In the Summa Theologiae, Thomas Aquinas includes five arguments answering the question of whether we can know if God exists. In his five ways, Aquinas uses human reason to conclude what faith has already revealed to us.
Aquinas drew on the philosophy of Aristotle in his theology, using both faith and reason to give us some of the most erudite as well as understandable explanations of revealed truths.
Pope Leo XIII said in his encyclical Aeterni Patris that "reason, borne on the wings of Thomas to its human height, can scarcely rise higher, while faith could scarcely expect more or stronger aids from reason than those which she has already obtained through Thomas."
In a time when some groups claimed that the truths of faith and the truths of science were separate and incongruent, Aquinas demonstrated that the sciences reveal the one Truth in harmony with revelation.
Aquinas' five arguments for God's existence are one of his most widely known contributions. Thanks to the work of this saint, we are all given the ability to raise our reason higher in contemplation of our Creator.
Thomas Aquinas, Part II: A Legacy
Although Thomas Aquinas accorded primacy to revelation, he recognized an autonomy proper to human reason and clearly delineated the spheres of faith and reason, maintaining the importance of philosophy and the sciences, even for theology. Despite a profound influence from Neoplatonism, his philosophical thought, contained in commentaries on Aristotle and independent treatises, is basically Aristotelian, empirical, and realist, or what G. K. Chesterton called “organized common sense.”
Following Aristotle’s maxim that knowledge presupposes an essential likeness between the knower and thing known, and because human nature is both corporeal and intellectual, Thomas held that knowledge necessarily begins with sense perception (nihil in intellectu quod non prius fuerit in sensu). This conviction that valid arguments must start with facts of the natural world gave his proofs for the existence of God their characteristic form. He emphasized the notion of “scientific” knowledge based on “first principles,” which are acquired either by experience or education, and deemed Aristotelian logic basic to an understanding of all other disciplines.
Also running through Thomas’s thought is the antithesis of potency and act. He held that the idea of “primary matter” or pure potency, existing without form, was self-contradictory, while God, on the other hand, was “pure act” (actus purus), in whom every perfection was fully realized. Intermediate in the scale of being were created things, composed of potency and act. Closely related is his distinction of matter and form. While all individuals in a species have the same form, the matter is proper to each individual. Fundamental, too, is his distinction between essence and existence.
Distinct from philosophy was sacred doctrine or theology. Thomas declared this to be a science, in as much as it is an ordered body of knowledge, even though it depends for its first principles on a higher knowledge that it cannot itself test or prove, namely the knowledge which God has, in which man has some share by means of divine revelation. The fundamental truths revealed by God are the Trinity of Persons in the Godhead and the Incarnation of the Word as a Person of human nature born of the Blessed Virgin Mary.
But Thomas held that revelation was also necessary for truths, such as the existence of God and the divine attributes of creative power and providence, which reason can attain unaided, because without revelation such truths “would be known only by a few, and that after a long time, and with an admixture of error” (Summa Theol. 1. 1. 1). He treated sacred doctrine as a single discipline at a time when it was often presented in a fragmented way which obscured its connection with the practice of religion; in Thomas’s teaching theology embraces the whole life of the Church, including worship, morals, and spiritual practice. All this is illustrated in his great theological synthesis, the Summa Theologiae, whose three parts treat respectively of God and creation, of the human person as a free moral agent, and of Christ as the way of man to God.
Thomas’s most striking insight, according to Étienne Gilson, is that whereas all creatures are composed of a nature and a borrowed existence, God alone is subsistent being, the necessary being, the necessary being who cannot not be. Seeing God as subsistent being, Thomas resolved the dichotomy between immanence and transcendence, positing God’s intimate presence at the center of every creature as the cause of its being.
Perhaps his most controversial proposition was that the rational soul is the only form of the human body, which Thomas insisted on, in spite of the difficulty it caused in explaining the continuity of Christ’s dead body with His living body; one consequence is that the human person has to be treated as a composite whole, rather than as a soul using a body. Thomas identified God’s image in human beings with their capacity to know and direct their own activity.
His moral theology is theological, dealing first with happiness as the end of human existence and then with the soul’s faculties and powers, which can be directed by virtue and guided by law and grace, as a means to this end. The first principles of moral knowledge (the Natural Law) are part of human nature; the “law” of the Gospel is the grace of the Holy Spirit which transforms and elevates (but does not negate) human nature.
Thomas emphasized the role of Christ’s humanity in the Incarnation, insisting on His human emotions and acquired knowledge. More importantly, he showed that Christ’s humanity had a causal relationship to the work of redemption. As the head of redeemed humanity, Christ continues the work of redemption through the sacraments which are an extension of His humanity, mediating His fullness of grace to His members.
Thomas held that all seven sacraments were instituted by Christ, that the Eucharist was the highest of them all (sacramentum sacramentorum), and that as the ultimate purpose of the sacrament of Order was the Eucharist, the Priesthood was the highest of the orders, and the Episcopate therefore not a separate order. He explored in a highly original way the possibility of exploiting the Aristotelian philosophy of substance and accidents to develop a systematic understanding of transubstantiation. The concomitance of the Body and Blood of Christ in both Eucharistic species afforded a theological justification for Communion in one kind.
Various of his teachings were attacked before and after his death, and a formal condemnation by Étienne Tempier, Archbishop of Paris, was averted only by the intervention of the Roman Curia in 1277. Propositions condemned by the Dominican R. Kilwardby, Archbishop of Canterbury, in 1277, and by his successor, the Franciscan J. Pecham in 1284, appeared to involve his teaching, while theologians hostile to Thomas tried to implicate him in Tempier’s 1277 condemnation of a long list of propositions allegedly being taught in the Arts Faculty at Paris.
From 1278, General Chapters of the Dominican Order insisted that his writings be respected and defended within the Order. In 1323 he was canonized by John XXII, and the Parisian condemnation “in so far as it touched or seemed to touch him” was lifted in 1325. His body was translated to the Dominican church in Toulouse on January 28, 1369.
Apart from the fifteenth-century Dominican John Capreolus, who used Thomas’s thought in his critique of Nominalism, it was not until the sixteenth century that Thomism was revived by Thomas Cajetan, Sylvester of Ferrara, John of St. Thomas, and others. By the time of the Council of Trent, the Roman Catholic Church had accepted the substance of his teaching as an authentic expression of doctrine.
In 1567 Pius V declared him a Doctor of the Church and ordered the first complete printed edition of his works. After another period of eclipse, a new era for Thomism was inaugurated by Leo XIII’s bull Aeterni Patris (1879), which enjoined the study of Aquinas on all theological students as a clear, systematic philosophy capable of defending Christian tradition from contemporary attack.
In 1880 Thomas was declared patron of all Catholic universities and in 1923 his authority as a teacher was reiterated by Pius XI in Studium Ducem. In his encyclical Lumen Ecclesiae (1974), Paul VI proposed Thomas as a model for theologians not only for his doctrinal position, but also for his openness to the world of his day. The period since the Second Vatican Council has witnessed a critical reappraisal of neo-scholasticism and has opened up new areas of research, notably on the value of Thomas’s thought on moral development.
Adapted from The Oxford dictionary of the Christian Church (3rd ed. rev., pp. 1625–1628) F.L. Cross & E.A. Livingstone, Eds. (2005). Oxford; New York: Oxford University Press, courtesy of Verbum.