Objection 1: It would seem that the will alone is the subject of sin. For Augustine says (De Duabus Anim. x, 10) that "no one sins except by the will." Now the subject of sin is the power by which we sin. Therefore the will alone is the subject of sin.
Obj. 2: Further, sin is an evil contrary to reason. Now good and evil pertaining to reason are the object of the will alone. Therefore the will alone is the subject of sin.
Obj. 3: Further, every sin is a voluntary act, because, as Augustine states (De Lib. Arb. iii, 18) [*Cf. De Vera Relig. xiv.], "so true is it that every sin is voluntary, that unless it be voluntary, it is no sin at all." Now the acts of the other powers are not voluntary, except in so far as those powers are moved by the will; nor does this suffice for them to be the subject of sin, because then even the external members of the body, which are moved by the will, would be a subject of sin; which is clearly untrue. Therefore the will alone is the subject of sin.
On the contrary, Sin is contrary to virtue: and contraries are about one same thing. But the other powers of the soul, besides the will, are the subject of virtues, as stated above (Q. 56). Therefore the will is not the only subject of sin.
I answer that, As was shown above (A. 1), whatever is the a principle of a voluntary act is a subject of sin. Now voluntary acts are not only those which are elicited by the will, but also those which are commanded by the will, as we stated above (Q. 6, A. 4) in treating of voluntariness. Therefore not only the will can be a subject of sin, but also all those powers which can be moved to their acts, or restrained from their acts, by the will; and these same powers are the subjects of good and evil moral habits, because act and habit belong to the same subject.
Reply Obj. 1: We do not sin except by the will as first mover; but we sin by the other powers as moved by the will.
Reply Obj. 2: Good and evil pertain to the will as its proper objects; but the other powers have certain determinate goods and evils, by reason of which they can be the subject of virtue, vice, and sin, in so far as they partake of will and reason.
Reply Obj. 3: The members of the body are not principles but merely organs of action: wherefore they are compared to the soul which moves them, as a slave who is moved but moves no other. On the other hand, the internal appetitive powers are compared to reason as free agents, because they both act and are acted upon, as is made clear in Polit.i, 3. Moreover, the acts of the external members are actions that pass into external matter, as may be seen in the blow that is inflicted in the sin of murder. Consequently there is no comparison.