On the Twelve Abuses of the World, with extracts from On the Soul and its Origin and On the Divination of Demons against the Pagans

They often receive permission and inflict diseases, and by corrupting the very air, they render it morbid; and they persuade boys and those who love earthly advantages to commit evil deeds original: "maleficia". It becomes certain from their habits that they are offended when these things are urged upon them by these spirits. They persuade, however, in wonderful and invisible ways, through the subtlety of their bodies, by penetrating into the bodies of those who do not perceive it, and by mixing themselves with their thoughts through certain imaginary visions, whether while they are waking or sleeping.

Sometimes, they do not announce what they themselves are going to do, but they foresee the future through natural signs. They predict these things before the signs can come into the senses of men. Demons are deceived when, as they announce their own plans, something suddenly intervenes from above which disturbs all their efforts. Demons are also deceived when a physician, or a sailor, or a farmer foresees some things here through natural causes, much more acutely and much more excellently, because of the more clever sense and training of their earthly bodies, because other matters—unknown to the demons—are suddenly and unexpectedly changed by the angels who piously serve God. Demons are also deceived by the desire to deceive and by their envious will, through which they rejoice in the errors of men. But so that they do not lose the weight of authority among their own worshippers, they act in such a way that the fault is attributed to their own interpreters—the guessers of their signs—whenever they have been deceived or have lied.