The magicians and astrologers who frequented the courts of Burgundy and France during the reign of Charles VI to render their dubious services to king and nobles, induced friar Laurens Pignon OP to write a treatise called Contre les devineurs (1411) which he dedicated to John the Fearless, duke of Burgundy. This book presents a critical edition of the treatise and tries to reconstruct its historical and intellectual context by examining the role of magic and astrology at court. By means of theological and philosophical arguments which he derives from Aquinas, Pignon demonstrates the dangers and deficiencies of divination. In three appendices editions of supplementary documents are supplied: a confession of a court-magician, two divinatory texts and a fictional prognostication on the house of Burgundy.