True or False Possession?: How to Distinguish the Demonic from the Demented

Unhinged or unholy? Fiend or fraud? That’s what authorities had to decide about the French nun Marie-Thérèse Noblet (1889–1930). She suffered sudden diseases that were as quickly cured, chokings, night beatings, unclean visions of blasphemous scenes, violent shakes witnessed by onlookers, foul assaults from filthy beasts, including one she recalled as “full of terrible beauty with eyes full of hate.”

Then there’s Sr. Jeanne of the Angels, the seventeenth-century prioress of her Ursuline conv...