Pierre D Ailly, or Petrus de Alliaeo, a French prelate and theologian, surnamed the Hammer of Heretics and the Eagle of the Doctors of France, born in 1350, died in 1420 or 1425. He was distinguished as a preacher and philosophical disputant (being a leader of the nominalists), and early became a doctor of the Sor-bonne, in 1384 grand master of the college of Navarre, in 1389 chancellor of the university, and in 1398 bishop of Cambrai; he was also almoner and confessor to Charles VI. His exertions led to the calling of the council of Pisa in 1409, for the healing of the papal schism, and he was one of its most active members. He was made cardinal by John XXIII. and sent as papal legate to Germany. In this capacity he took a prominent part in the council of Constance, 1414 - '18, where he promoted the condemnation of John Huss and Jerome of Prague, but zealously advocated a reform in the church, maintained the superiority of councils to the popes, and aided in the election of Martin V. in place of the three rival popes. He was afterward papal legate at Avignon till his death.

His published writings are numerous, including Concordantia Astronomice cum Theologia, etc, written in accordance with the astrological views of the age.