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. ________________________

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.

Obj. 2:  Further, a thing is not its own cause.  But the internal movements of a man are sins.  Therefore they are not the cause of sin.

Obj. 3:  Further, whatever is within man is either natural or voluntary.  Now that which is natural cannot be the cause of sin, for sin is contrary to nature, as Damascene states (De Fide Orth. ii, 3; iv, 21); while that which is voluntary, if it be inordinate, is already a sin.  Therefore nothing intrinsic can be the cause of the first sin.

On the contrary, Augustine says (De Duabus Anim. x, 10, 11; Retract. i, 9) that “the will is the cause of sin.”

I answer that, As stated above (A. 1), the direct cause of sin must be considered on the part of the act.  Now we may distinguish a twofold internal cause of human acts, one remote, the other proximate.  The proximate internal cause of the human act is the reason and will, in respect of which man has a free-will; while the remote cause is the apprehension of the sensitive part, and also the sensitive appetite.  For just as it is due to the judgment of reason, that the will is moved to something in accord with reason, so it is due to an apprehension of the senses that the sensitive appetite is inclined to something; which inclination sometimes influences the will and reason, as we shall explain further on (Q. 77, A. 1).  Accordingly a double interior cause of sin may be assigned; one proximate, on the part of the reason and will; and the other remote, on the part of the imagination or sensitive appetite.

But since we have said above (A. 1, ad 3) that the cause of sin is some apparent good as motive, yet lacking the due motive, viz. the rule of reason or the Divine law, this motive which is an apparent good, appertains to the apprehension of the senses and to the appetite; while the lack of the due rule appertains to the reason, whose nature it is to consider this rule; and the completeness of the voluntary sinful act appertains to the will, so that the act of the will, given the conditions we have just mentioned, is already a sin.