'Summary of Theology'), often referred to simply as the Summa, is the best-known work of Thomas Aquinas (1225–1274), a scholastic theologian and Doctor of the Church.

[1] It remains Aquinas's "most perfect work, the fruit of his mature years, in which the thought of his whole life is condensed".

[2] Throughout the Summa, Aquinas cites Christian, Muslim, Hebrew, and Pagan sources, including, but not limited to: Christian Sacred Scripture, Aristotle, Augustine of Hippo, Avicenna, Averroes, Al-Ghazali, Boethius, John of Damascus, Paul the Apostle, Pseudo-Dionysius, Maimonides, Anselm of Canterbury, Plato, Cicero, and John Scotus Eriugena.

The Summa Theologiae intended to explain the Christian faith to beginning theology students, whereas the Summa contra Gentiles, to explain the Christian faith and defend it in hostile situations, with arguments adapted to the intended circumstances of its use, each article refuting a certain belief or a specific heresy.

[4] The Summa Theologiae continues to be a major reference in Western and Eastern Catholic Churches, and the mainline Protestant denominations (Anglicanism, Lutheranism, Methodism, and Reformed Christianity) for those seeking ordination to the diaconate or priesthood, for professed male or female religious life, or for laypersons studying philosophy and theology at the collegiate level.

It was while teaching at the Santa Sabina studium provinciale—the forerunner of the Santa Maria sopra Minerva studium generale and College of Saint Thomas, which in the 20th century would become the Pontifical University of Saint Thomas Aquinas, Angelicum—that Aquinas began to compose the Summa.

He completed the Prima Pars ('first part') in its entirety and circulated it in Italy before departing to take up his second regency as professor at the University of Paris (1269–1272).

[5] Aquinas conceived the Summa specifically as a work suited to beginning students: Quia Catholicae veritatis doctor non solum provectos debet instruere, sed ad eum pertinet etiam incipientes erudire, secundum illud apostoli I ad Corinth.

III, tanquam parvulis in Christo, lac vobis potum dedi, non escam; propositum nostrae intentionis in hoc opere est, ea quae ad Christianam religionem pertinent, eo modo tradere, secundum quod congruit ad eruditionem incipientium Because a doctor of catholic truth ought not only to teach the proficient, but to him pertains also to instruct beginners.

As the Apostle says in 1 Corinthians 3: 1–2, as to infants in Christ, I gave you milk to drink, not meat, our proposed intention in this work is to convey those things that pertain to the Christian religion, in a way that is fitting to the instruction of beginners.

The method of exposition undertaken in the articles of the Summa is derived from Averroes, to whom Aquinas refers respectfully as "the Commentator".

The contents are as follows:[9] The Summa makes many references to certain thinkers held in great respect in Aquinas's time.

Some were called by special names: St. Thomas's greatest work was the Summa, and it is the fullest presentation of his views.

[9] What was lacking was added afterwards from the fourth book of his commentary on the Sentences of Peter Lombard as a supplementum, which is not found in manuscripts of the 13th and 14th centuries.

[9] The structure of the Summa Theologiae is meant to reflect the cyclic nature of the cosmos, in the sense of the emission and return of the Many from and to the One in Platonism, cast in terms of Christian theology: The procession of the material universe from divine essence; the culmination of creation in man; and the motion of man back towards God by way of Christ and the Sacraments.

Determinism is deeply grounded in the system of St. Thomas; things (with their source of becoming in God) are ordered from eternity as means for the realization of his end in himself.

On moral grounds, St. Thomas advocates freedom energetically; but, with his premises, he can have in mind only the psychological form of self-motivation.

Since God has only the functions of thinking and willing, only two processiones can be asserted from the Father; but these establish definite relations of the persons of the Trinity, one to another.

It is the capacity to form concepts and to abstract the mind's images (species) from the objects perceived by sense; but since what the intellect abstracts from individual things is universal, the mind knows the universal primarily and directly and knows the singular only indirectly by virtue of a certain reflexio (cf.

As certain principles are immanent in the mind for its speculative activity, so also a "special disposition of works"—or the synderesis (rudiment of conscience)—is inborn in the "practical reason", affording the idea of the moral law of nature so important in medieval ethics.

The thought is involved here by the fact that St. Thomas, like other scholastics, believed in creationism; he therefore taught that souls are created by God.

The consequence of this loss is the disorder and maiming of man's nature, which shows itself in "ignorance; malice, moral weakness, and especially in concupiscentia, which is the material principle of original sin."

Grace is a supernatural ethical character created in man by God, which comprises in itself all good, both faith and love.

A creative act of God enters, which executes itself as a spiritual motive in a psychological form corresponding to the nature of man.

Human law is not all-powerful; it cannot govern a man's conscience, nor prohibit all vices, nor can it force all men to act according to its letter, rather than its spirit.

Furthermore, it is possible that an edict can be issued without any basis in law as defined in Question 90; in this case, men are under no compulsion to act, save as it helps the common good.

The one side of the work of redemption consists herein, that Christ as head of humanity imparts ordo, perfectio, and virtus to his members.

In this way, Christ as head of humanity effects the forgiveness of their sins, their reconciliation with God, their immunity from punishment, deliverance from the devil, and the opening of heaven's gate; but inasmuch as all these benefits are already offered through the inner operation of the love of Christ, Aquinas has combined the theories of Anselm and Abelard by joining the one to the other.

St. Thomas attempts to remove the difficulty of a sensuous thing producing a creative effect, by distinguishing between the causa principalis et instrumentalis.

[14] From the 16th century, numerous commentaries on the Summa were published, notably by Peter Crockaert (d. 1514), Francisco de Vitoria and by Thomas Cajetan (1570).