Objection 1. It would seem that the will cannot be a subject of sin. For Dionysius says (Div. Nom. iv) that "evil is outside the will and the intention." But sin has the character of evil. Therefore sin cannot be in the will.
Objection 2. Further, the will is directed either to the good or to what seems good. Now from the fact that will wishes the good, it does not sin: and that it wishes what seems good but is not truly good, points to a defect in the apprehensive power rather than in the will. Therefore sin is nowise in the will.
Objection 3. Further, the same thing cannot be both subject and efficient cause of sin: because "the efficient and the material cause do not coincide" (Phys. 2, text. 70). Now the will is the efficient cause of sin: because the first cause of sinning is the will, as Augustine states (De Duabus Anim. x, 10,11). Therefore it is not the subject of sin.
On the contrary, Augustine says (Retract. i, 9) that "it is by the will that we sin, and live righteously."
I answer that, Sin is an act, as stated above (71, A1,6). Now some acts pass into external matter, e.g. "to cut" and "to burn": and such acts have for their matter and subject, the thing into which the action passes: thus the Philosopher states (Phys. iii, text. 18) that "movement is the act of the thing moved, caused by a mover." On the other hand, there are acts which do not pass into external matter, but remain in the agent, e.g. "to desire" and "to know": and such are all moral acts, whether virtuous or sinful. Consequently the proper subject of sin must needs be the power which is the principle of the act. Now since it is proper to moral acts that they are voluntary, as stated above (1, 1; 18, 6), it follows that the will, which is the principle of voluntary acts, both of good acts, and of evil acts or sins, is the principle of sins. Therefore it follows that sin is in the will as its subject.
Reply to Objection 1. Evil is said to be outside the will, because the will does not tend to it under the aspect of evil. But since some evil is an apparent good, the will sometimes desires an evil, and in this sense is in the will.
Reply to Objection 2. If the defect in the apprehensive power were nowise subject to the will, there would be no sin, either in the will, or in the apprehensive power, as in the case of those whose ignorance is invincible. It remains therefore that when there is in the apprehensive power a defect that is subject to the will, this defect also is deemed a sin.
Reply to Objection 3. This argument applies to those efficient causes whose actions pass into external matter, and which do not move themselves, but move other things; the contrary of which is to be observed in the will; hence the argument does not prove.
Source: Summa Theologica (New Advent)