Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XV. The answer what devils can and what they cannot do in regard to the thoughts of men.

Chapter XV.

The answer what devils can and what they cannot do in regard to the thoughts of men.

Serenus: Nobody doubts that unclean spirits can influence the character of our thoughts, but this is by affecting them from without by sensible influences, i.e., either from our inclinations or from our words, and those likings to which they see that we are especially disposed. But they cannot possibly come near to those which have not yet come forth from the inmost recesses of the soul. And the thoughts too, which they suggest, whether they are actually or in a kind of way embraced, are discovered by them not from the nature of the soul itself, i.e., that inner inclination which lies concealed so to speak in the very marrow, but from motions and signs given by the outward man, as for example, when they suggest gluttony, if they have seen a monk raising his eyes anxiously to the window or to the sun, or inquiring eagerly what o’clock it is, they know that he has admitted the feeling of greediness. If when they suggest fornication they find him calmly submitting to the attack of lust, or see him perturbed in body, or at any rate not groaning as he ought under the wantonness of an impure suggestion, they know that the dart of lust is already fixed in his very soul. If they stir up incitements to grief, or anger, or rage, they can tell whether they have taken root in the heart by the movements of the body, and visible disturbances, when, for instance, they have noticed him either groaning silently, or panting with indignation or changing colour; and so they cunningly discover the fault to which he is given over. For they know that every one of us is enticed in a regular way by that one, to the incitement of which they see, by a sort of assenting motion of the body, that he has yielded his consent and agreement. And it is no wonder that this is discovered by those powers of the air, when we see that even clever men can often discover the state of the inner man from his mien and look and external bearing. How much more surely then can this be discovered by those who as being of a spiritual nature are certainly much more subtle and cleverer than men.