As regards the object, a thing may be understood as moving the will in three ways.  First, the object itself which is proposed to the will:  thus we say that food arouses man’s desire to eat.  Secondly, he that proposes or offers this object.  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.  But in none of these three ways can anything be the direct cause of sin, because the will is not, of necessity, moved by any object except the last end, as stated above (Q. 10, AA. 1, 2).  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.

Reply Obj. 1:  All these, and other like authorities, if we meet with them, are to be understood as denoting that the devil induces man to affection for a sin, either by suggesting to him, or by offering him objects of appetite.

Reply Obj. 2:  This comparison is true in so far as the devil is somewhat the cause of our sins, even as God is in a certain way the cause of our good actions, but does not extend to the mode of causation:  for God causes good things in us by moving the will inwardly, whereas the devil cannot move us in this way.

Reply Obj. 3:  God is the universal principle of all inward movements of man; but that the human will be determined to an evil counsel, is directly due to the human will, and to the devil as persuading or offering the object of appetite. ________________________

SECOND ARTICLE [I-II, Q. 80, Art. 2]

Whether the Devil Can Induce Man to Sin, by Internal Instigations?

Objection 1:  It would seem that the devil cannot induce man to sin, by internal instigations.  Because the internal movements of the soul are vital functions.  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.

Obj. 2:  Further, all the internal movements arise from the external senses according to the order of nature.  Now it belongs to God alone to do anything beside the order of nature, as was stated in the First Part (Q. 110, A. 4).  Therefore the devil cannot effect anything in man’s internal movements, except in respect of things which are perceived by the external senses.