John Cassian, Conferences I–X VIII.12: Of the crowd of the devils, and the disturbance which they always raise in our atmosphere

But the atmosphere which extends between heaven and earth is ever filled with a thick crowd of spirits, which do not fly about in it quietly or idly, so that most fortunately the divine providence has withdrawn them from human sight. For through fear of their attacks, or horror at the forms, into which they transform and turn themselves at will, men would either be driven out of their wits by an insufferable dread, and faint away, from inability to look on such things with bodily eyes, or else would daily grow worse and worse, and be corrupted by their constant example and by imitating them, and thus there would arise a sort of dangerous familiarity and deadly intercourse between men and the unclean powers of the air, whereas those crimes which are now committed among men, are concealed either by walls and enclosures or by distance and space, or by some shame and confusion: but if they could always look on them with open face, they would be stimulated to a greater pitch of insanity, as there would not be a single moment in which they would see them desist from their wickedness, since no bodily weariness, or occupation in business or care for their daily food (as in our case) forces them sometimes even against their will to desist from the purposes they have begun to carry out.