Objection 1. It would seem that the external action does not add any goodness or malice to that of the interior action. For Chrysostom says (Hom. xix in Matt.): "It is the will that is rewarded for doing good, or punished for doing evil." Now works are the witnesses of the will. Therefore God seeks for works not on His own account, in order to know how to judge; but for the sake of others, that all may understand how just He is. But good or evil is to be estimated according to God's judgment rather than according to the judgment of man. Therefore the external action adds no goodness or malice to that of the interior act.
Objection 2. Further, the goodness and malice of the interior and external acts are one and the same, as stated above (Article 3). But increase is the addition of one thing to another. Therefore the external action does not add to the goodness or malice of the interior act.
Objection 3. Further, the entire goodness of created things does not add to the Divine Goodness, because it is entirely derived therefrom. But sometimes the entire goodness of the external action is derived from the goodness of the interior act, and sometimes conversely, as stated above (1,2). Therefore neither of them adds to the goodness or malice of the other.
On the contrary, Every agent intends to attain good and avoid evil. If therefore by the external action no further goodness or malice be added, it is to no purpose that he who has a good or an evil will, does a good deed or refrains from an evil deed. Which is unreasonable.
I answer that, If we speak of the goodness which the external action derives from the will tending to the end, then the external action adds nothing to this goodness, unless it happens that the will in itself is made better in good things, or worse in evil things. This, seemingly, may happen in three ways. First in point of number; if, for instance, a man wishes to do something with a good or an evil end in view, and does not do it then, but afterwards wills and does it, the act of his will is doubled and a double good, or a double evil is the result. Secondly, in point of extension: when, for instance, a man wishes to do something for a good or an evil end, and is hindered by some obstacle, whereas another man perseveres in the movement of the will until he accomplish it in deed; it is evident that the will of the latter is more lasting in good or evil, and in this respect, is better or worse. Thirdly, in point of intensity: for these are certain external actions, which, in so far as they are pleasurable, or painful, are such as naturally to make the will more intense or more remiss; and it is evident that the more intensely the will tends to good or evil, the better or worse it is.
On the other hand, if we speak of the goodness which the external action derives from its matter and due circumstances, thus it stands in relation to the will as its term and end. And in this way it adds to the goodness or malice of the will; because every inclination or movement is perfected by attaining its end or reaching its term. Wherefore the will is not perfect, unless it be such that, given the opportunity, it realizes the operation. But if this prove impossible, as long as the will is perfect, so as to realize the operation if it could; the lack of perfection derived from the external action, is simply involuntary. Now just as the involuntary deserves neither punishment nor reward in the accomplishment of good or evil deeds, so neither does it lessen reward or punishment, if a man through simple involuntariness fail to do good or evil.
Reply to Objection 1. Chrysostom is speaking of the case where a man's will is complete, and does not refrain from the deed save through the impossibility of achievement.
Reply to Objection 2. This argument applies to that goodness which the external action derives from the will as tending to the end. But the goodness which the external action takes from its matter and circumstances, is distinct from that which it derives from the end; but it is not distinct from that which it has from the very act willed, to which it stands in the relation of measure and cause, as stated above (1,2).
From this the reply to the Third Objection is evident.
Source: Summa Theologica (New Advent)