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. ________________________

FOURTH ARTICLE [I-II, Q. 75, Art. 4]

Whether One Sin Is a Cause of Another?

Objection 1:  It would seem that one sin cannot be the cause of another.  For there are four kinds of cause, none of which will fit in with one sin causing another.  Because the end has the character of good; which is inconsistent with sin, which has the character of evil.  In like manner neither can a sin be an efficient cause, since “evil is not an efficient cause, but is weak and powerless,” as Dionysius declares (Div.  Nom. iv).  The material and formal cause seems to have no place except in natural bodies, which are composed of matter and form.  Therefore sin cannot have either a material or a formal cause.

Obj. 2:  Further, “to produce its like belongs to a perfect thing,” as stated in Meteor. iv, 2 [Cf. _De Anima_ ii.].  But sin is essentially something imperfect.  Therefore one sin cannot be a cause of another.

Obj. 3:  Further, if one sin is the cause of a second sin, in the same way, yet another sin will be the cause of the first, and thus we go on indefinitely, which is absurd.  Therefore one sin is not the cause of another.

On the contrary, Gregory says on Ezechiel (Hom. xi):  “A sin that is not quickly blotted out by repentance, is both a sin and a cause of sin.”