Germanus and Cassian continue to engage the elder Serenus about the action of evil spirits. Serenus with great patience and eloquence shows them that evil spirits only have the power to incite and that we as human beings remain capable of either rejecting or accepting their suggestions. We either choose to be deceived or fail quickly to oppose them. So called "possession" is only due to the weakening of the body that comes from the acceptance and embrace of sin; much akin to the effects of wine or fever on the human person. God alone is incorporeal and has access to the deepest part of our soul. Evil spirits, however, discern from bodily gestures and from perceptible movements whether temptation or suggestion has taken hold of the heart: for example, when a person has been silent, or sighing with a certain indignation, or his face pale or blush and thus they have a subtle knowledge of who is given to what vice.
Philokalia Ministries