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."
I answer that, Forasmuch as a sin has a cause on the part of the act of sin, it is possible for one sin to be the cause of another, in the same way as one human act is the cause of another. Hence it happens that one sin may be the cause of another in respect of the four kinds of causes. First, after the manner of an efficient or moving cause, both directly and indirectly. Indirectly, as that which removes an impediment is called an indirect cause of movement: for when man, by one sinful act, loses grace, or charity, or shame, or anything else that withdraws him from sin, he thereby falls into another sin, so that the first sin is the accidental cause of the second. Directly, as when, by one sinful act, man is disposed to commit more readily another like act: because acts cause dispositions and habits inclining to like acts. Secondly, after the manner of a material cause, one sin is the cause of another, by preparing its matter: thus covetousness prepares the matter for strife, which is often about the wealth a man has amassed together. Thirdly, after the manner of a final cause, one sin causes another, in so far as a man commits one sin for the sake of another which is his end; as when a man is guilty of simony for the end of ambition, or fornication for the purpose of theft. And since the end gives the form to moral matters, as stated above (Q. 1, A. 3; Q. 18, AA. 4, 6), it follows that one sin is also the formal cause of another: because in the act of fornication committed for the purpose of theft, the former is material while the latter is formal.
Reply Obj. 1: Sin, in so far as it is inordinate, has the character of evil; but, in so far as it is an act, it has some good, at least apparent, for its end: so that, as an act, but not as being inordinate, it can be the cause, both final and efficient, of another sin. A sin has matter, not of which but about which it is: and it has its form from its end. Consequently one sin can be the cause of another, in respect of the four kinds of cause, as stated above.
Reply Obj. 2: Sin is something imperfect on account of its moral imperfection on the part of its inordinateness. Nevertheless, as an act it can have natural perfection: and thus it can be the cause of another sin.
Reply Obj. 3: Not every cause of one sin is another sin; so there is no need to go on indefinitely: for one may come to one sin which is not caused by another sin.