French theologian. He taught at Paris and in 1397 he became Bp. of Cambrai. His main concern was to find a means of ending the Great Schism. He attended the Council of Pisa, where he supported the newly elected Alexander V; he was made a cardinal by his successor, John XXIII. From 1414 to 1418 he attended the Council of Constance, where he upheld the Conciliar theory, without, however, entirely approving the ‘Decrees of Constance’. In 1416 he published his influential Tractatus super Reformatione Ecclesiae.
In his doctrinal teaching d'Ailly usually accepted the views of William of Ockham. He held that the existence of God was not a rationally demonstrable truth, and that sin was not inherently evil but sinful only because God wills it to be so. He maintained that bishops and priests received their jurisdiction directly from Christ and not mediately through the Pope, and that neither Pope nor Council was infallible. His views were developed by the Reformers and influenced Gallicanism.