Giles (Aegidius) of Lessines

(b. Lessines [now Hainaut, Belgium], ca. 1235; d. 1304 or later)

astronomy, natural philosophy.

Giles entered the Dominican order, possibly at the priory of Valenciennes, and in all likelihood studied under Albertus Magnus at Cologne and under Thomas Aquinas at Paris during the latter’s second professorship there (1269–1272). He was one of the first to develop, not merely expound, Thomistic doctrine, particularly on the unicity of substantial form (De unitate formae, completed July 1278). His De usuris, written between 1278 and 1284, is the most complete study of usury in the Middle Ages. He composed a letter to Albert asking his judgment on fifteen points of doctrine, thirteen of which were condemned by Étienne Tempier, bishop of Paris, on 10 December 1270; this elicited a reply from Albert, the important De quindecim problematibus. Giles wrote also a number of theological treatises and De concordia temporum, a concordance of historical chronology that ends with the year 1304—from which is conjectured the date of his death.

Works of scientific interest include a lost treatise, De geometia; a work on twilights, De crepusculis; and the classic De essentia, motu et significatione cometarum (“On the Nature, Movement, and Significance of Comets”), occasioned by a comet that was seen from the latter part of July to early October 1264. The work depends heavily on Aristotle’s Meteorologia, which Giles knew in a translation made from the Greek in 1260 by his fellow Dominican, William of Moerbeke, and which shows Giles’s awareness of problems of textual criticism. Giles cites classical authors as well as al-Bitrüjī, Abu Ma’shar, Robert Grosseteste, and Albertus Magnus, among others; evidently “he had access to a remarkably extensive library”¹ and used his sources intelligently. The treatise is divided into ten chapters: the first seven, concerned with the nature, causes, and properties of comets, are astronomical and meteorological in intent; and the last three, dealing with the significance of comets, are chiefly astrological. The final chapter includes a history of comets and their sequels; apart from those of antiquity, only the comets of 840, 1062 or 1066, 1222, 1239, and 1264 are mentioned. Incidental details of the comet of 1264, based on Giles’s own observations, were used by him to falsify theories proposed by others; they were sufficiently precise to enable Richard Dunthorne, working from a manuscript of Giles’s treatise in 1751, to compute the orbit of the comet.² Both Giles and Dunthorne were assailed by “an enlightened eighteenth-century philosophe,” Alexandre Guy Pingré, whose own work is criticized by Thorndike.³

NOTES

1. Thorndike, p. 95.

2.Philosophical Transactions of the Royal Society, 47 (1753), 282

3. Pp. 97–99.

BIBLIOGRAPHY

The Latin text of Giles’s treatise on comets, together with a critical intro., is in L. Thorndike, Latin Treatises on Comets Between 1238 and 1368 A. D. (Chicago, 1950), pp. 87–184. F. J. Roensch, Early Thomistic School (Dubuque, Iowa, 1964), lists all of Giles’s works, summarizes his philosophical thought, and provides a bibliographical guide.

William A. Wallace, O.P.