Incubus visits are explained by sixteenth-century medicine

In 1569, the physician Jean Wier published a treatise explaining that incubus attacks were illusions caused by diseases and not by demons. He illustrated his point by analyzing the case of a fourteen-year-old young nun named Gertrude who believed she slept with Satan every night. According to him, vapors resulting from melancholy rose to the brain to oppress the animal spirits and disturb the sleeper's imaginative faculty.