How Unclean Spirits Are Joined to Human Souls

It is no wonder that spirit can be joined to spirit unnoticed, and can exercise an unseen power of persuasion toward whatever it is allowed. For between them, just as between men, there is a certain likeness and kinship of substance, since the description given of the nature of the soul applies equally well to their substance. But it is impossible for spirits to be planted inside spirits, or united with them in such a way that one can contain the other. That is truly the prerogative of God alone, the only nature that is simple and without a body.