Ailly, Pierre D’

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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

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