In 1252 Aquinas went to Paris for the master's degree, but met with some difficulty owing to attacks on the mendicant orders by the professoriate of the University. Ultimately, however, he received the degree and entered upon his office of teaching in 1257; he taught in Paris for several years and there wrote some of his works and began others. In 1259 he was present at an important chapter of his order at Valenciennes. At the solicitation of Pope Urban IV (therefore not before the latter part of 1261), he took up his residence in Rome. In 1269-71 he was again active in Paris. In 1272 the provincial chapter at Florence empowered him to found a new ''studium generale'' at such place as he should choose, and he selected Naples.
Aquinas had a mystical experience while celebrating Mass on December 6, 1273, after which he stopped writing, leaving his great work, the ''Summa Theologiae'', unfinished. When asked why he had stopped writing, Aquinas replied, "I cannot go on...All that I have written seems to me like so much straw compared to what I have seen and what has been revealed to me." He died on March 7, 1274.
Contemporaries described Thomas as a big man, corpulent and dark-complexioned, with a large head and receding hairline. His manners showed his breeding; he is described as refined, affable, and lovable. In argument he maintained self-control and won over opponents by his personality and great learning. His tastes were simple. His associates were specially impressed by his power of memory. When absorbed in thought, he often forgot his surroundings. The ideas he developed by such strenuous absorption he was able to express for others systematically, clearly and simply. Because of the keen grasp he had of his materials, in his writings Thomas does not, like Duns Scotus, make the reader his associate in the search for truth, but teaches it authoritatively. On the other hand, the consciousness of the insufficiency of his works in view of the revelation which he believed he had received was a cause of dissatisfaction for him.
===Death and canonization===
Early in 1274 the Pope directed him to attend the Second Council of Lyons and, though far from well, he undertook the journey. On the way he stopped at the castle of a niece and there became seriously ill. He wished to end his days in a monastery and not being able to reach a house of the Dominicans he was taken to the Cistercians. He died at the monastery of Fossanova, one mile from Sonnino, on March 7, 1274.
Aquinas had made a remarkable impression on all who knew him. He was placed on a level with the [[Apostle Paul]] and [[Augustine of Hippo|Augustine]], receiving the title ''doctor angelicus'' ('''''Angelic Doctor''''').