Thomas Aquinas (1225–1274) was the son of an Italian noble family; they planned for him to become abbot of Monte Cassino, but he joined the newly-founded mendicant Dominican Order instead. In Paris and Cologne, under his teacher Albertus Magnus, he encountered Aristotle — whose works were only then arriving in Latin via Arabic and Jewish commentaries.

He began the Summa Theologiae in Rome in 1265 as a handbook for his students. It comprises three principal parts: I (God, creation, angels, the human being), II (human action, the virtues, law, grace), III (Christ, the sacraments, last things). Every article carries Abelard's method: a question, opposing positions ('videtur quod non...'), Aquinas' own reply ('respondeo dicendum...'), and an answer to each objection. About three thousand articles in total.

Aquinas' original contribution is this: he does not pit Aristotelian reason against Christian revelation, but treats them as distinct layers — reason and revelation are in harmony, and theology takes up where philosophy cannot go. His 'five ways' to God's existence (quinque viae) — motion, cause, necessity, gradation, finality — are the visible face of that framework: natural reason, without revelation, can reach God's existence.

On 6 December 1273, during Mass, something happened (a mystical experience or a small stroke, modern interpretation still argues). After that event he put down his pen and said 'all that I have written seems to me like straw'. He died three months later. The final part of the Summa was assembled by his students from earlier notes. In 1879 Pope Leo XIII declared the work the official philosophical foundation of the Catholic Church.