Summa Theologiae

The Summa Theologiae is a theological treatise written by Thomas Aquinas in the last years of his life, roughly between 1265 and 1274. Conceived as a teaching manual for beginning students, including seminarians and educated laypeople, rather than as a polemic aimed at non-Catholics, it sets out in an orderly way much of the fundamental teaching of the Catholic Church. Aquinas began it while teaching at the Santa Sabina studium in Rome, a forerunner of the institutions that later gave rise to the Pontifical University of Saint Thomas Aquinas.

The work is divided into three parts, the second of which is further subdivided into two. The first treats of God, creation, angels and man; the second, of humanity's ultimate end, human acts, the passions, the virtues, vices, law and grace; the third, of Christ and the sacraments. It was left unfinished: Aquinas stopped writing after an experience he described as a revelation, and the supplement that closes the third part was added afterwards by his disciples from earlier writings. Its method, with questions divided into articles, is to state objections, cite an authority, give Aquinas's own answer and finally reply to each objection. Among its sources are Scripture, the Church Fathers, Aristotle, whom he calls simply "the Philosopher", Averroes, referred to as "the Commentator", and other Islamic and Jewish thinkers.

The work is regarded as one of the classics of the history of philosophy and one of the most influential works of Western literature. It remains a standard reference in Catholic theological education and in other Christian traditions, and it strongly shaped Dante's Divine Comedy, which has sometimes been called the "Summa in verse".