JOHN XXI, Pope. - Born in Lisbon, probably between 1210 and 1220, elected on 8 September 1276, died on 20 May 1277. Peter of Julian, a Portuguese, had studied in Paris and taught medicine in Siena between 1247 and 1252; he had then been among the familiars of Cardinal Ottobono de' Fieschi, the future Adrian V, then dean of Lisbon (1261), archbishop of Braga (1273), archiater of Gregory X and by him promoted cardinal bishop of Tusculum (1273).
After a tumultuous Conclave, he was elected at Viterbo as successor of Adrian V and crowned on 20 September. As soon as he ascended the pontificate, he punished the authors of the disturbances; but he confirmed a decree of Adrian, which suspended the constitution of Gregory X concerning the promptness of the papal election, proposing to replace it with new norms (30 September 1276). A man of study rather than of government, dominated by Cardinal Orsini, who was later Nicholas III, G. nevertheless, in the few months of his pontificate, displayed considerable activity. Following the path already taken by Gregory X, he sought to reconcile Rudolf of Habsburg and Charles of Anjou in order to make possible the coronation of Rudolf and the peace of Italy, but he wished at the same time to prevent the excessive power of Charles and asked Rudolf for the promise not to encroach upon the dominion of the Church in Romagna. He sent two friars to reconcile Alfonso X of Castile and Philip III of France, threatening them with excommunication if they moved war. He wished to unite all the Christian princes for the Crusade and collected tithes for the enterprise; he also thought of a league with the Tatar sovereigns, who had sent him an embassy. He strove to make effective the union between the Greek and the Roman Church, deliberated at the Council of Lyon. He ordered Stephen Tempier, bishop of Paris, to investigate the new errors spread in that University (18 January 1277); and, when the bishop had pronounced the condemnation of Averroism and of the theory of double truth (7 March 1277), he ordered him to proceed with the purging of the masters of arts as well as of theology (28 April 1277). The collapse of a floor in the papal Palace of Viterbo interrupted a pontificate that gave promise of being beneficial to the Church and to the peace of the world.
More than for the works of his brief pontificate, "Peter the Spaniard" was famed for his scientific and philosophical doctrine, on account of which Fra Salimbene calls him "magnus sophista, loycus et disputator atque theologus" and Dante places him in the second "garland" of theologians in the heaven of the Sun (Par., XII, 134-35). He composed books of medicine, following the Greek-Arab tradition; among them a De oculo and a Thesaurus pauperum, seu de medendis humani corporis membris, where, with a sense of Christian piety for those who lack means of fortune, the manner of curing every ailment is indicated. He wrote a treatise De anima and commented on Aristotle's De animalibus in the version of Michael Scot. But, already according to Dante, he "shines" above all for the "twelve little books" of the Summulae logicales, a manual of dialectic, for which he made use of the Summa of Lambert of Auxerre and of the work of Michael Psellos. On the basis of Aristotelian-Boethian logic, he sets forth all the forms of dialectical disputation and adds to them the contributions of what was called logica modernorum, dealing de proprietatibus terminorum, that is, with the properties of logical terms and their relations to grammatical terms. The work had enormous diffusion already in manuscript; printed for the first time at Alost in 1474, it had very numerous other editions (Prantl records as many as 84); it was also translated into Greek.