Pierre d Ailly

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Ailly, Pierre D’

also known as Petrus de Alliaco

(b. Compiègne, France, 1350; d. Avignon, France, 1420)

theology, cosmography.

D’Ailly studied at the College of Navarre of the University of Paris, where he received his doctorate of theology in 1381. He was grand master of Navarre from 1384 to 1389, and from 1389 to 1395 he was chancellor of the University of Paris. In 1395 d’Ailly became bishop of LePuy, and in 1397 bishop of Cambrai. He was made a cardinal in 1411. D’Ailly wrote commentaries on Aristotle (the De anima and the Meteorologica), as well as a number of astronomical and astrological works, including a commentary on the De sphaera of Sacrobosco. In his treatises on astrology, he reflects a more lenient attitude than that of either Nicole Oresme or Henry of Hesse. He was also concerned with the problem of calendar reform, and wrote a work on this subject for the Council of Constance (1414).

His most significant scientific work is a collection of cosmographical and astronomical treatises with the collective title Imago mundi. The Imago includes sixteen treatises on geography and astronomy, and the concordance of astrology, astronomy, and theology with historical events; only the first of these is the Imago mundi properly speaking. In the geographical portion of the Imago (the first treatise), d’Ailly makes use of the newly translated Geography of Ptolemy. It was thought that d’Ailly’s work caused Columbus to underestimate his distance from the supposed coast of Asia, but it is now known that Columbus did not read the Imago until after his first voyage.

In his philosophical and scientific outlook, d’Ailly is considered a nominalist; however, his scientific writing shows little originality and much unacknowledged borrowing. He has a more significant claim to historical prominence as a leader of the conciliar movement.


I. Original Works. There is a modern edition of d’Ailly’s Imago mundi in Latin-French, edited by Edmond Buron (Paris, 1930). This is only the first of the sixteen treatises in the medieval Imago mundi.

II. Secondary Literature. Louis Selembier, Pierre d’Ailly (Tourcoing, 1931), a French version of a Latin dissertation, Petrus de Alliaco (1886). There is a list of secondary material on d’Ailly following the entry on Pierre d’Ailly in the Lexikon für Theologie und Kirche, VIII (Freiburg, 1963), 330; also in Francis Oakley, The Political Thought of Pierre d’Ailly (New Haven-London, 1964), bibliographical note, pp. 350–356. Other works that discuss d’Ailly are Pierre Duhem, Le système du monde, IV (Paris, 1953), 168–183; George H. T. Kimble, Geopgraphy in the Middle Ages (London, 1938), pp. 92, 208–212, 218, n. 4; Lynn Thorndike, A History of Magic and Experimental Science, IV (New York, 1934), 101–113, 322, and The Sphere of Sacrobosco and Its Commentators (Chicago, 1949), pp. 38–40, 49–51.

Claudia Kren

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Pierre d' Ailly (pyĕr dāyē´), 1350–1420, French theologian and writer, cardinal of the Roman Catholic Church. He was the teacher of John Gerson and was Gerson's predecessor as chancellor of the Univ. of Paris (1385–95). Ailly figured prominently among the conciliarists working to end the Great Schism (see Schism, Great). He urged that an ecumenical council be called to name a new pope as the only means of settling the schism. He seems to have been more concerned with a practical solution than with the implications of the conciliar theory. He participated in both the Council of Pisa (see Pisa, Council of) and the Council of Constance (see Constance, Council of). At Constance Ailly took part in the trial and condemnation of John Hus. His vast writings embrace theology, philosophy, cosmography, plans for ecclesiastical reform, and French religious verse. One of his works, the Imago mundi, an astronomical compendium, was studied by Columbus.

See studies by J. P. McGowan (1936) and F. Oakley (1964).