Aquinas is held in the Catholic Church to be the model teacher for those studying for the priesthood. The works for which he is best-known are the Summa Theologica and the Summa Contra Gentiles. One of the 33 Doctors of the Church, he is considered by many Catholics to be the Church's greatest theologian and philosopher.

The philosophy of Aquinas has exerted enormous influence on subsequent Christian theology, especially that of the Roman Catholic Church, extending to Western philosophy in general, where he stands as a vehicle and modifier of Aristotelianism, which he fused with the thought of Augustine. Philosophically, his most important and enduring work is the Summa Theologica, in which he expounds his systematic theology of the quinque viae.

Aquinas believed "that for the knowledge of any truth whatsoever man needs divine help, that the intellect may be moved by God to its act." However, he believed that human beings have the natural capacity to know many things without special divine revelation, even though such revelation occurs from time to time, "especially in regard to [topics of] faith.

Aquinas believed that truth is known through reason (natural revelation) and faith (supernatural revelation). Supernatural revelation has its origin in the inspiration of the Holy Spirit and is made available through the teaching of the prophets, summed up in Holy Scripture, and transmitted by the Magisterium, the sum of which is called "Tradition". Natural revelation is the truth available to all people through their human nature; certain truths all men can attain from correct human reasoning. For example, he felt this applied to rational ways to know the existence of God.

Though one may deduce the existence of God and his Attributes (One, Truth, Good, Power, Knowledge) through reason, certain specifics may be known only through special revelation (such as the Trinity). In Aquinas' view, special revelation is equivalent to the revelation of God in Jesus Christ. The major theological components of Christianity, such as the Trinity and the Incarnation, are revealed in the teachings of the Church and the Scriptures and may not otherwise be deduced.

Supernatural revelation (faith) and natural revelation (reason) are complementary rather than contradictory in nature, for they pertain to the same unity: truth.

Aquinas's ethics are based on the concept of "first principles of action." In his Summa Theologica, he wrote:
“ Virtue denotes a certain perfection of a power. Now a thing's perfection is considered chiefly in regard to its end. But the end of power is act. Wherefore power is said to be perfect, according as it is determinate to its act.”

Aquinas defined the four cardinal virtues as prudence, temperance, justice, and fortitude. The cardinal virtues are natural and revealed in nature, and they are binding on everyone. There are, however, three theological virtues: faith, hope, and charity. These are supernatural and are distinct from other virtues in their object, namely, God:
“     Now the object of the theological virtues is God Himself, Who is the last end of all, as surpassing the knowledge of our reason. On the other hand, the object of the intellectual and moral virtues is something comprehensible to human reason. Wherefore the theological virtues are specifically distinct from the moral and intellectual virtues.     ”

Furthermore, Aquinas distinguished four kinds of law: eternal, natural, human, and divine. Eternal law is the decree of God that governs all creation. Natural law is the human "participation" in the eternal law and is discovered by reason. Natural law, of course, is based on "first principles":
“     . . . this is the first precept of the law, that good is to be done and promoted, and evil is to be avoided. All other precepts of the natural law are based on this . . .    ”

The desires to live and to procreate are counted by Aquinas among those basic (natural) human values on which all human values are based. However, Aquinas was vehemently opposed to non-procreative sexual activity; not only did this lead him to view masturbation, oral sex, and even coitus interruptus, as being worse than incest and rape, but also he condemned all sexual positions other than the missionary position, on the assumption that they made conception more difficult.

Human law is positive law: the natural law applied by governments to societies. Divine law is the specially revealed law in the scriptures.

Aquinas also greatly influenced Catholic understandings of mortal and venial sins.

Aquinas denied that human beings have any duty of charity to animals because they are not persons. Otherwise, it would be unlawful to use them for food. But this does not give us license to be cruel to them, for "cruel habits might carry over into our treatment of human beings."

Aquinas contributed to economic thought as an aspect of ethics and justice. He dealt with the concept of a just price, normally its market price or a regulated price sufficient to cover seller costs of production. He argued it was immoral for sellers to raise their prices simply because buyers were in pressing need for a product.

Aquinas viewed theology, or the sacred doctrine, as a science, the raw material data of which consists of written scripture and the tradition of the Catholic Church. These sources of data were produced by the self-revelation of God to individuals and groups of people throughout history. Faith and reason, while distinct but related, are the two primary tools for processing the data of theology. Aquinas believed both were necessary — or, rather, that the confluence of both was necessary — for one to obtain true knowledge of God. Aquinas blended Greek philosophy and Christian doctrine by suggesting that rational thinking and the study of nature, like revelation, were valid ways to understand truths pertaining to God. According to Aquinas, God reveals himself through nature, so to study nature is to study God. The ultimate goals of theology, in Aquinas’ mind, are to use reason to grasp the truth about God and to experience salvation through that truth.

Aquinas believed that the existence of God is neither obvious nor unprovable. In the Summa Theologica, he considered in great detail five reasons for the existence of God. These are widely known as the quinque viae, or the "Five Ways."

Concerning the nature of God, Aquinas felt the best approach, commonly called the via negativa, is to consider what God is not. This led him to propose five statements about the divine qualities:

   1. God is simple, without composition of parts, such as body and soul, or matter and form.
   2. God is perfect, lacking nothing. That is, God is distinguished from other beings on account of God's complete actuality.
   3. God is infinite. That is, God is not finite in the ways that created beings are physically, intellectually, and emotionally limited. This infinity is to be distinguished from infinity of size and infinity of number.
   4. God is immutable, incapable of change on the levels of God's essence and character.
   5. God is one, without diversification within God's self. The unity of God is such that God's essence is the same as God's existence. In Aquinas's words, "in itself the proposition 'God exists' is necessarily true, for in it subject and predicate are the same."

In this approach, he is following, among others, the Jewish philosopher Maimonides.

Following St. Augustine of Hippo, Aquinas defines sin as "a word, deed, or desire, contrary to the eternal law." In other words, anything that disobeys God's will is said to be a sin, and is synonymous with "evil" (privation of good, or privatio boni).

Aquinas argued that God, while perfectly united, also is perfectly described by Three Interrelated Persons. These three persons (Father, Son, and Holy Spirit) are constituted by their relations within the essence of God. The Father generates the Son (or the Word) by the relation of self-awareness. This eternal generation then produces an eternal Spirit "who enjoys the divine nature as the Love of God, the Love of the Father for the Word."

This Trinity exists independently from the world. It transcends the created world, but the Trinity also decided to communicate God's self and God's goodness to human beings. This takes place through the Incarnation of the Word in the person of Jesus Christ and through the indwelling of the Holy Spirit (indeed, the very essence of the Trinity itself) within those who have experienced salvation by God.

In the Summa Theologica, Aquinas begins his discussion of Jesus Christ by recounting the biblical story of Adam and Eve and by describing the negative effects of original sin. The purpose of Christ's Incarnation was to restore human nature by removing "the contamination of sin", which humans cannot do by themselves. "Divine Wisdom judged it fitting that God should become man, so that thus one and the same person would be able both to restore man and to offer satisfaction." Aquinas argued in favor of the satisfaction view of atonement; that is, that Jesus Christ died "to satisfy for the whole human race, which was sentenced to die on account of sin."

Aquinas argued against several specific contemporary and historical theologians who held differing views about Christ. In response to Photinus, Aquinas stated that Jesus was truly divine and not simply a human being. Against Nestorius, who suggested that Son of God was merely conjoined to the man Christ, Aquinas argued that the fullness of God was an integral part of Christ's existence. However, countering Apollinaris' views, Aquinas held that Christ had a truly human (rational) soul, as well. This produced a duality of natures in Christ. Aquinas argued against Eutyches that this duality persisted after the Incarnation. Aquinas stated that these two natures existed simultaneously yet distinguishably in one real human body, unlike the teachings of Manichaeus and Valentinus.

In short, "Christ had a real body of the same nature of ours, a true rational soul, and, together with these, perfect Deity." Thus, there is both unity (in his one hypostasis) and diversity (in his two natures, human and Divine) in Christ.

In Aquinas's thought, the goal of human existence is union and eternal fellowship with God. Specifically, this goal is achieved through the beatific vision, an event in which a person experiences perfect, unending happiness by seeing the very essence of God. This vision, which occurs after death, is a gift from God given to those who have experienced salvation and redemption through Christ while living on earth.

This ultimate goal carries implications for one's present life on earth. Aquinas stated that an individual's will must be ordered toward right things, such as charity, peace, and holiness. He sees this as the way to happiness. Aquinas orders his treatment of the moral life around the idea of happiness. The relationship between will and goal is antecedent in nature "because rectitude of the will consists in being duly ordered to the last end [that is, the beatific vision]." Those who truly seek to understand and see God will necessarily love what God loves. Such love requires morality and bears fruit in everyday human choices.

Thomas Aquinas belonged to the Ordo Praedicatorum (commonly known as the Dominicans), whose original primary goal was the peaceful conversion of the Albigensian heretics. In the Summa Theologica, he wrote:

With regard to heretics two points must be observed: one, on their own side; the other, on the side of the Church. On their own side there is the sin, whereby they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death.

On the part of the Church, however, there is mercy which looks to the conversion of the wanderer, wherefore she condemns not at once, but "after the first and second admonition," as the Apostle directs: after that, if he is yet stubborn, the Church no longer hoping for his conversion, looks to the salvation of others, by excommunicating him and separating him from the Church, and furthermore delivers him to the secular tribunal... (Summa, II-II, Q.11, art.3.)

Heresy was also against the secular law in most European countries of the 13th century, therefore Aquinas' suggestion must be understood in the context of the times (hence why he specifically demands that heretics be handed to a "secular tribunal" rather than magisterial authority). Furthermore, the fact that Aquinas specifically says that heretics "deserve... death" should also be understood in a theological context, where all sinners do not deserve life ("For the wages of sin is death; but the free gift of God is eternal life in Christ Jesus our Lord"[74]) but continue to live by the grace and mercy of God. He elaborates on his opinion regarding heresy in the next article, when he says:

In God's tribunal, those who return are always received, because God is a searcher of hearts, and knows those who return in sincerity. But the Church cannot imitate God in this, for she presumes that those who relapse after being once received, are not sincere in their return; hence she does not debar them from the way of salvation, but neither does she protect them from the sentence of death. (Summa, op. cit., art.4.)
Many modern ethicists both within and outside the Catholic Church (notably Philippa Foot and Alasdair MacIntyre) have recently commented on the possible use of Aquinas's virtue ethics as a way of avoiding utilitarianism or Kantian "sense of duty" (called deontology). Through the work of twentieth century philosophers such as Elizabeth Anscombe (especially in her book Intention), Aquinas's principle of double effect specifically and his theory of intentional activity generally have been influential.

It is remarkable that Aquinas's aesthetic theories, especially the concept of claritas, deeply influenced the literary practice of modernist writer James Joyce, who used to extol Aquinas as being second only to Aristotle among Western philosophers. The influence of Aquinas's aesthetics also can be found in the works of the Italian semiotician Umberto Eco, who wrote an essay on aesthetic ideas in Aquinas (published in 1956 and republished in 1988 in a revised edition).

Thomas Aquinas. (2010, February 11). In Wikipedia, The Free Encyclopedia. Retrieved 19:13, February 13, 2010, from http://en.wikipedia.org/w/index.php?title=Thomas_Aquinas&oldid=343275649

{include creative-commons}