In 1563 Dutch physician and demonologist Johann Weyer published in Basel at the press of Johannes Oporinus De praestigiis daemonum, et incantationibus ac veneficiis, libri V. In this celebrated exposé of the witchcraft delusion Weyer presented one of the first scientific approaches to the study of mental illness. Defying the authorities of the Inquisition and the doctrines of the Malleus maleficarum (noticed in this database), Weyer asserted the most witches were actually suffering from mental illness. He backed his claim with careful descriptions of a number of case histories from his own clinical experience, containing some of the earliest references to purely psychological treatment. To emphasize the superstitious ignorance of doctors who adhered to demonological theory, Weyer analyzed the effects of the stupefying and hallucinatory drugs used in sixteenth-century medicine, attributing many aspects of witchcraft to their effects. He recognized the relationship between a highly suggestible temperament and mental instability, and described the phenomenon of mass contagion of mental illness.
Like many innovators during the sixteenth century Weyer held positions relative to witchcraft and demonology that were both traditional and new.
"While he defended the idea that the Devil's power was not as strong as claimed by the Christian church in De Praestigiis Daemonum, he defended also the idea that demons did have power and could appear before people who called upon them, creating illusions; but he commonly referred to magicians and not to witches when speaking about people who could create illusions, saying they were heretics who were using the Devil's power to do it, and when speaking on witches, he used the term mentally ill" (Wikipedia article on Johann Weyer, accessed 02-28-2009).
Weyer "was the first clinical and the first descriptive psychiatrist to leave to succeeding generations a heritage which was accepted, developed, and perfected into an observational branch of medicine. . . . He reduced the clinical problems of psychopathology to simple terms of everyday life and of everyday human, inner experience" (Zilboorg & Henry, A History of Medical Psychology  228).
Hook & Norman, The Haskell F. Norman Library of Science and Medicine (1991) no. 2209.