at which something ought naturally to have, such an inordinateness
must needs have an accidental efficient cause. For that which
naturally is and ought to be in a thing, is never lacking except on
account of some impeding cause. And accordingly we are wont to say
that evil, which consists in a certain privation, has a deficient
cause, or an accidental efficient cause. Now every accidental cause
is reducible to the direct cause. Since then sin, on the part of its
inordinateness, has an accidental efficient cause, and on the part of
the act, a direct efficient cause, it follows that the inordinateness
of sin is a result of the cause of the act. Accordingly then, the
will lacking the direction of the rule of reason and of the Divine
law, and intent on some mutable good, causes the act of sin directly,
and the inordinateness of the act, indirectly, and beside the
intention: for the lack of order in the act results from the lack of
direction in the will.

Reply Obj. 1: Sin signifies not only the privation of good, which
privation is its inordinateness, but also the act which is the
subject of that privation, which has the nature of evil: and how this
evil has a cause, has been explained.

Reply Obj. 2: If this definition is to be verified in all cases, it
must be understood as applying to a cause which is sufficient and not
impeded. For it happens that a thing is the sufficient cause of
something else, and that the effect does not follow of necessity, on
account of some supervening impediment: else it would follow that all
things happen of necessity, as is proved in _Metaph._ vi, text. 5.
Accordingly, though sin has a cause, it does not follow that this is
a necessary cause, since its effect can be impeded.

Reply Obj. 3: As stated above, the will in failing to apply the rule
of reason or of the Divine law, is the cause of sin. Now the fact of
not applying the rule of reason or of the Divine law, has not in
itself the nature of evil, whether of punishment or of guilt, before
it is applied to the act. Wherefore accordingly, evil is not the
cause of the first sin, but some good lacking some other good.
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SECOND ARTICLE [I-II, Q. 75, Art. 2]

Whether Sin Has an Internal Cause?

Objection 1: It would seem that sin has no internal cause. For that
which is within a thing is always in it. If therefore sin had an
internal cause, man would always be sinning, since given the cause,
the effect follows.