quality:corruption, structural:summarization, structural:scholar_edition, information:extraction
Late career and cessation of writing (1272–1274)

In 1272, Thomas took leave from the University of Paris when the Dominicans from his home province called upon him to establish a studium generale wherever he liked and staff it as he pleased. He chose to establish the institution in Naples and moved there to take his post as regent master. He took his time at Naples to work on the third part of the Summa while giving lectures on various religious topics. He also preached to the people of Naples every day in Lent of 1273. These sermons on the Commandments, the Creed, the Our Father, and Hail Mary were very popular.

Thomas has been traditionally ascribed with the ability to levitate and as having had various mystical experiences. For example, G. K. Chesterton wrote that "His experiences included well-attested cases of levitation in ecstasy; and the Blessed Virgin appeared to him, comforting him with the welcome news that he would never be a Bishop." It is traditionally held that on one occasion, in 1273, at the Dominican convent of Naples in the chapel of Saint Nicholas, after Matins, Thomas lingered and was seen by the sacristan Domenic of Caserta to be levitating in prayer with tears before an icon of the crucified Christ. Christ said to Thomas, "You have written well of me, Thomas. What reward would you have for your labor?" Thomas responded, "Nothing but you, Lord."

On 6 December 1273, another mystical experience took place. While Thomas was celebrating Mass, he experienced an unusually long ecstasy. Because of what he saw, he abandoned his routine and refused to dictate to his socius Reginald of Piperno. When Reginald begged him to get back to work, Thomas replied: "Reginald, I cannot, because all that I have written seems like straw to me" (mihi videtur ut palea). As a result, the Summa Theologica would remain uncompleted. What exactly triggered Thomas's change in behaviour is believed by some to have been some kind of supernatural experience of God. After taking to his bed, however, he did recover some strength.

In 1274, Pope Gregory X summoned Thomas to attend the Second Council of Lyon. The council was to open 1 May 1274, and it was Gregory's attempt to try to heal the Great Schism of 1054, which had divided the Catholic Church in the West from the Eastern Orthodox Church. At the meeting, Thomas's work for Pope Urban IV concerning the Greeks, Contra errores graecorum, was to be presented. However, on his way to the council, riding on a donkey along the Appian Way, he struck his head on the branch of a fallen tree and became seriously ill again. He was then quickly escorted to Monte Cassino to convalesce. After resting for a while, he set out again but stopped at the Cistercian Fossanova Abbey after again falling ill. The monks nursed him for several days, and as he received his last rites he prayed: "I have written and taught much about this very holy Body, and about the other sacraments in the faith of Christ, and about the Holy Roman Church, to whose correction I expose and submit everything I have written." He died on 7 March 1274 while giving commentary on the Song of Songs.