Reply Obj. 1:  That which is within a thing as its natural power, is always in it:  but that which is within it, as the internal act of the appetitive or apprehensive power, is not always in it.  Now the power of the will is the potential cause of sin, but is made actual by the preceding movements, both of the sensitive part, in the first place, and afterwards, of the reason.  For it is because a thing is proposed as appetible to the senses, and because the appetite is inclined, that the reason sometimes fails to consider the due rule, so that the will produces the act of sin.  Since therefore the movements that precede it are not always actual, neither is man always actually sinning.

Reply Obj. 2:  It is not true that all the internal acts belong to the substance of sin, for this consists principally in the act of the will; but some precede and some follow the sin itself.

Reply Obj. 3:  That which causes sin, as a power produces its act, is natural; and again, the movement of the sensitive part, from which sin follows, is natural sometimes, as, for instance, when anyone sins through appetite for food.  Yet sin results in being unnatural from the very fact that the natural rule fails, which man, in accord with his nature, ought to observe. ________________________

THIRD ARTICLE [I-II, Q. 75, Art. 3]

Whether Sin Has an External Cause?

Objection 1:  It would seem that sin has no external cause.  For sin is a voluntary act.  Now voluntary acts belong to principles that are within us, so that they have no external cause.  Therefore sin has no external cause.

Obj. 2:  Further, as nature is an internal principle, so is the will.  Now in natural things sin can be due to no other than an internal cause; for instance, the birth of a monster is due to the corruption of some internal principle.  Therefore in the moral order, sin can arise from no other than an internal cause.  Therefore it has no external cause.

Obj. 3:  Further, if the cause is multiplied, the effect is multiplied.  Now the more numerous and weighty the external inducements to sin are, the less is a man’s inordinate act imputed to him as a sin.  Therefore nothing external is a cause of sin.

On the contrary, It is written (Num. 21:16):  “Are not these they, that deceived the children of Israel by the counsel of Balaam, and made you transgress against the Lord by the sin of Phogor?” Therefore something external can be a cause of sin.

I answer that, As stated above (A. 2), the internal cause of sin is both the will, as completing the sinful act, and the reason, as lacking the due rule, and the appetite, as inclining to sin.  Accordingly something external might be a cause of sin in three ways, either by moving the will itself immediately, or by moving the reason, or by moving the sensitive appetite.  Now, as stated above (Q. 9, A. 6; Q. 10, A. 4), none can move the will inwardly