nothing to do with its explanatory function, if all its members be finite. Finally, any such series requires a first mover (primary in the sense of causality, not necessarily of chronological priority). This first mover would be a Supreme Being. It is obvious that many of the attributes of God are already implied in the argument for divine existence.

knowledge of god

Regarding the nature and attributes of God, Aquinas's greatest emphasis fell on how little we really know about the Supreme Being. In a series of articles (Summa Theologiae I, 8688) on the objects of human knowledge, he reiterated his position that man is naturally equipped to understand directly the natures of material things; further, that man is aware of his own psychic functions as they occur but that all man's understanding of the nature of his own soul, of immaterial substances such as angels, and of infinite immaterial being (God) is achieved by dint of discursive and indirect reasoning. There is, of course, a wide gap between material and immaterial substances. Yet both these types of finite beings fall within the same logical genus, as substances, and thus bodies and created spirits have some aspects in common. On the other hand, God is an immaterial being of an entirely different nature from that of bodies or even of created spirits. Between God and creatures there is no univocal community: That is to say, God does not fall within the same genus, either real or logical, as any other being. Hence, God's nature transcends all species and genera. Man's natural knowledge of God's nature is therefore very imperfect, achieved by negating various imperfections found in finite beings: Thus, God is not in time, not in place, not subject to change, and so on. Furthermore, man may reach some semipositive knowledge of God by way of analogy: Thus, God is powerful but not in the finite manner of other beings; he is knowing, willing, and so on.


Divine providence is that attribute of God whereby he intelligently orders all things and events in the universe. As Aquinas explained it in the Summa Contra Gentiles (III), God both establishes the plan (ratio ) in accord with which all creatures are kept in order and executes this plan through continued governance of the world. Literally, providence means "foresight," and this required Aquinas to face certain problems traditionally associated with any theory of divine foreknowledge. First of all, he insisted that such a view of divine providence does not exclude chance events from the universe. In one sense, a chance event occurs apart from the intention of the agent. However, what is intended by one agent may involve another agent who is unaware of the intention of the first. Hence, a plurality of real but imperfect agents sets the stage for chance: God knows this and permits it to occur.


In the Quaestiones Disputatae de Malo and elsewhere Aquinas agreed with Augustine that evil (both physical and moral) is a privation of goodness, of perfection, in being or in action. This does not deny the fact that evil really occurs but asserts that it is like a wound in being (the phrase is Maritain's); and, like any defect, evil is important by virtue of what is lacking. As to why a perfectly good God will allow evil to occur, Thomas argued that the possibility of evil is necessary so that many goods may be possible. "If there were no death of other animals, there would not be life for the lion; if there were no persecution from tyrants, there would be no occasion for the heroic suffering of the martyrs" (Summa Theologiae I, 22, 2, ad 2).


Aquinas also did not admit that divine foreknowledge is opposed to the exercise of human freedom. His explanation of this point (in Summa Theologiae I, 103, 7 and 8) is complicated and not easy to state briefly. In effect, human freedom does not imply absolute indeterminism (action that is uncaused). What a man does freely is caused by himself, as a knowing and willing agent. God makes man capable of choosing well or ill, permits man to do so freely, and knows what man will accomplish. What appears to be necessitated from one point of view may be quite contingent and free from another viewpoint. From God's vantage point in eternity, human actions are not affairs of past or future but are events within the all-inclusive present of a divine observer who witnesses these events but does not determine them.

Ethics and Political Philosophy

The foregoing problems and considerations fall within Aquinas's speculative philosophy. His practical philosophy, aimed at the intelligent performance of actions, is divided into ethics, economics (treating problems of domestic life), and politics. In all three areas the thinking is teleological; finality, purposiveness, and the means-end relation all are aspects of Thomistic teleology. Rationally controlled activities must be directed to some goal; they are judged good or bad in terms of their attainment of that goal and in terms of the means by which they attain (or fail to attain) that end.

Aquinas dealt with the theoretical analysis of ethical activities in a long series of works: the Scriptum in IV Libros Sententiarum, Book III; Summa Contra Gentiles III, 114138; the In X Libros Ethicorum; Quaestiones Disputatae de Malo ; and the Summa Theologiae, Part II. Most of these works take the approach of moral theology, viewing moral good and evil in terms of accord or discord with divine law, which is revealed in Scripture and developed and interpreted in Christian tradition. Thomas himself did not consider moral theology to be a part of philosophy, and it will not be further considered here, except as throwing incidental light on his ethical position.

voluntary action

Aquinas's ethics consists of a study of good and evil in human conduct, from the point of view of man's achievement of ultimate happiness. Not all the actions in which man is involved are truly human but only those accomplished under control of man's intellect and will. The primary characteristic of human conduct, according to Aquinas, is not so much freedom as voluntariness. His description of voluntary activity is a development of the teaching of Aristotle. Several factors are required for a voluntary action. There must be sufficient knowledge on the part of a moral agent that a given action is within his power; he cannot be entirely ignorant of the kind of action that he is performing or of the means, circumstances, and end of his action. Violence, under certain conditions, modifies the voluntariness of one's actionsas do certain kinds of uncontrollable feelings. Furthermore, as Aquinas saw it there are two opposites to what is voluntary. The "involuntary" is a contrary: It represents a diminution of voluntariness. Thus, an action that is partly involuntary is also partly voluntary and is, to a greater or lesser extent, imputable to the agent. On the other hand, the "not-voluntary" is the contradictory of what is voluntary, and an agent who is not voluntary is not morally responsible for his action.

natural law

Most surveys of ethical theories classify Aquinas's ethics as a natural law theory. He described natural law as a rational participation in the eternal law of God and suggested that all men have a sufficient knowledge of what is morally right (the justum ) to be able to regulate their own actions. In a famous passage (Summa Theologiae III, 94, 2) Aquinas explained the way in which he thought that rules of natural law are known. The judgment of synderesis (an intellectual quality enabling any man to intuit the first principle of practical reasoning) is simply the proposition "Good should be done and sought after; evil is to be avoided." (Most modern Thomists take this rule as a formal principle in the Kantian sense, requiring further knowledge to fill in the content of specific moral rules.) Aquinas then proceeded to describe three kinds of inclinations natural to man: that of man's substantial nature toward the conservation of its own existence and physical well-being, that of man's animal nature to seek such biological goods as sexual reproduction and the care of offspring, and that of man's reason whereby he tends toward universal goods, such as consideration of the interests of other persons and the avoidance of ignorance. All three kinds of inclinations are presented as natural and good, provided they are reasonably pursued. They form the bases from which one may conclude to a number of rules of natural moral law. Aquinas never attempted to make an exhaustive listing of the precepts of such a law; nor did he consider such a codification advisable.

In point of fact, the natural law approach to moral theory is not the only, and not the best, classification of Aquinas's ethics. Particularly in view of various shifts in the meaning of "law" since the time of Aquinas (notably a growing stress on law as a fiat of legislative will), it can be positively misleading to limit Aquinas's ethics to a natural law position. He defines law in general as "any ordinance of reason that is promulgated for the common good by one who has charge of a community" (Summa Theologiae III, 90, 4 c). "Reason" is the key word in this definition. Right reason (recta ratio ) is the justification of ethical judgment in Aquinas's thought. "In the case of volitional activities, the proximate standard is human reason (regula proxima est ratio humana ) but the supreme standard is eternal law. Therefore, whenever a man's action proceeds to its end in accord with the order of reason and of eternal law, then the act is right; but when it is twisted away from this rightness, then it is called a sin" (21, 1 c).

reason, goodness, and justice

Thomistic ethics requires a person to govern his actions as reasonably as he can, keeping in mind the kind of agent that he is and the position that he occupies in the total scheme of reality. Man's own good is achieved by the governance of his actions and feelings under rational reflectionand God does not require anything else. "For we do not offend God, except by doing something contrary to our own good" (Summa Contra Gentiles III, 121122). It is a part of being reasonable to respect the good of others. The moral good, then, is not so much what men are obligated to do by an all-powerful legislator; rather, it is that which is in accord with the reasonable perfecting of man. In becoming a better agent within himself, man is making himself more fit for ultimate happiness and for the vision of God. This kind of ethics resembles a self-perfectionist theory, without idealist overtones.

Aquinas based much of his teaching on ethical rules on the theory of natural justice found in Book V of the Nicomachean Ethics. All things have specific natures that do not change: Dogs are dogs and stones are stones. Certain functions are taken as natural and appropriate to given natures: Eating is an act expected of a dog but not of a stone. Human nature shares certain functions with the higher brutes but is distinguished by the performance of rational activities. Some of these typical functions are always the same in relation to man's nature and ethical rules pertaining to these do not change. Aquinas's example of such an immutable rule of justice is simply "Theft is unjust." Other ethical judgments, however, are not essential to justice (for example, detailed ordinances that contain many variable factors); these secondary rules are by no means absolute and immutable. Examples would be rules concerned with taxation, buying and selling, and other such circumstantially variable regulations. Moral law is composed of both types of rules and is neither absolute nor immutable in all its requirements.


In De Veritate (XVII) Aquinas referred to moral conscience as a concrete intellectual judgment whereby the individual agent decides for himself that a given action or feeling is good or bad, right or wrong, to be done or not to be done. Conscience was not considered a special power or moral sense, nor was it viewed as the source of universal moral convictions. For Aquinas it was simply a man's best practical judgment concerning a concrete moral problem. As such, moral conscience is a person's internal guide to good action; one acts immorally in going against his conscience, for it is his best judgment on a matter. If it is not his best judgment, then the person is clearly required to make a better effort to reach a conscientious decision. Reasonable consideration of a proposed action includes thinking of the kind of action that it is (the formal object), the purpose to which it is directed (the end), and the pertinent circumstances under which it is to be performed. These three moral determinants were used by Aquinas to complete the theory of right reasoning in De Malo (II, 4 c, ad 2, ad 5).


Aquinas also considered man in his social relations. In the Summa Contra Gentiles (III, cc. 122126) the family is regarded as a natural and reasonable type of small society, designed to provide for the procreation and raising of children and for the mutual good of husband and wife. (The material on matrimony in the so-called Supplement to the Summa Theologiae was excerpted from Book IV of the Scriptum in IV Libros Sententiarum and does not represent Aquinas's mature thought.) The main reason why people get married, Aquinas thought, is to raise children, so his approach to the family was child-oriented. There should be but one husband and wife in a family; they should stay together until the children are fully grown and educated; they should deal honestly and charitably with each other as marriage partners. Many of Aquinas's arguments for monogamy and the indissolubility of the marriage bond are but restatements of similar reasonings in Aristotle's Politics.

political theory

Aquinas's family, living in southern Italy, had been closely allied with the imperial government: His father and at least two of his brothers were in the service of Emperor Frederick II. Aquinas thus grew up with monarchic loyalties. However, early in life he joined the Dominicans, a religious community remarkable for its democratic and liberal practices. As a result Aquinas's political philosophy (in De Regno, in In Libros Politicorum, and in Summa Theologiae, III, passim) stressed the ideal of the limited monarchy, or that kind of state which Aristotle had called the politeia. The purpose of the state is described as to provide for temporal peace and welfare. Political society is quite different from ecclesiastical society (the church), whose end is otherworldly. Here again Aquinas always stressed the central role of reason: "Divine justice (ius divinum ) which stems from grace does not cancel human justice which comes from natural reason." There is no detailed theory of government in Aquinas's writings.

Art and Aesthetics

In his theory of art Aquinas was quite abstract and intellectualistic, taking Aristotle's Rhetoric, Poetics, and Nicomachean Ethics (Book VI) as his major sources. He used a new awareness of the spiritual and moral dimensions of the beautiful, found seminally in the mystical Neoplatonism of Dionysius the Pseudo-Areopagite, to develop the fragmentary aesthetics of Aristotelianism. Most of these precisions are found in Aquinas's commentary on the fourth chapter of Dionysius's De Divinis Nominibus.

Art is understood to be a special habit, or acquired skill, of the practical intellect, which is simply man's possible intellect applied to problems of action. Prudence, the key practical habit in moral discourse, is defined as right reason in doing things (recta ratio agibilium ). Similarly, art is defined as right reason in making things (recta ratio factibilium ). These two practical habits are not confused. Elsewhere it is explained: "The principle of artifacts is the human intellect which is derived by some sort of similitude from the divine intellect, and the latter is the principle of all things in nature. Hence, not only must artistic operations imitate nature but even art products must imitate the things that exist in nature" (In I Politicorum 1). Some artifacts are merely useful; others may be beautiful; and still others may exist only in the order of thought (Aquinas took seriously the dictum that logic is an art).

He regarded the beautiful and the good as really identical but insisted that they differ in their formal meanings (rationes ). Where the good is simply that which all desire, the beautiful is that which gives pleasure when perceived (quod visum placet ). Three aspects of the beautiful are distinguished: integrity (integritas sive perfectio ), due proportion (debita proportio sive consonantia ), and brilliance (claritas ). Each of these aesthetic factors is taken as capable of variation in degree and appeal.

These notions on the general meaning of Beauty were used not to describe the attraction of a life of sacrifice but of spiritual perfection as a member of a religious community, such as the Dominicans. "In fact," Aquinas wrote, "there are two kinds of beauty. One is spiritual and it consists in a due ordering and overflowing of spiritual goods. Hence, everything that proceeds from a lack of spiritual good, or that manifests intrinsic disorder, is ugly. Another kind is external beauty which consists in a due ordering of the body" (Contra Impugnantes Dei Cultum et Religionem 7, ad 9). He was actually defending the practice of begging, as used in the mendicant orders. Aquinas agreed that there is something distasteful about begging but argued that it is an admirable exercise of humility, when religiously motivated. Here again the concept of purpose, teleological order, is central.

Metaphysical participation recurs as a key theme in Aquinas's discussion of the manner in which the manifold of creation shares in the transcendent beauty of God. All lower beauties are but imperfect manifestations of one highest pulchritudo. This is Dionysian mystical aesthetics and is presented in In Dionysii de Divinis Nominibus (IV, 56).

Authority and Influence

Aquinas has been given a special position of respect in the field of Catholic scholarship, but this does not mean that all Catholic thinkers agree with him on all points. Within three years of his death a number of propositions closely resembling his philosophic views were condemned as errors by Bishop Tempier of Paris. This episcopal condemnation was formally revoked in 1325. Thomistic thought met much criticism in the later Middle Ages. Since the Renaissance nearly all the popes have praised Aquinas's teaching; the one who provided for the first collected edition of his works (St. Pius V) also did the same for St. Bonaventure, a Franciscan, and proclaimed both Doctors of the Church. In the ecclesiastical law of the Catholic Church, revised in 1918, canon 589:1 states that students for the priesthood are required to study at least two years of philosophy and four of theology, "following the teaching of St. Thomas." Further, canon 1366:2 directs professors in seminaries to organize their teaching "according to the method, teaching and principles of the Angelic Doctor."

Actually, Thomism has never been the only kind of philosophy cultivated by Catholics, and from the fourteenth century to the Enlightenment, Thomism was rivaled and sometimes obscured by Scotism and Ockhamism.

In 1879, with the publication of the Encyclical Aeterni Patris by Pope Leo XIII, the modern revival of Thomism started. While this document praised Thomism throughout, Pope Leo added this noteworthy qualification: "If there be anything that ill agrees with the discoveries of a later age, or, in a word, improbable in whatever wayit does not enter Our mind to propose that for imitation to our age" (Étienne Gilson, ed., The Church Speaks to the Modern World, New York, 1954, p. 50.)

In 1914 a group of Catholic teachers drew up a set of twenty-four propositions that, they felt, embodied the essential points in the philosophy of Aquinas. The Sacred Congregation of Studies, with the approval of Pope Pius X, published these "Twenty-four Theses" as clear expressions of the thought of the holy Doctor. (Original Latin text in Acta Apostolicae Sedis 6 [1914]: 384386; partial English version in Charles Hart, Thomistic Metaphysics, Englewood Cliffs, NJ, 1959, passim.)

The first six theses attempt a formulation of the general metaphysical position of Aquinas. All beings are composed of potential and actual principles, with the exception of God, who is pure act. The divine esse (act of being) is utterly simple (that is, without parts or constituents) and infinite in every way. Other beings are composite; their acts of existing are limited in character and merely participated. In general, metaphysical being may be understood in terms of analogy: God's being and that of created things do not belong within the same genus, but there is some remote resemblance between divine and nondivine beings. To satisfy competing theories of analogy that developed in Renaissance Thomism, the theses describe this metaphysical analogy in terms of both attribution (following Francisco Suárez) and proportionality (following Cardinal Cajetan). The real distinction between essence and esse is stressed in the fifth thesis, while the difference between substance and accidents is stated in the sixth (accidents exist in some substance but never, in the natural course of things, exist by themselves). Marking a transition to special metaphysics (cosmology and philosophical psychology), the seventh proposition treats a spiritual creature as composed of essence and esse, and also of substance and accidents, but denies that there is any composition of matter and form in spirits.

A series of theses (VIII to XIII) describe bodily beings as constituted of prime matter and substantial form, neither of which may exist by itself. As material, bodies are extended in space and subject to quantification. Matter as quantified is proposed as the principle that individuates bodies. The location of a body in place is also attributed to quantity. Thesis XIII distinguishes nonliving from living bodies and makes the transition to a group of propositions concerned with human nature and its activities. The life principle in any plant or animal is called a soul, but, in the case of the human animal, the soul is found to be a principle of a very special kind. Theses XIV to XXI focus on the vital nature and functions of man. His soul is capable of existing apart from the human body; it is brought into existence directly by God's creative action; it is without constituent parts and so cannot be disintegrated, that is to say, the human soul is immortal. Moreover, man's soul is the immediate source of life, existence, and all perfection in the human body. Subsequent propositions emphasize the higher human functions of cognition and volition, and they distinguish sensitive knowledge of individual bodies and their qualities from intellectual understanding of the universal features of reality. Willing is subsequent to intellectual cognition, and the free character of volitional acts of choice is strongly asserted.

The last three theses offer a summary of Aquinas's philosophic approach to God. The divine existence is neither directly intuited by the ordinary man nor demonstrable on an a priori basis. It is capable of a posteriori demonstration using any of the famous arguments of the Five Ways; these arguments are briefly summarized. Thesis XXIII reaffirms the simplicity of God's being and maintains the complete identity between the divine essence and esse. The final thesis asserts the creation by God of all things in the universe and stresses the point that the coming into existence and the motion of all creatures are to be attributed ultimately to God as First Cause.

These twenty-four theses represent a rigid and conservative type of Thomism. Many modern Catholic philosophers, while recognizing that these propositions do express some of the basic themes in the speculative thought of Aquinas, doubt that it is possible to put the wisdom of any great philosopher into a few propositions and prefer to emphasize the open-minded spirit with which Aquinas searched for information among his predecessors and approached the problems of his own day. After all, it was Aquinas who remarked that arguments from authority are appropriate in sacred teaching but are the weakest sort of evidence in philosophic reasoning.

See also Abelard, Peter; Aesthetics, History of; Albert the Great; Aristotelianism; Aristotle; Augustine, St.; Averroes; Avicenna; Being; Boethius, Anicius Manlius Severinus; Bonaventure, St.; Cajetan, Cardinal; Cicero, Marcus Tullius; Duns Scotus, John; Empiricism; Enlightenment; Essence and Existence; Eternal Return; Ethics, History of; Faith; Gilson, Étienne Henry; Ibn Gabirol, Solomon ben Judah; John of Damascus; Liber de Causis; Maimonides; Maritain, Jacques; Metaphysics, History of; Neoplatonism; Ockhamism; Plato; Pseudo-Dionysius; Roscelin; Ross, William David; Scientia Media and Molinism; Scotism; Stoicism; Suárez, Francisco; Taylor, Alfred Edward; Thomism; Universals, A Historical Survey; William of Champeaux.


works by thomas aquinas

Collected Editions

Opera Omnia, 25 vols. Parma, 18521873. Reprinted, New York, 19481950. A noncritical but almost complete edition.

Opera Omnia, 34 vols. Paris, 18711882. A noncritical but almost complete edition.

S. Thomae Aquinatis, Opera Omnia. Rome, 1882. The Leonine Edition is recognized by scholars as the standard edition of the works of Aquinas.

Individual Works

Scriptum in IV Libros Sententiarum (12521257). 4 vols, edited by Pierre Mandonnet and M. F. Moos. Paris, 19291947. No English version.

De Ente et Essentia (1253). Vol. XLIII of Leonine edition. Translated by Armand Maurer as On Being and Essence. Toronto, 1949.

De Principiis Naturae (1253). Vol. XLIII of Leonine edition. Translated by V. J. Bourke in The Pocket Aquinas, edited by V. J. Bourke, 6177. New York: Washington Square Press, 1960.

Contra Impugnantes Dei Cultum et Religionem (1256). Vol. XLI of Leonine edition. Translated by J. Proctor as An Apology for the Religious Orders. Westminster, MD, 1950.

Quaestiones Disputatae de Veritate (12561259). Vol. XXII of Leonine edition. Translated by R. W, Mulligan et al. as Truth, 3 vols. Chicago, 19521954.

Quaestiones Quodlibetales (12561272). Vol. XXV of Leonine edition. Partially translated by S. Edwards as Quodlibetal Questions 1 and 2. Medieval Sources in Translation, Vol. 27. Toronto, 1983.

In Librum Boethii de Trinitate Expositio (12571258). Vol. L of Leonine edition. Translated by R. E. Brennan as The Trinity and the Unicity of the Intellect. St. Louis, 1946.

In Librum Dionysii de Divinis Nominibus (12581265), edited by Ceslas Pera. Turin, 1950. Partially translated by V. J. Bourke in The Pocket Aquinas, 269278. New York: Washington Square Press, 1960.

Summa de Veritate Catholicae Fidei Contra Gentiles (12591264), Vols. XIIIXV of the Leonine edition. Rome, 19181930. Also published in a one-volume "manual" edition. Turin and Rome, 1934. Translated by A. C. Pegis, J. F. Anderson, V. J. Bourke, and C. J. O'Neil as On the Truth of the Catholic Faith, 5 vols. New York, 19551957.

De Emptione et Venditione (1263). Vol. XLII of Leonine edition. Translated and edited by Alfred O'Rahilly as "Notes on St. Thomas on Credit." Irish Ecclesiastical Record 31 (1928): 164165. Translation reprinted in The Pocket Aquinas, 223225. New York: Washington Square Press, 1960.

Quaestiones Disputatae de Potentia Dei (1265). Edited by R. M. Spiazzi. Turin, 1949. Translated by Lawrence Shapcote as On the Power of God. Westminster, MD: Newman Press, 1952.

Commentaries on Aristotle (12651273), listed in next section.

De Regno (12651266). Vol. XLII of Leonine edition. Translated by G. B. Phelan and I. T. Eschmann as On Kingship. Toronto: Pontifical Institute of Mediaeval Studies, 1949.

Compendium Theologiae (12651269). Vol. XLII of Leonine edition. Translated by Cyril Vollert as Compendium of Theology. St. Louis, 1957.

Summa Theologiae (12651273). Vols. IVXII of the Leonine edition. Rome, 19181930. Reprinted, Turin, 1934. Translated by the English Dominican Fathers as The Summa Theologica, 22 vols. London, 19121936. Revision of part of the English Dominican Fathers' translation appears in Basic Writings of Saint Thomas Aquinas, edited by A. C. Pegis. New York: Random House, 1945.

Quaestiones Disputatae de Spiritualibus Creaturis (1267). Vol. XXIV of Leonine edition. Translated by John Wellmuth and Mary Fitzpatrick as On Spiritual Creatures. Milwaukee, 1949.

Quaestio Disputata de Anima (1269). Vol. XXIV of Leonine edition. Translated by J. P. Rowan as The Soul. St. Louis, 1949.

Quaestiones Disputatae de Malo (12691272). Vol. XXIII of Leonine edition. Translated by J. and J. Oesterle as Disputed Questions on Evil. Notre Dame, IN: University of Notre Dame Press, 1983.

Quaestiones Disputatae de Virtutibus (12691272). Edi Vol. XXIV of Leonine edition. Partially translated by J. P. Reid as The Virtues in General. Providence, RI, 1951.

In Job Expositio (12691272). In Vol. XVII of Leonine edition. Rome, 1962. Translated by A. Damico as The Literal Exposition on Job: A Scriptural Commentary concerning Providence. Classics in Religious Studies 7. Atlanta, 1989.

In Evangelium Joannis Expositio (12691272), edited by Raphael Cai. Turin, 1952. Partially translated by J. A. Weisheipl and F. R. Larcher as Commentary on the Gospel of Saint John, Part I. Albany, NY, 1980.

De Unitate Intellectus (1270). Edited by Vol. XLIII of Leonine edition. Translated by Sister Rose E. Brennan as The Unicity of the Intellect. St. Louis: Herder, 1946.

De Substantiis Separatis (1271). Vol. XL of Leonine edition. Translated by F. J. Lescoe as Treatise on Separate Substances. West Hartford, CT: Saint Joseph College, 1963.

De Aeternitate Mundi (1271). Vol. XLIII of Leonine edition. Translated by Cyril Vollert as On the Eternity of the World. Milwaukee: Marquette University Press, 1964.

In Librum de Causis (1271), edited by H. D. Saffrey. Fribourg, 1954. Translated by V. A. Guagliardo et al. as Commentary on the Book of Causes. Thomas Aquinas in Translation Series. Washington, DC, 1996.

Commentaries on Aristotle

In Libros Posteriorum Analyticorum. In Vol. I of the Leonine edition. Rome, 1882. Translated by Pierre Conway. Quebec, 1956.

In Libros de Anima. In Vol. XLV of the Leonine edition. Also edited by R. M. Spiazzi. Turin, 1955. Translated by Kenelm Foster and Silvester Humphries as Aristotle's De Anima with the Commentary of St. Thomas. London and New Haven, CT, 1951.

In Libros de Caelo et Mundo. In Vol. III of the Leonine edition. Rome, 1886. Also edited by R. M. Spiazzi. Turin, 1952. No English version.

In X Libros Ethicorum. In Vol. XLVII of the Leonine edition. Also edited by R. M. Spiazzi. Turin, 1949. Translated by C. I. Litzinger as Commentary on the Nicomachean Ethics, 2 vols. Chicago: Regnery, 1964.

In Libros de Generatione et Corruptione. In Vol. III of the Leonine edition. Rome, 1886. Also edited by R. M. Spiazzi. Turin, 1952. No English version.

In Libros Peri Hermeneias. In Vol. I of the Leonine edition. Rome, 1882. Also edited by R. M. Spiazzi. Turin, 1955. Translated by Jean Oesterle as Aristotle on InterpretationCommentary by St. Thomas and Cajetan. Milwaukee: Marquette University Press, 1962.

In Libros de Memoria et Reminiscentia, et de Sensu et Sensato. In Vol. XLV of the Leonine edition. Also edited by R. M. Spiazzi. Turin, 1949. No English version.

In XII Libros Metaphysicorum. In Vol. XLVI of the Leonine edition. Also edited by R. M. Spiazzi. Turin, 1950. Translated by J. P. Rowan as Commentary on the Metaphysics of Aristotle, 2 vols. Chicago: Regnery, 1961.

In Libros Meteorologicorum. in Vol. III of the Leonine edition. Rome, 1886. Also edited by R. M. Spiazzi. Turin, 1952. No English version.

In VIII Libros Physicorum. In Vol. II of the Leonine edition. Rome, 1884. Also edited by P. M. Maggiolo. Turin, 1954. Translated by R. J. Blackwell and others as Commentary on Aristotle's Physics. New Haven, CT: Yale University Press, 1963.

In Libros Politicorum. In Vol. XLVIII of the Leonine edition. Also edited by R. M. Spiazzi. Turin, 1951. Translation of Book III, Lectures 16 by E. L. Fortin and Peter D. O'Neill in Medieval Political Philosophy, edited by Ralph Lerner and Muhsin Mahdi, 297334. New York: Free Press of Glencoe, 1963.

works on thomas aquinas


Bergomo, Petri de. Tabula Aurea in Omnia Opera S. Thomae Aquinatis. Bologna, 1485. Reprinted in Vol. XXV of S. Thomae, Opera Omnia. Parma, 1873.

Busa, Roberto. Index Thomisticus. Stuttgart-Bad Cannstatt: Fromann-Holzboog, 19741980.

Deferrari, Roy J., Sister M. Inviolata Barry, and Ignatius McGuiness. A Lexicon of St. Thomas Aquinas, Based on the Summa Theologica and Selected Passages of His Other Works. Washington, DC: Catholic University of America Press, 1949.

Indices in Summam Theologiae et Summam Contra Gentiles. In Vol. XVI of the Leonine edition. Reprinted with some omissions in Editio Leonina Manualis. Rome, 1948.

Schütz, Ludwig. Thomas-Lexikon. Paderborn, 1895. Reprinted New York, 1949.


Aertsen, Jan. Nature and Creature: Thomas Aquinas' Way of Thought. Leiden: Brill, 1988.

Anderson, J. F. An Introduction to the Metaphysics of St. Thomas Aquinas. Chicago: Regnery, 1953. Collection of basic texts with clear explanations; useful for the theory of analogy.

Bourke, V. J. Aquinas' Search for Wisdom. Milwaukee: Bruce, 1965. A factual biography excluding pious legends and situating Thomas's thought in the context of his life.

Burrell, David. Knowing the Unknowable God: Ibn Sina, Maimonides, Aquinas. Notre Dame: University of Notre Dame Press, 1986.

Callahan, Leonard. A Theory of Esthetic according to St. Thomas. Washington, DC: Catholic University of America Press, 1927. Rethinks the original theory but also interprets the fragmentary maxims of Aquinas on beauty and art.

Chenu, M. D. Toward Understanding Saint Thomas. Chicago: Regnery, 1964. Masterful introduction to the advanced study of the works of Thomas Aquinas.

Copleston, F. C. Aquinas. Harmondsworth, U.K.: Penguin, 1955. This is a well-written exposition of the philosophy of Thomas, with some effort to meet the criticisms of British analysts.

Davies, Brian. The Thought of Thomas Aquinas. Oxford: Clarendon Press, 1992.

De Koninck, Charles. "Introduction à l'étude de l'âme." Laval théologique et philosophique 3 (1947): 965. Reprinted in Stanislas Cantin, Précis de psychologie thomiste. Quebec, 1948. LXXXIII, 173.

Descoqs, Pedro. Thomisme et scolastique. Paris: G. Beauchesne, 1927. Representative Suarezian criticism of the metaphysics of Aquinas; insists that there are other valid positions in philosophy.

Duns Scotus, John. De Primo Principio. Translated and edited by Evan Roche. St. Bonaventure, NY: Franciscan Institute, 1949. Important for fourteenth-century criticism by Duns Scotus of the general metaphysics and natural theology of Thomas.

Fabro, Cornelio. La nozione metafisica di partecipazione secondo S. Tommaso. Milan, 1939; 2nd ed., Turin, 1950. Outstanding study of the metaphysics of Thomism, stressing the Platonic elements.

Finnis, John. Aquinas: Moral, Political, and Legal Theory. New York: Oxford University Press, 1998.

Flannery, Kevin. Acts Among Precepts: The Aristotelian Structure of Thomas Aquinas's Moral Theory. Washington DC: Catholic University of America Press, 2001.

Garrigou-Lagrange, Réginald. Reality: A Synthesis of Thomistic Thought. St. Louis: Herder, 1950. Adapted from a long article in the Dictionnaire de théologie catholique, this work represents a most conservative type of Thomism.

Geiger, L. B. La participation dans la philosophie de saint Thomas. Paris, 1942. Excellent discussion of the Platonic themes in Thomas's metaphysics; with Fabro and Geiger it has become clear that Thomism is not a "baptized" Aristotelianism.

Gilby, Thomas. The Political Thought of Thomas Aquinas. Chicago: University of Chicago Press, 1958. Work of popularization that offers, however, a rather balanced view of the monarchic and democratic elements in Thomas's ideas on government.

Gilson, Étienne. The Christian Philosophy of St. Thomas Aquinas. New York: Random House, 1956. With "Catalogue of St. Thomas's Works" by I. T. Eschmann. A highly esteemed exposition; the French original followed the order of the Summa Contra Gentiles ; this English revision stresses the idea that Thomism is a Christian philosophy.

Grabmann, Martin. Die Werke des hl. Thomas von Aquin. 3rd ed. Münster, 1949. The most complete study of the chronology and authenticity of the writings of Aquinas.

Henle, R. J. Saint Thomas and Platonism. The Hague: Nijhoff, 1956. Good example of a careful textual study using the method of parallel passages taken in chronological sequence.

Hughes, Christopher. On a Complex Theory of a Simple God: An Investigation in Aquinas' Philosophical Theology. Ithaca: Cornell University Press, 1989.

Jaffa, H. V. Thomism and Aristotelianism. A Study of the Commentary by Thomas Aquinas on the Nicomachean Ethics. Chicago: University of Chicago Press, 1952. A sharp criticism of Aquinas as an interpreter of Aristotle's practical philosophy.

Kenny, Anthony, ed. Aquinas: A Collection of Critical Essays. Garden City: Doubleday, 1969.

Kenny, Anthony. Aquinas on Mind. London: Routledge, 1993.

Klubertanz, G. P. St. Thomas Aquinas on Analogy. Chicago: Loyola University Press, 1960. Advanced and difficult study of all that Aquinas has written on metaphysical analogy; stresses the pluralism inherent in the theory: several types of analogy are at work.

Kluxen, Wolfgang. Philosophische Ethik bei Thomas von Aquin. Mainz, 1964. Important discussion of the relation between moral theology and ethics.

Kretzmann, Norman. The Metaphysics of Creation: Aquinas's Natural Theology in Summa Contra Gentiles II. Oxford: Clarendon Press, 1999.

Kretzmann, Norman. The Metaphysics of Theism: Aquinas's Natural Theology in Summa Contra Gentiles I. Oxford: Clarendon Press, 1997.

Kretzmann, Norman, and Stump, Eleonore, eds. The Cambridge Companion to Aquinas. Cambridge, U.K.: Cambridge University Press, 1993.

Lonergan, Bernard. Verbum: Word and Idea in Aquinas. Edited by David Burrell. Notre Dame, IN: University of Notre Dame Press, 1967.

Maritain, Jacques. The Angelic Doctor: The Life and Thought of St. Thomas Aquinas, rev. ed. New York, 1958. Popular work stressing papal approval of Thomistic philosophy and the special status of Aquinas in Catholic educational programs.

Maritain, Jacques. Art and Scholasticism. New York: Scribners, 1930. An almost classic account of the aesthetic theory of Thomas, with some adaptation to artistic problems in the twentieth century.

Maurer, Armand, ed. St. Thomas Aquinas, 12741974: Commemorative Studies, 2 vols. Toronto: Pontifical Institute of Medieaval Studies, 1974.

McInerny, Ralph. Aquinas and Analogy. Washington, DC: Catholic University of America Press, 1996.

Meyer, Hans. The Philosophy of St. Thomas Aquinas. St. Louis: Herder, 1944. In the guise of an exposition of Aquinas's thought, this is actually a trenchant critique of Thomism.

Oesterle, John A. Ethics: The Introduction to Moral Science. Englewood Cliffs, NJ: Prentice-Hall, 1957. Representative textbook emphasizing the autonomy and strictly philosophical character of Thomistic ethics.

Owens, Joseph. An Elementary Christian Metaphysics. Milwaukee: Bruce, 1963. Advanced textbook in the tradition of the Christian philosophy interpretation of Aquinas.

Owens, Joseph. St. Thomas Aquinas on the Existence of God: Collected Papers of Joseph Owens, edited by John Catan. Albany: State University of New York Press, 1980.

Pasnau, Robert. Thomas Aquinas on Human Nature: A Philosophical Study of Summa Theologiae Ia 7589. Cambridge, U.K.: Cambridge University Press, 2001.

Pieper, Josef. Guide to Thomas Aquinas. Translated by Richard and Clara Winston. New York: Pantheon, 1962. A readable and elementary introduction by a noted German Thomist.

Rahner, Karl. Geist im Welt. Zur Metaphysik der endliche Erkenntnis bei Thomas von Aquin. Innsbruck and Leipzig, 1939. Outstanding German study of the theory of knowledge.

Ramírez, J. M. De Auctoritate Doctrinae Sancti Thomae Aquinatis. Salamanca, 1952. Representative of the well-informed and broadminded views of some Spanish Dominicans on the status of Thomism in Catholic philosophy.

Rommen, Heinrich. The State in Catholic Thought. St. Louis: Herder, 1945. Situates Thomistic political philosophy in the broader context of Catholic thinking as a whole.

Reith, Herman. The Metaphysics of St. Thomas Aquinas. Milwaukee: Bruce, 1958. Standard exposition, not difficult to read. Includes 200 pages of quotations from many works of Thomas.

Ryan, J. K. "St. Thomas and English Protestant Thinkers." New Scholasticism 22 (1) (1948): 133; (2) (1948): 126208. Excellent survey of the reactions to, and criticisms of, Aquinas in post-Reformation British thought.

Smith. Gerard. Natural Theology: Metaphysics II. New York: Macmillan, 1951. On Thomas's philosophical approach to God; Ch. 16 discusses divine foreknowledge and human freedom.

Stump, Eleonore. Aquinas. London: Routledge, 2003.

Torrell, Jean-Pierre. Saint Thomas Aquinas, Vol. 1: The Person and His Work. Translated by Robert Royal. Washington, DC: Catholic University of America Press, 1996.

Ude, Johannes. Die Autorität des hi. Thomas von Aquin als Kirchenlehrer. Salzburg, 1932. Basic study of the question "Must Catholics agree in philosophy and theology with Thomas Aquinas?" The answer is "No."

Walz, Angelus, Saint Thomas d'Aquin. French adaptation by Paul Novarina. Louvain and Paris, 1962. One of the best biographies; this French printing has much more information than the English version, Saint Thomas Aquinas, translated by S. T. Bullough. Westminster, MD: Newman Press, 1951.

Westberg, Daniel. Right Practical Reason: Aristotle, Action, and Prudence in Aquinas. Oxford: Clarendon Press, 1994.

Wippel, John. The Metaphysical Thought of Thomas Aquinas: From Finite Being to Uncreated Being. Washington, DC: Catholic University of America Press, 2000.

Wittmann, Michael. Die Ethik des hl. Thomas von Aquin. Munich, 1933. Good historical study of Thomistic ethics in relation to Aristotle, the Stoics, and the Fathers of the Church.

works on thomism

Bourke, V. J. Thomistic Bibliography, 19201940. St. Louis: Modern Schoolman, 1945.

Bulletin thomiste (since 1921).

Ingardia, Richard. Thomas Aquinas: International Bibliography 19771990. Bowling Green, OH: Philosophy Documentation Center, 1993.

Mandonnet, Pierre, and Jean Destrez. Bibliographie thomiste. Le Saulchoir, Belgium: Revue des Sciences Philosophiques et Thélogiques, 1921; rev. ed., edited by M. D. Chenu. Paris: Vrin, 1960.

Répertoire bibliographique. Annual supplement to Revue philosophique de Louvain (Belgium).

Wyser, Paul. Thomas von Aquin and Der Thomismus, fascicles 1314 and 1516 of Bibliographische Einführungen in das Studium der Philosophie. Fribourg, Switzerland, 1948ff.

Vernon J. Bourke (1967)

Bibliography updated by Christian B. Miller (2005)

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Thomas Aquinas, St. (c. 1224–1274)

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