By 1256 he was a master of theology. Some of his works from this period include 'On Being and Essence', 'On Truth', 'Summa Contra Gentiles', and 'Summa Theologica'. In 1272 he returned to Naples for good after traveling between cities in Italy and Paris for a number of years. He died in 1274, en route to the Second Council of Lyons.

Many of Aquinas' works were theological, though he also wrote widely on Aristotle, producing the first major commentaries of the middle ages on a Greek other than Plato. His metaphysics were at the center of his philosophy. The main focus of his writings on this topic was being. He mentions five ways that man can know the existence of God. A major thrust of his works was the idea that human knowledge can come to a position that God exists.

Perhaps the most influential of the medieval thinkers, Aquinas was able to combine Aristotilian and neo-Platonist elements of philosophy in a Christian context. Born in Italy, he studied at the University of Naples, and joined the Dominican order. He went to Cologne to study under Albertus Magnus, the first major reader of Aristotle in the medieval era, around 1248. In 1252 he went to Paris, to lecture as a bachelor of theology.