D'Ailly was born in Compiègne in 1350 or 1351 of a prosperous bourgeois family. D'Ailly studied in Paris, receiving the licentiate in arts in 1367 and the masters a year later. He was active in university affairs by 1372. D'Ailly taught the Bible in 1375 and the Sentences of Peter Lombard in 1376–1377. D'Ailly received the licentiate and doctorate in theology in 1381. He was affiliated with the Collège de Navarre, University of Paris, serving as rector in 1384, At the College, he taught Jean Gerson and Nicholas of Clémanges.
D'Ailly rose to prominence by leading the university's effort to secure removal of John Blanchard as chancellor. Blanchard was accused by d'Ailly before the Avignon antipope Clement VII of abuse of office. D'Ailly served as chancellor of the University from 1389 to 1395, and Gerson succeeded him. Both were involved in expelling the Dominican Order from the university for refusing to embrace the idea of the Virgin Mary's Immaculate Conception and in the effort to end the Great Schism by means of an ecumenical council. D'Ailly, although slow at first to embrace the conciliar solution to the Schism, participated in both the Council of Pisa (1409) and the Council of Constance (1414–1418). With Gerson, d'Ailly was one of the leading theologians at the Council of Constance.
D'Ailly's ecclesiastical career prospered. He served as bishop of Le Puy (1395), bishop of Noyon, and bishop of Cambrai (1397). The antipope John XXIII made him a cardinal in 1411. He became known as the Cardinal of Cambrai. D'Ailly also was a royal chaplain and almoner. This acceptability to many interests helps explain his advancement.
D'Ailly wrote extensively on the Schism, reform, astrology and other topics. His ideas on the powers of the college of cardinals and the infallibility of the general council were very influential. D'Ailly's Imago Mundi (1410), a work of cosmography, influenced Christopher Columbus in his estimates of the size of world land-mass. His views on astrology, expressed in several works, attempted to balance divine omniscience and human free will. D'Ailly's writings on the Schism put the crisis and the need for reform into an apocalyptic context. His astrology also was tied to the Schism, attempting to determine whether the division of the church was a sign of the coming of the Antichrist.
After the council, d'Ailly returned to Paris. When in France's civil discord the Burgundian faction seized Paris in 1419, killing some professors in the process, he fled south and retired to Avignon. His former pupil Gerson settled nearby at a house of the Celestine Order. D'Ailly, known as the Cardinal of Cambrai, died in 1420. His works began appearing in print before the end of the fifteenth century.
The crater Aliacensis on the Moon is named after him.
- Pascoe, p. 8.
- Smoller, p. 7.
- Bernstein, pp. 60-176.
- Stump, p. 8.
- "Ailly, Pierre". New International Encyclopedia. 1905.
- Smoller, p. 7.
- Francis Oakley, "Pierre d'Ailly and Papal Infallibility," Mediaeval Studies 26 (1964), 353-358.
- See the chronology of these works in Smoller, pp. 136-137.
- Pascoe, pp. 11-51.
- Smoller, pp. 85-86.
- Guenée, pp. 252-253.
- Smoller, pp. 133-134.
Further reading 
- Alan E. Bernstein, Pierre d'Ailly and the Blanchard affair : University and Chancellor of Paris at the beginning of the Great Schism. Leiden: Brill, 1978.
- Bernard Guenée, Between Church and State: The Lives of Four French Prelates in the Late Middle Ages, Translated by Arthur Goldhammer, Chicago: University of Chicago Press, 1991.
- Francis Oakley, Political Thought of Pierre d’Ailly: The Voluntarist Tradition (New Haven: Yale University Press, 1964)
- Louis B. Pascoe, Church and Reform: Bishops, Theologians, and Canon Lawyers in the Thought of Pierre d'Ailly (1351-1420). Leiden: Brill, 2005.
- Laura A. Smoller, History, Prophecy, and the Stars: The Christian Astrology of Pierre D'Ailly, 1350-1420. Princeton University Press (Princeton, NJ; 1994) ISBN 0-691-08788-1.
- Philip H. Stump, The Reforms of the Council of Constance (1414-1418), Leiden: Brill, 1994. ISBN 90-04-09930-1
- Pierre D'Ailly Imago Mundi
See also