St. Thomas's student nickname was the Dumb Ox, because he was slow in manner and quite stout. He was, however, a brilliant lecturer and a clear, sharp thinker, as his works show—not only in their rigid application of reason, but also in their Latin diction, which is admirably exact and simple. His spiritual character is manifest in the humility and charity of his conduct and the use to which he put his theories in his devotional works, notably in the Mass and office for the feast of Corpus Christi (June 21), which he wrote at Urban IV's request (1264). The four hymns of this Mass and office, Laude Sion Salvatorem, Pange Lingua, Sacris solemniis, and Verbum supernum (ending with O Salutaris Hostia ), are classed among the greatest of Christian hymns.
No single work of St. Thomas can be said fully to reveal his philosophy. His works may be classified according to their form and purpose. The principal ones are Commentary in the Sentences (a series of public lectures; 1254–56), his earliest great work; seven quaestiones disputatae (public debates; 1256–72); philosophical commentaries on Aristotle's Physics, Metaphysics, De anima, Ethics, part of the De interpretatione, and the Posterior Analytics; treatises on many subjects, including the Summa contra Gentiles (1258–60); and, most important of all, Summa theologica (1267–73), an incomplete but systematic exposition of theology on philosophical principles. St. Thomas's philosophy is avowedly Aristotelian; the methods and distinctions of Aristotle are adapted to revelation.
The 13th cent. was a critical period in Christian thought, which was torn between the claims of the Averroists and Augustinians. Thomas opposed both schools, the Averroists led by Siger de Brabant, who would separate faith and truth absolutely, and the Augustinians, who would make truth a matter of faith. St. Thomas held that reason and faith constitute two harmonious realms in which the truths of faith complement those of reason; both are gifts of God, but reason has an autonomy of its own. Thus he vindicated Aristotle against those who saw him as the inspiration of Averroës and heresy.
The first principle of philosophy according to St. Thomas is the affirmation of being. From this he proceeded to a consideration of the manner in which the intellect achieves knowledge. For humans all knowledge begins by way of the senses, which are the medium through which he grasps the intelligible world, the universal. According to the position of Thomas, which is known as moderate realism, the form or the universal may be said to exist in three ways: in God, in things, and in the mind (see universals). He argues that it is by the knowledge of things that we come to know of God's existence. In the natural order what God is can be known only by analogy and negation.
Thomas's conviction that the existence of God can be discovered by reason is shown by his proofs of the existence of God. His metaphysics relies on the Aristotelian concepts of potency and act, matter and form, being and essence. A thing that requires completion by another is said to be in potency to that other; the realization of potency is called actuality. The universe is conceived of as a series of things arranged in an ascending order of potency, an act at once crowned and created by God, who alone is pure act. Two other pairs of metaphysical concepts—matter and form, essence and being—are special cases of potency and act. St. Thomas's moral philosophy is derived from these distinctions as well, since the opposite of being does not exist and since the good is identical with being, evil is but the absence of good.