Summa Theologica (originally Summa Theologiae) is the principal work of Saint Thomas Aquinas (1225-1274), an Italian friar, philosopher, and theologian and one of the central figures in medieval Christian thought. Aquinas wrote the Summa between 1265 and 1273, intending it to serve as a summation of all known theological learning for seminarians. He never finished the massive Latin work, but what he completed has influenced Roman Catholic theology and Western thought in general.

Aquinas’ central idea was that reason and faith, or reason and revelation, complement each other. He sought to reconcile Christian teachings with the insights of classical Greek philosophers, especially Aristotle (384-322 B.C.), whom Western Europe was rediscovering during Aquinas’ lifetime. Aquinas drew from Jewish, Islamic, and pagan Greek sources, as well as the Christian Bible and the Church Fathers to form his arguments for the relationship between reason and revelation. Although the Summa is a theological work, it is deeply grounded in philosophical concepts and arguments. Aquinas frequently cites the works of “The Philosopher” (Aristotle), as well as such authorities as the Arab philosopher Averroes and the Jewish rabbi and philosopher Moses Maimonides.

The Summa is organized into three Parts. The First Part addresses God, gradually working its way through God’s creation and the angels to man. The Second Part deals with man in greater depth, and the Third Part discusses Jesus Christ, who serves as mediator between God and man in Christian thought.

The Parts subdivide into Treatises, Questions, and Articles—reflecting Aquinas’ distinctive method of philosophical argument now known as Thomism or the Scholastic method: He states the case for the opposing position, then refutes the points one by one while expounding his own point of view, citing relevant authorities. Some passages of the Summa have become particularly well-known and widely studied, such as the section on the five ways to demonstrate God’s existence (Part 1, Question 2). This guide will cover the First Part and the first two treatises of the Second Part of the Summa, in the translation by the English Dominican Fathers.