The Summa Theologiae (written 1265–1274 and also known as the Summa Theologica or simply the Summa) is the best-known work of Thomas Aquinas (c. 1225–1274). Although unfinished, the Summa is "one of the classics of the history of philosophy and one of the most influential works of Western literature." It was intended as an instructional guide for theology students, including seminarians and the literate laity. It was a compendium of all of the main theological teachings of the Catholic Church. It presents the reasoning for almost all points of Christian theology in the West. The Summa's topics follow a cycle: the existence of God; Creation, Man; Man's purpose; Christ; the Sacraments; and back to God.
Among non-scholars, the Summa is perhaps most famous for its five arguments for the existence of God, which are known as the "five ways" (Latin: quinque viae). The five ways, however, occupy under two pages of the Summa's approximately 3,500 pages.
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, Dionysius the Areopagite, Maimonides, Anselm, Plato, Cicero, and Eriugena.
The Summa is a more structured and expanded version of Aquinas's earlier Summa contra Gentiles, though these works were written for different purposes, the Summa Theologiae to explain the Christian faith to beginning theology students, and 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.
Aquinas conceived the Summa specifically as a work suited to beginning students: "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."
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).