he 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.
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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