1911 Encyclopædia Britannica/John XXI.
JOHN XXI. (Pedro Giuliano-Rebulo), pope from the 8th of September 1276 to the 20th of May 1277 (should be named John XX., but there is an error in the reckoning through the insertion of an antipope), a native of Portugal, educated for the church, became archdeacon and then archbishop of Braga, and so ingratiated himself with Gregory X. at the council of Lyons (1274) that he was taken to Rome as cardinal-bishop of Frascati, and succeeded Gregory after an interregnum of twenty days. As pope he excommunicated Alphonso III. of Portugal for interfering with episcopal elections and sent legates to the Great Khan. He was devoted to secular science, and his small affection for the monks awakened the distrust of a large portion of the clergy. His life was brought to a premature close through the fall of the roof in the palace he had built at Viterbo. His successor was Nicholas III.
John XXI. has been identified since the 14th century, most probably correctly, with Petrus Hispanus, a celebrated Portuguese physician and philosopher, author of several medical works—notably the curious Liber de oculo, trans. into German and well edited by A. M. Berger (Munich, 1899), and of a popular textbook in logic, the Summulae logicales. John XXI. is constantly referred to as a magician by ignorant chroniclers.
See Les Registres de Grégoire X. et Jean XXI., published by J. Guiraud and E. Cadier in Bibliothèque des écoles françaises d’Athènes et de Rome (Paris, 1898); A. Potthast, Regesta pontif. Roman., vol. 2 (Berlin, 1875); F. Gregorovius, Rome in the Middle Ages, vol. v., trans. by Mrs G. W. Hamilton (London, 1900–1902); R. Stapper, Papst Johann XXI. (Münster, 1898); J. T. Köhler, Vollständige Nachricht von Papst Johann XXI. (Göttingen, 1760). (C. H. Ha.)