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, later dean of Lisbon (1261), archbishop of Braga (1273), physician to Gregory X, and promoted by him to cardinal bishop of Tusculum (1273).
After a tumultuous conclave, he was elected at Viterbo as successor to Adrian V and crowned on 20 September. Soon after ascending the pontificate, he punished the authors of the disturbances; but he confirmed a decree of Adrian suspending the constitution of Gregory X for the speed 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, John, however, displayed considerable activity in the few months of his pontificate. Following the path already taken by Gregory X, he sought to reconcile Rudolph of Habsburg and Charles of Anjou to make possible the coronation of Rudolph and the peace of Italy, but at the same time wished to prevent the excessive power of Charles and demanded from Rudolph a promise not to infringe 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 waged war. He wished to unite all Christian princes for the Crusade and collected tithes for the enterprise; he also thought of an alliance with the Tartar sovereigns, who had sent him an embassy. He worked to make effective the union between the Greek and Roman Churches, decided at the Council of Lyons. He ordered Stephen Tempier, bishop of Paris, to investigate the new errors spreading in that university (18 January 1277); and when the bishop had pronounced the condemnation of Averroism and the theory of double truth (7 March 1277), he ordered him to proceed with the purging of errors in both arts and theology (28 April 1277). The collapse of a floor in the papal palace at Viterbo cut short a pontificate that promised to be beneficial to the Church and to world peace.
More than for the works of his brief pontificate, "Peter the Spaniard" was renowned for his scientific and philosophical learning, for 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 medical works, following the Greco-Arab tradition; among them a De oculo and a Thesaurus pauperum, seu de medendi humani corporis membris, in which, with a sense of Christian charity toward those without means, he indicates how to cure every ailment. He wrote a treatise De anima and commented on Aristotle’s De animalibus in the translation by Michael Scot. But, as Dante already says, he "shines" above all for the "sweet little books" of the Summulae logicales, a manual of dialectic, for which he drew on the Summa of Lambert of Auxerre and the work of Michael Psellus. Based on Aristotelian-Boethian logic, he expounds all the forms of dialectical disputation and adds the contributions of what was called the logica modernorum, dealing with the proprietatibus terminorum, i.e., the properties of logical terms and their relations to grammatical terms. The work enjoyed enormous diffusion even in manuscript form; printed for the first time at Alost in 1474, it had many subsequent editions (Prantl mentions no fewer than 84); it was also translated into Greek.