AILLY, PIERRE D' or PETRUS DE ALLIACO (1350-1420), French churchman and schoolman, was born at Compiegne and studied at the College of Navarre of the University of Paris. He became master of the college in 1384 and later obtained the highest dignity in the university, the chancellorship of Notre Dame. John Gerson was one of Ailly's pupils and his lifelong friend. The two men played an important part in the later history of the papal schism. Pierre d'Ailly acted as spokesman of the university before the king's council (May 20 '1381) with the pro posal that the best way of ending the scandal of the existence of two popes, Urban VI. at Rome and Clement VII. at Avignon, was the summoning of a general council. The suggestion was unacceptable and Ailly retired from Paris for some time. As master of the College of Navarre he was twice sent by the uni versity on missions to Clement VII., and on the death of that pope, he was sent by the king to congratulate Clement's suc cessor, Benedict XIII. His acceptance of that mission displeased the university, which was already beginning to demand the resig nation of both popes as a preliminary to the re-unification of the church. Suspicion of his attitude was deepened by his nomina tion by Benedict XIII. as bishop of Le Puy (1395) and then bishop of Cambrai (1397). At the ecclesiastical council, held in Paris in 1406, Ailly, by the king's order, defended Benedict XIII., and it was not until the convocation of the general Council of Pisa (1409) that he forsook Benedict's cause and returned to the point of view of the University of Paris, of which he had been the champion before his preferment. He and Gerson urged that the unity of the church could only be restored by a general council which, in case of schism, would be supreme and legitimate even though it were not convoked by the pope. The nominee of the Council of Pisa died within a year. His successor, John XXIII., gave Ailly the bishopric of Orange and made him a cardiXxiii., gave Ailly the bishopric of Orange and made him a cardi- nal. Nevertheless Ailly was one of the most formidable adver saries of John XXIII. at the Council of Constance (Nov. 14I4 April 1418). He maintained that, as the decisions of the Council of Pisa had not been accepted and there were now three "popes," it was necessary to begin de novo and treat the claims of all three alike, and he made serious depositions against John. The theory put forward by Ailly and Gerson that the general council had jurisdiction over the pope himself was a dangerous one, and, according to later decisions, even heretical ; but both men were seeking an outlet from an impossible situation. Ailly found in the end an ingenious compromise on the rights of the Sacred College and the council and the schism was healed by the appointment of Martin V.
During the Council of Constance, Ailly had earned the hatred of the Anglo-Burgundians by trying to reduce the power of the English by contesting their right to form one of the four sepa rate nations. He was compelled to retire to Avignon, where he died on Aug. 9 142o.
Pierre d'Ailly's written works are numerous. Many of them were published with the works of Gerson (by Ellies du Pin, Ant werp, 1706) ; others appeared in the 15th century, probably at Brussels; and there are many treatises and sermons still unpub lished. In philosophy he was a nominalist.
BIBLIOGRAPHY.-P. Tschackert, Peter von Ailli (Gotha, 1877) ; L. Bibliography.-P. Tschackert, Peter von Ailli (Gotha, 1877) ; L. Salembier, Petrus de Alliaco (Lille, 1886) ; H. Denifle et Em. Chatelain, Chartularium Universitatis Parisiensis, t. (1894) ; N. Valois, La France et le grand schisme d'Occident (1896-1902) ; and Bibliot heque de l'ecole des chartes, vol. lxv., P.