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.
Objection 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.
Objection 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 (Article 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 (77, 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 (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 to Objection 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 proposed as appetible to the senses, and because the appetite is inclined, that the reason sometimes fails to consider the due rule, so that the will produces the act of sin. Since therefore the movements that precede it are not always actual, neither is man always actually sinning.
Reply to Objection 2. It is not true that all the internal acts belong to the substance of sin, for this consists principally in the act of the will; but some precede and some follow the sin itself.
Reply to Objection 3. That which causes sin, as a power produces its act, is natural; and again, the movement of the sensitive part, from which sin follows, is natural sometimes, as, for instance, when anyone sins through appetite for food. Yet sin results in being unnatural from the very fact that the natural rule fails, which man, in accord with his nature, ought to observe.
Source: Summa Theologica (New Advent)