Objection 1. It would seem that good and evil are in the external action prior to being in the act of the will. For the will derives goodness from its object, as stated above (19, 1,2). But the external action is the object of the interior act of the will: for a man is said to will to commit a theft, or to will to give an alms. Therefore good and evil are in the external action, prior to being in the act of the will.
Objection 2. Further, the aspect of good belongs first to the end: since what is directed to the end receives the aspect of good from its relation to the end. Now whereas the act of the will cannot be an end, as stated above (1, 1, ad 2), the act of another power can be an end. Therefore good is in the act of some other power prior to being in the act of the will.
Objection 3. Further, the act of the will stands in a formal relation to the external action, as stated above (Question 18, Article 6). But that which is formal is subsequent; since form is something added to matter. Therefore good and evil are in the external action, prior to being in the act of the will.
On the contrary, Augustine says (Retract. i, 9) that "it is by the will that we sin, and that we behave aright." Therefore moral good and evil are first in the will.
I answer that, External actions may be said to be good or bad in two ways. First, in regard to their genus, and the circumstances connected with them: thus the giving of alms, if the required conditions be observed, is said to be good. Secondly, a thing is said to be good or evil, from its relation to the end: thus the giving of alms for vainglory is said to be evil. Now, since the end is the will's proper object, it is evident that this aspect of good or evil, which the external action derives from its relation to the end, is to be found first of all in the act of the will, whence it passes to the external action. On the other hand, the goodness or malice which the external action has of itself, on account of its being about due matter and its being attended by due circumstances, is not derived from the will, but rather from the reason. Consequently, if we consider the goodness of the external action, in so far as it comes from reason's ordination and apprehension, it is prior to the goodness of the act of the will: but if we consider it in so far as it is in the execution of the action done, it is subsequent to the goodness of the will, which is its principle.
Reply to Objection 1. The exterior action is the object of the will, inasmuch as it is proposed to the will by the reason, as good apprehended and ordained by the reason: and thus it is prior to the good in the act of the will. But inasmuch as it is found in the execution of the action, it is an effect of the will, and is subsequent to the will.
Reply to Objection 2. The end precedes in the order of intention, but follows in the order of execution.
Reply to Objection 3. A form as received into matter, is subsequent to matter in the order of generation, although it precedes it in the order of nature: but inasmuch as it is in the active cause, it precedes in every way. Now the will is compared to the exterior action, as its efficient cause. Wherefore the goodness of the act of the will, as existing in the active cause, is the form of the exterior action.
Source: Summa Theologica (New Advent)