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 save God alone, who cannot be a cause of sin, as we shall prove further on (Q. 79, A. 1). Hence it follows that nothing external can be a cause of sin, except by moving the reason, as a man or devil by enticing to sin; or by moving the sensitive appetite, as certain external sensibles move it. Yet neither does external enticement move the reason, of necessity, in matters of action, nor do things proposed externally, of necessity move the sensitive appetite, except perhaps it be disposed thereto in a certain way; and even the sensitive appetite does not, of necessity, move the reason and will. Therefore something external can be a cause moving to sin, but not so as to be a sufficient cause thereof: and the will alone is the sufficient completive cause of sin being accomplished.
Reply Obj. 1: From the very fact that the external motive causes of sin do not lead to sin sufficiently and necessarily, it follows that it remains in our power to sin or not to sin.
Reply Obj. 2: The fact that sin has an internal cause does not prevent its having an external cause; for nothing external is a cause of sin, except through the medium of the internal cause, as stated.
Reply Obj. 3: If the external causes inclining to sin be multiplied, the sinful acts are multiplied, because they incline to the sinful act in both greater numbers and greater frequency. Nevertheless the character of guilt is lessened, since this depends on the act being voluntary and in our power.