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.

Reply Obj. 1: That which is within a thing as its natural power, is
always in it: but that which is within it, as the internal act of the
appetitive or apprehensive power, is not always in it. Now the power
of the will is the potential cause of sin, but is made actual by the
preceding movements, both of the sensitive part, in the first place,
and afterwards, of the reason. For it is because a thing is propo