The Summa Theologiae or Summa Theologica (transl. '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. It is a compendium of all of the main theological teachings of the Catholic Church, intended to be an instructional guide for theology students, including seminarians and the literate laity. Presenting the reasoning for almost all points of Christian theology in the West, topics of the Summa follow the following cycle: God; Creation, Man; Man's purpose; Christ; the Sacraments; and back to God.
Although unfinished, it is "one of the classics of the history of philosophy and one of the most influential works of Western literature." It remains Aquinas' "most perfect work, the fruit of his mature years, in which the thought of his whole life is condensed".
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 is a more-structured and expanded version of Aquinas's earlier Summa contra Gentiles, though the two were written for different purposes. The Summa Theologiae intended to explain the Christian faith to beginning theology students, In contrast, the Summa contra Gentiles to explains 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 also 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 fits to the instruction of beginners.