Thomas Aquinas

“ It also happens, through the rousing of a passion, that what is put before the imagination, is judged, as being something to be pursued, because, to him who is held by a passion, whatever the passion inclines him to, seems good. In this way the devil induces man inwardly to sin. ”

“ Now no vital functions can be exercised except by an intrinsic principle, not even those of the vegetal soul, which are the lowest of vital functions. Therefore the devil cannot instigate man to evil through his internal movements. ”

“ Thirdly, he that persuades the will that the object proposed has an aspect of good, because he also, in a fashion, offers the will its proper object, which is a real or apparent good of reason. Accordingly, in the first way the sensible things, which approach from without, move a man's will to sin. In the second and third ways, either the devil or a man may incite to sin, either by offering an object of appetite to the senses, or by persuading the reason. ”

“ Consequently neither the thing offered from without, nor he that proposes it, nor he that persuades, is the sufficient cause of sin. Therefore it follows that the devil is a cause of sin, neither directly nor sufficiently, but only by persuasion, or by proposing the object of appetite. ”

“ He is not, however, the direct cause of all the sins of men, as though each were the result of his suggestion. Origen proves this (Peri Archon iii, 2) from the fact that even if the devil were no more, men would still have the desire for food, sexual pleasures and the like; which desire might be inordinate, unless it were subordinate to reason, a matter that is subject to the free-will. ”

“ Although vital functions are always from an intrinsic principle, yet an extrinsic agent can cooperate with them, even as external heat cooperates with the functions of the vegetal soul, that food may be more easily digested. ”