Objection 2: Further, man's reason cannot be moved except in respect of things that are offered outwardly to the senses, or are represented to the imagination: because "all our knowledge arises from the senses, and we cannot understand without a phantasm" (De Anima iii, text.30.39). Now the devil can move man's imagination, as stated above (A); and also the external senses, for Augustine says (Qq. lxxxiii, qu.12) that "this evil," of which, to wit, the devil is the cause, "extends gradually through all the approaches to the senses, it adapts itself to shapes, blends with colors, mingles with sounds, seasons every flavor." Therefore it can incline man's reason to sin of necessity.
Objection 3: Further, Augustine says (De Civ. Dei xix, 4) that "there is some sin when the flesh lusteth against the spirit." Now the devil can cause concupiscence of the flesh, even as other passions, in the way explained above (A). Therefore he can induce man to sin of necessity.
On the contrary, It is written (1 Pet.5:8): "Your adversary the devil, as a roaring lion, goeth about seeking whom he may devour." Now it would be useless to admonish thus, if it were true that man were under the necessity of succumbing to the devil. Therefore he cannot induce man to sin of necessity.
Further, it is likewise written (Jam.4:7): "Be subject . . . to God, but resist the devil, and he will fly from you," which would be said neither rightly nor truly, if the devil were able to compel us, in any way whatever, to sin; for then neither would it be possible to resist him, nor would he fly from those who do. Therefore he does not compel to sin.
I answer that, The devil, by his own power, unless he be restrained by God, can compel anyone to do an act which, in its genus, is a sin; but he cannot bring about the necessity of sinning. This is evident from the fact that man does not resist that which moves him to sin, except by his reason; the use of which the devil is able to impede altogether, by moving the imagination and the sensitive appetite; as is the case with one who is possessed. But then, the reason being thus fettered, whatever man may do, it is not imputed to him as a sin. If, however, the reason is not altogether fettered, then, in so far as it is free, it can resist sin, as stated above (Q, A). It is consequently evident that the devil can nowise compel man to sin.
Reply to Objection 2: That which is apprehended by the senses or the imagination does not move the will, of necessity, so long as man has the use of reason; nor does such an apprehension always fetter the reason.
Reply to Objection 3: The lusting of the flesh against the spirit, when the reason actually resists it, is not a sin, but is matter for the exercise of virtue. That reason does not resist, is not in the devil's power; wherefore he cannot bring about the necessity of sinning.