Trance states, prophesying, convulsions, fasting, and other physical manifestations were often regarded as signs that a person was seized by spirits. In a book that sets out the prehistory of the early modern European witch craze, Nancy Caciola shows how medieval people decided whom to venerate as a saint infused with the spirit of God and whom to avoid as a demoniac possessed of an unclean spirit. This process of discrimination, known as the discernment of spirits, was central to the religious culture of Western Europe between 1200 and 1500.

Since the outward manifestations of...