In Freud's assessment of the scientific achievements of Johann Weier, the sixteenth century Dutch physician equalled Copernicus and Darwin in his contributions to human thought. The author traces Weier's intellectual pursuits in an effort to understand Freud's possibly ambivalent stance: he considered Weier to be an outstanding contributor to the advancement of science yet apparently mentioned his name only once. Weier was able to give precise, detailed clinical descriptions and to investigate in a scientific, humanistic manner, dismissing artifice, bigotry, and supernatural explanations. One result was the exoneration of some suspected witches. In Weier's hands, medical knowledge replaced ignorance and magic. Weier's interest extended beyond purely physical phenomena: he acknowledged the role of wish fulfillment in fantasy, the power of suggestibility in groups, and the