John XXI

John XXI d. 1277, pope (1276-77), a Portuguese named Pedro Giuliano; successor of Adrian V. Known generally as Peter of Spain (Petrus Hispanus), he is the only Portuguese pope. Peter's reputation as a scholastic philosopher was widespread, and he was the reputed author of an extensively used book on logic. Nine months after his election the ceiling of his library at Viterbo fell and killed him. Due to confusion in the medieval listing of popes named John, he called himself John XXI instead of John XX; this numbering is usually maintained. He was succeeded by Nicholas III.

John XXI (d. 1277), Pope from 1276. Peter of Spain was born in Lisbon, probably not later than c.1205. In 1273 he was elected Abp. of Braga and created Cardinal Bishop of Frascati; in this capacity he attended the Second Council of Lyons (1274). As Pope he was concerned to preserve the union with the Greek Church achieved at Lyons, and in 1277 he required the Bp. of Paris to report to him those who were teaching errors prejudicial to the faith.

Peter of Spain's Tractatus or Summulae logicales was probably composed in N. Spain in the early 1230s; it is the most influential medieval manual on logic. He also wrote other works on logic and medicine and was among the first to comment on various works of Aristotle.