Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XII. The answer how it is that unclean spirits can lord it over those possessed.

Chapter XII.

The answer how it is that unclean spirits can lord it over those possessed.

Serenus: What you speak of as taking place in the case of demoniacs is not opposed to our assertion; viz., that those possessed by unclean spirits say and do what they do not want to, and are forced to utter what they know not; for it is perfectly clear that they are not subject to the entrance of the spirits all in the same way: for some are affected by them in such a way as to have not the slightest conception of what they do and say, while others know and afterwards recollect it. But we must not imagine that this is done by the infusion of the spirit in such a way that it penetrates into the actual substance of the soul and, being as it were united to it and somehow clothed with it, utters words and sayings through the mouth of the sufferer. For we ought not to believe that this can possibly be done by them. For we can clearly see that this results from no loss of the soul but from weakness of the body, when the unclean spirit seizes on those members in which the vigour of the soul resides, and laying on them an enormous and intolerable weight overwhelms it with foulest darkness, and interferes with its intellectual powers: as we see sometimes happen also from the fault of wine and fever or excessive cold, and other indispositions affecting men from without; and it was this which the devil was forbidden to attempt to inflict on the blessed Job, though he had received power over his flesh, when the Lord commanded him saying: “Lo, I give him into thine hands: only preserve his soul,” 1470 i.e., do not weaken the seat of his soul and make him mad, and overpower the understanding and wisdom of what remains, by smothering the ruling power in his heart with your weight.

Footnotes

Job ii. 6 (LXX.).