This is useful for understanding the relation of body and soul in Aquinas for several reasons. First, the spiritual governs the corporeal, so temptation comes through the body. Second, the movements of appetite make perception more acute, or the body prepares the soul’s cognition. Third, imagination produces appetite, or the soul prepares the body, at least in part.
Summa Theologica, I-II.80.2
Hence, the entirety of the devil’s interior operation seems to focus on the imagination and sentient appetite.
HOW HE TEMPTS THE TWO FACULTIES
By moving the two of them, he is able to induce a man to sin, since he can operate in such a way as
to present certain imagined forms to the imagination, and he can likewise
bring it about that the sentient appetite is excited with some passion.
For it was explained in the First Part (ST 1, q. 110, a. 3) that a corporeal nature naturally obeys a spiritual nature with respect to local motion.
PRINCIPLE APPLIED TO DEVIL
Hence, the devil, too, is able to cause everything that can arise from the local motion of lower bodies, unless he is held back by God’s power.
(1) IMAGINATION THROUGH MOTION
Now the fact that forms are represented to the imagination follows sometimes from local motion.
For in De Somno et Vigilia the Philosopher says, “When an animal sleeps and the blood descends to the sentient principle in a large quantity, the movements or impressions that are left by sensible motions and that are conserved in the sensible species descend at the same time and move the apprehensive principle in such a way that the impressions appear in the way they would if the sentient principle were at that time being affected by the exterior things themselves.”
Hence, this sort of local motion on the part of the humors or [animal] spirits can be procured by demons whether the man in question is asleep or awake, and in this way it follows that the man imagines certain things.
(2) SENSE APPETITE THROUGH MOTIONS
Similarly, the sentient appetite is likewise excited toward certain passions in accord with certain determinate movements of the heart and [animal] spirits; hence, the devil can likewise operate to effect this.
(a) PASSIONS PRODUCE MORE ACCUTE PERCEPTION
And from the fact that certain passions are excited in the sentient appetite, it follows that the man perceives more acutely the sensible movement or tendency that is traced back, in the way just explained, to the apprehensive principle;
for as the Philosopher says in the same book, “Lovers are moved to the apprehension of the beloved even by a slight similarity.”
(b) PASSIONS PRODUCE A GREATER ACTION TENDENCY WHEN COUPLED WITH IMAGINATION
Again, because the passion is excited, it happens that what is proposed to the imagination is judged to be something that should be pursued;
for someone who is in the grips of a passion (ei qui a passione detinetur) is such that what he is inclined toward by the passion seems good to him. And it is in this way that the devil induces a man interiorly to sin.