The French scholar and cardinal Pierre d'Ailly (1350-1420) is known chiefly for his efforts in healing the Western Schism of the Church. His scientific and philosophical writings are also important.

Pierre d'Ailly was born at Compiègne. He spent most of his life in association with the University of Paris, graduating in theology from the College of Navarre in 1380 and becoming master of the college in 1384 and chancellor of the university in 1389.

One of the university's chief concerns was the Western Schism (1378-1417), in which rival popes claimed legitimacy. At first D'Ailly supported the Avignon pope Benedict XIII, but he soon became a radical leader of the Conciliar movement. The Conciliarists argued that a general council of the Church is superior to the pope and that therefore a general council could end the schism by choosing a new pope satisfactory to all parties. D'Ailly played a prominent part at the Council of Pisa (1409), which elected a new pope, Alexander V. In 1411 Alexander's successor, John XXIII, made D'Ailly a cardinal. When the rival popes refused to resign, however, the Council of Constance (1414-1418) was called. D'Ailly was an acknowledged leader and effected the decision to have the contending popes abdicate. The council then elected a new pope, Martin V, and the schism was ended. D'Ailly himself was a candidate for the papal throne, but he lost the election because of opposition from France's enemies, England and Burgundy. He retired for safety to Avignon, where he served Martin V.

Pierre d'Ailly wrote prolifically. His works on the nature of the Church had the most lasting influence. He developed the theory of conciliarism and the concept that the only infallible body in the Church is the whole of the faithful. These ideas were later shared by the Protestant reformers. He was an advocate of the calendar reform later made by Pope Gregory XII; and, like many important thinkers of his day, he took great interest in astrology, which he felt was consistent with religion. His book on geography, Imago mundi, was read carefully by Columbus, who said that it inspired his voyage of 1492 by suggesting the feasibility of sailing from Spain west to India. D'Ailly also wrote on astronomy, meteorology, mathematics, logic, metaphysics, and psychology. He died in Avignon in 1420.