Thomism is the philosophical school that followed in the legacy of Thomas Aquinas. The word comes from the name of its originator, whose summary work Summa Theologiae has arguably been second only to the Bible in importance to the Catholic Church. During the thirteenth century, the philosophy of ancient Greece was introduced to European scholars through the works of Arabian and Jewish scholars, and the works of Aristotle became available for the first time in Latin translation. Thomas Aquinas synthesized the principles of Aristotle with the doctrines of the Christian faith, using logic and dialectic to produce an explanation of Catholic dogma. The thought of Thomas Aquinas was important in shifting medieval philosophy (also known as Scholasticism) away from the influence of Plato and towards Aristotle. The ensuing school of thought became one of the most influential philosophies of all time, through its influence on Roman Catholicism and Catholic ethics, and through the sheer number of people who lived by its teachings.
In the Encyclical Doctoris Angelici (1914), Pope Pius X cautioned that the teachings of the Catholic Church cannot be understood without the basic philosophical underpinning of Aquinas's major theses. The Second Vatican Council described Aquinas' system as the "Perennial Philosophy" .
The thirteenth century, the high point of the Middle Ages, witnessed three important theological developments: Duns Scotus laid the foundation for modern theology by emphasizing the primacy of will over intellect; Bonaventura (Italian, San Bonaventura, original name Giovanni Di Fidanza), represented the spirit of Augustine and Saint Francis, placing God first as the source of illumination; and Thomas Aquinas adapted the principles of Aristotle to Christian doctrine and became the classical theologian of the Roman Catholic Church.
European philosophy of the thirteenth century was characterized by several formative influences. The Crusades brought Christianity into contact with three highly developed cultures, Islam, the original Judaism, and ancient Greek culture, which was introduced to the medieval world by Arabian theologians. The works of Aristotle became available to scholars for the first time, giving rise to a new scientific methodology in philosophy. Two new monastic orders appeared, the Dominican and the Franciscan, each of which took the new influences and used them in a different way. Francis belonged to the tradition of Augustine and Anselm, which sought the mystical union of Christianity with nature and culture. Dominic took on the mission of preaching to the people and defending the Catholic faith. The Dominican order produced Thomas Aquinas’ classical system of apologetic theology, and the greatest preachers, among whom was Meister Eckhart. The Dominicans were responsible more than any other school for bringing Aristotle to the West, stressing intellect over will even in their mysticism.
Aquinas worked to create a philosophical system which integrated Christian doctrine with elements taken from the philosophy of Aristotle. He synthesized the Neo-Platonic view of philosophy which, after Augustine, had become tremendously influential amongst medieval philosophers, with insights drawn from Aristotle. Aquinas was greatly influenced by his reading of contemporary Arabic philosophers, especially Averroes, though he rejected Averroes' primary conclusions and themes. Inspired by Aristotle, Aquinas set out to provide a logical philosophical explanation for many of the tenets of the Christian faith, an explanation which could satisfy the intellect at a time when there was a revival, also inspired by Aristotle, of interest in the natural sciences and methods of scientific inquiry. Aquinas is credited with moving the focus of medieval Scholastic philosophy from Plato to Aristotle.
A famous statement by Thomas Aquinas reads, “Grace does not remove nature, but fulfills it.” Aquinas taught that, in addition to all of his natural abilities, at the time of creation God gave Adam a “supernature,” a gift of grace by which man could persist in union with God. Man’s nature could thus be fulfilled by this “supernature,” or grace. A similar principle applied to revelation and reason, with revelation being the fulfillment, or extension, of reason by supernatural grace.
Aquinas accepted Aristotle’s doctrine of hylomorphism, defining prime matter as pure potentiality and substantial form as the “first act” of a physical body, which placed it in a specific class and determined its essence. The union of the substantial form with physical matter produced the individual qualities of each physical existence. Forms of being existed in a hierarchy of scale from the lowest inorganic substances to the human being, at the summit of the physical world, and finally to the ultimate simplicity of God. The essence of any corporeal being was composed of matter and form; that by which it was a real being was “existence” (“esse”). Existence was to essence as “act” was to potentiality.
“In intellectual substances, which are not composed of matter and form (in them the form is a subsistent substance), the form is that which is; but existence is the act by which form is; and on that account there is in them only on e composition of act and potentiality, namely the composition of substance and existence… In substances composed of matter and form, however, there is a double composition of act and potentiality; the first a composition in the substance itself, which is composed of matter and form, the second a composition of the substance itself, which is already composite, with existence.” Thomas Aquinas, Contra Gentiles, 2. 54
In his Summa theologiae (Ia, q. 2, a. 3), Aquinas offers five "ways" of proving the existence of God through logic. Though termed "proofs" of God's existence, they are better understood as "reasonings." These reasonings observe certain effects, and from them infer the cause. Aquinas would argue that God's existence cannot be "proven" per se because thinking of an object does not prove its existence, but that God's existence can be inferred based on these effects. Aquinas recognized that many religious mysteries could not be explained through logic, but must be understood only through faith; he regarded these “proofs” of God’s existence as preambles to faith.
"It is clear that there are in this world things which are moved. Now, every object which is moved receives that movement from another. If the motor is itself moved, there must be another motor moving it, and after that yet another, and so on. But it is impossible to go on indefinitely, for then there would be no first motor at all, and consequently no movement" ("Contra Gentiles," ii. 33). This proof, like much of Thomas Aquinas's thought, is taken from Aristotle, whose "unmoved mover" was the first recorded example of a cosmological argument for God's existence.
"We discern in all sensible things a certain chain of efficient causes. We find, however, nothing which is its own efficient cause, for that cause would then be anterior to itself. On the other side, it is impossible to ascend from cause to cause indefinitely in the series of efficient causes…. There must therefore exist one self-sufficient, efficient cause, and that is God" ("Contra Gent." i. 22).
"Find in nature things which may be and may not be, since there are some who are born and others who die; they consequently can exist or not exist. But it is impossible that such things should live for ever, for there is nothing which may be as well as not be at one time. Thus if all beings need not have existed, there must have been a time in which nothing existed. But, in that case, nothing would exist now; for that which does not exist can not receive life but from one who exists; … there must therefore be in nature a necessarily existent being."
Any category has its degrees, such as good and better, warm and warmer. Each also has one thing that is the ultimate of that measure, like good and "best," warm and "hottest." And whatever is the most of that category is the source of that category, as fire (or, in modern terms, energy itself) is the source of heat, and God must therefore be the ultimate source of goodness.
Everything, sentient or otherwise, progresses in an orderly way. Planets move in their orbits, light breaks from and combines into its spectrum, and so on. Reality has a natural order, which could not have come from nothing, yet which precedes mere humans. 
This is essentially the teleological argument for God's existence. Some scholars believe that this argument is equivalent to what is now called "Intelligent Design." However, this is not an accurate presentation of Aquinas' thought.
Aquinas adopted the Aristotelian view of the soul as the “form” of the body and therefore viewed physical body and soul as a coherent unity. The soul needed the body in order to acquire knowledge and experience through the senses of the physical body; since body and soul existed in a natural unity, the soul required the body in order to perform its natural function. At the same time, Aquinas argued that the soul was not dependent on the physical body for its existence, but a subsistent form, because it was capable of knowing the natures of all bodies. As proof of the immortality of the soul, Aquinas cited man’s universal desire for immortality, which he argued must have been implanted in man by the Creator, God, because immortality existed.
Aquinas derived a system of ethics from his concept of nature and grace, consisting of a rational substructure and a theological superstructure, and combining the pagan virtues of classical philosophy with Christian values. The rational substructure contained the four Platonic virtues of courage, temperance, wisdom and justice, which by themselves would produce natural happiness, or natural blessedness, the fulfillment of man’s nature. The supernatural virtues were the Christian virtues of faith, hope and love, virtues given not by nature, but by grace.
Aquinas viewed the ethical purpose of man as the fulfillment of his intellect; the intellect was what distinguished man from animals, and made him able to live within a meaningful structure of reason.
Thomas Aquinas was the first philosopher of the Middle Ages to create a theological aesthetics, in which beauty was a reflection of virtue. “The beautiful is that kind of the good in which the soul rests without possessing.”
Aquinas also developed a political structure, based on his concept of “nature” and “supernature,” which had a profound influence on Europe during the Middle Ages, and whose repercussions are still seen in modern political organization. Aquinas distinguished between the secular values represented by the state, and the higher supernatural values embodied in the church. The church had authority over the state because it represented higher values, and in certain circumstances could ask the people to disobey the dictates of the state.
The thought of Thomas Aquinas was important in shifting medieval philosophy (also known as Scholasticism) away from the influence of Plato and towards Aristotle. The ensuing school of thought became one of the most influential philosophies of all time, through its influence on Roman Catholicism and Catholic ethics, and through the sheer number of people who lived by its teachings.
Thomist theology was not immediately affirmed. Some of its theses were condemned in 1277 by the ecclesiastical authorities of the most important theological schools in Middle Age Europe, the University of Paris and Oxford University, and the Franciscan Order vehemently opposed the ideas of the Dominican Thomas. The canonization of Thomas in 1323 led to revoking the condemnation of 1277 and ended the controversy over Thomist theology.
For a long time, Thomism remained the doctrine of only Dominican theologians, such Giovanni Capreolo (1380-1444) and Tommaso de Vio (1468-1534). In the sixteenth century, Spanish Jesuit theologians (including F. Suárez, F. Vitoria, F. Toledo, and others) wholeheartedly adopted Thomism, which became the official philosophy of the Catholic Church, offering a coherent, logical, and clear metaphysical picture of both the material and spiritual worlds. It prevailed as a coherent system until the discovery of Newtonian mechanics, and the rise of rationalism and empiricism as philosophical schools.
After the 1879 encyclical Aeterni Patris, which sanctioned the revival of Thomism, the ethical aspects of Thomism, as well as a many of its concepts of life, humanity, and theology, transferred to the various schools of Neothomism that are the official philosophy of the modern Roman Catholic Church. Thomism remains a vibrant and challenging school of philosophy. According to one of its most famous and controversial proponents, Alasdair MacIntyre (born January 12, 1929 in Scotland), a Thomistic Aristotelianism is the philosophical theory which best explains human knowledge of external reality and human practice.
Thomas Aquinas made a clear distinction between philosophy and theology, saying that philosophy and the rational sciences relied on principles known naturally by human reason, while theology based its reasoning on principles received by supernatural revelation, or faith. The use of logic, dialectic and other philosophical methods helped a theologian to better explain doctrines of faith, and led to a Scholastic theology, but did not turn theology into philosophy. Aquinas argued that a philosopher worked from principles known to natural reason to infer the existence of God, while a theologian began with the premise that God existed and worked to understand the structure of the world. Only theology could understand the ultimate supernatural end for which man was created, but philosophy could help man achieve a natural happiness while on earth by discovering the natural virtues and how to attain them, and offering analogical knowledge of God. Without revelation, it was impossible to work out a complete and adequate metaphysical understanding.
Aquinas always made Aristotelian concepts subservient to the religious doctrines of the Catholic Church, and found a way to synthesize them effectively. However, the tradition of philosophical inquiry through logic and dialectic which he initiated eventually led to the dominance of philosophy and to the rise of secular schools of thought in Europe.
Aquinas’ main work, "Summa Theologiæ," shows a profound knowledge not only of the writings of Avicebron (Solomon Ibn Gabirol), whose name he mentions, but also of most Jewish philosophical works then existing.
Thomas pronounced himself energetically against the hypothesis that the world was eternal, without beginning or end. He sought to demonstrate that Aristotle, who promoted this hypothesis, did not express himself categorically on this subject.
"The argument," said he, "which Aristotle presents to support this thesis is not properly called a demonstration, but is only a reply to the theories of those ancients who supposed that this world had a beginning and who gave only impossible proofs. There are three reasons for believing that Aristotle himself attached only a relative value to this reasoning…." (Summa Theologiæ, i. 46, art. 1 ). (Thomas copied these words from Maimonides's Guide for the Perplexed, which gives the three reasons for believing that Aristotle’s reasoning was only relative.(I:2,15)).
Aquinas' doctrines, because of their close relationship with those of Jewish philosophy, found great favor among Jews. Judah Romano (born 1286) translated Aquinas' ideas from Latin into Hebrew under the title "Ma'amar ha-Mamschalim," together with other small treatises extracted from the "Contra Gentiles" ("Neged ha-Umot").
Eli Hobillo (1470) translated, without Hebrew title, the "Quæstiones Disputatæ," "Quæstio de Anima," his "De Animæ Facultatibus," under the title "Ma'amar be-Koĵot ha-Nefesh," (edited by Jellinek); his "De Universalibus" as "Be-Inyan ha-Kolel"; "Shaalot Ma'amar beNimĵa we-biMehut."
Abraham Nehemiah ben Joseph (1490) translated Thomas' Commentarii in Metaphysicam. According to Moses Almosnino, Isaac Abravanel desired to translate the "Quæstio de Spiritualibus Creaturis." Abravanel indeed seems to have been well acquainted with the philosophy of Thomas Aquinas, whom he mentions in his work Mif'alot Elohim (vi. 3). The physician Jacob Zahalon (d. 1693) translated some extracts from the Summa Theologiæ Contra Gentiles.
In the decree "Postquam sanctissumus" of July 27, 1914, Pope Pius X declared that twenty-four theses formulated by "teachers from various institutions … clearly contain the principles and more important thoughts" of Aquinas.
The capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.
These twenty-four theses represent a summary of Aquinas's system:
1. Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.
2. Since act is perfection, it is not limited except through a potency which itself is a capacity for perfection. Hence in any order in which an act is pure act, it will only exist, in that order, as a unique and unlimited act. But whenever it is finite and manifold, it has entered into a true composition with potency.
3. Consequently, the one God, unique and simple, alone subsists in absolute being. All other things that participate in being have a nature whereby their being is restricted; they are constituted of essence and being, as really distinct principles.
4. A thing is called a being because of "esse." God and creature are not called beings univocally, nor wholly equivocally, but analogically, by an analogy both of attribution and of proportionality.
5. In every creature there is also a real composition of the subsisting subject and of added secondary forms, i.e. accidental forms. Such composition cannot be understood unless being is really received in an essence distinct from it.
6. Besides the absolute accidents there is also the relative accident, relation. Although by reason of its own character relation does not signify anything inhering in another, it nevertheless often has a cause in things, and hence a real entity distinct from the subject.
7. A spiritual creature is wholly simple in its essence. Yet there is still a twofold composition in the spiritual creature, namely, that of the essence with being, and that of the substance with accidents.
8. However, the corporeal creature is composed of act and potency even in its very essence. These act and potency in the order of essence are designated by the names form and matter respectively.
9. Neither the matter nor the form have being of themselves, nor are they produced or corrupted of themselves, nor are they included in any category otherwise than reductively, as substantial principles.
10. Although extension in quantitative parts follows upon a corporeal nature, nevertheless it is not the same for a body to be a substance and for it to be quantified. For of itself substance is indivisible, not indeed as a point is indivisible, but as that which falls outside the order of dimensions is indivisible. But quantity, which gives the substance extension, really differs from the substance and is truly an accident.
11. The principle of individuation, i.e., of numerical distinction of one individual from another with the same specific nature, is matter designated by quantity. Thus in pure spirits there cannot be more than individual in the same specific nature.
12. By virtue of a body's quantity itself, the body is circumscriptively in a place, and in one place alone circumscriptively, no matter what power might be brought to bear.
13. Bodies are divided into two groups; for some are living and others are devoid of life. In the case of the living things, in order that there be in the same subject an essentially moving part and an essentially moved part, the substantial form, which is designated by the name soul, requires an organic disposition, i.e., heterogeneous parts.
14. Souls in the vegetative and sensitive orders cannot subsist of themselves, nor are they produced of themselves. Rather, they are no more than principles whereby the living thing exists and lives; and since they are wholly dependent upon matter, they are incidentally corrupted through the corruption of the composite.
15. On the other hand, the human soul subsists of itself. When it can be infused into a sufficiently disposed subject, it is created by God. By its very nature, it is incorruptible and immortal.
16. This rational soul is united to the body in such a manner that it is the only substantial form of the body. By virtue of his soul a man is a man, an animal, a living thing, a body, a substance and a being. Therefore the soul gives man every essential degree of perfection; moreover, it gives the body a share in the act of being whereby it itself exists.
17. From the human soul there naturally issue forth powers pertaining to two orders, the organic and the non-organic. The organic powers, among which are the senses, have the composite as their subject. The non-organic powers have the soul alone as their subject. Hence, the intellect is a power intrinsically independent of any bodily organ.
18. Intellectuality necessarily follows upon immateriality, and furthermore, in such manner that the father the distance from matter, the higher the degree of intellectuality. Any being is the adequate object of understanding in general. But in the present state of union of soul and body, quiddities abstracted from the material conditions of individuality are the proper object of the human intellect.
19. Therefore, we receive knowledge from sensible things. But since sensible things are not actually intelligible, in addition to the intellect, which formally understands, an active power must be acknowledged in the soul, which power abstracts intelligible likeness or species from sense images in the imagination.
20. Through these intelligible likenesses or species we directly know universals, i.e., the natures of things. We attain to singulars by our senses, and also by our intellect, when it beholds the sense images. But we ascend to knowledge of spiritual things by analogy.
21. The will does not precede the intellect but follows upon it. The will necessarily desires that which is presented to it as a good in every respect satisfying the appetite. But it freely chooses among the many goods that are presented to it as desirable according to a changeable judgment or evaluation. Consequently, the choice follows the final practical judgment. But the will is the cause of it being the final one.
22. We do not perceive by an immediate intuition that God exists, nor do we prove it a priori. But we do prove it a posteriori, i.e., from the things that have been created, following an argument from the effects to the cause: namely, from things which are moved and cannot be the adequate source of their motion, to a first unmoved mover; from the production of the things in this world by causes subordinated to one another, to a first uncaused cause; from corruptible things which equally might be or not be, to an absolutely necessary being; from things which more or less are, live, and understand, according to degrees of being, living and understanding, to that which is maximally understanding, maximally living and maximally a being; finally, from the order of all things, to a separated intellect which has ordered and organized things, and directs them to their end.
23. The metaphysical motion of the Divine Essence is correctly expressed by saying that it is identified with the exercised actuality of its own being, or that it is subsistent being itself. And this is the reason for its infinite and unlimited perfection.
24. By reason of the very purity of His being, God is distinguished from all finite beings. Hence it follows, in the first place, that the world could only have come from God by creation; secondly, that not even by way of a miracle can any finite nature be given creative power, which of itself directly attains the very being of any being; and finally, that no created agent can in any way influence the being of any effect unless it has itself been moved by the first Cause.
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