Objection 1. It would seem that the goodness and malice of the interior act of the will are not the same as those of the external action. For the principle of the interior act is the interior apprehensive or appetitive power of the soul; whereas the principle of the external action is the power that accomplishes the movement. Now where the principles of action are different, the actions themselves are different. Moreover, it is the action which is the subject of goodness or malice: and the same accident cannot be in different subjects. Therefore the goodness of the interior act cannot be the same as that of the external action.
Objection 2. Further, "A virtue makes that, which has it, good, and renders its action good also" (Ethic. ii, 6). But the intellective virtue in the commanding power is distinct from the moral virtue in the power commanded, as is declared in Ethic. i, 13. Therefore the goodness of the interior act, which belongs to the commanding power, is distinct from the goodness of the external action, which belongs to the power commanded.
Objection 3. Further, the same thing cannot be cause and effect; since nothing is its own cause. But the goodness of the interior act is the cause of the goodness of the external action, or vice versa, as stated above (1,2). Therefore it is not the same goodness in each.
On the contrary, It was shown above (Question 18, Article 6) that the act of the will is the form, as it were, of the external action. Now that which results from the material and formal element is one thing. Therefore there is but one goodness of the internal and external act.
I answer that, As stated above (Question 17, Article 4), the interior act of the will, and the external action, considered morally, are one act. Now it happens sometimes that one and the same individual act has several aspects of goodness or malice, and sometimes that it has but one. Hence we must say that sometimes the goodness or malice of the interior act is the same as that of the external action, and sometimes not. For as we have already said (1,2), these two goodnesses or malices, of the internal and external acts, are ordained to one another. Now it may happen, in things that are subordinate to something else, that a thing is good merely from being subordinate; thus a bitter draught is good merely because it procures health. Wherefore there are not two goodnesses, one the goodness of health, and the other the goodness of the draught; but one and the same. On the other hand it happens sometimes that that which is subordinate to something else, has some aspect of goodness in itself, besides the fact of its being subordinate to some other good: thus a palatable medicine can be considered in the light of a pleasurable good, besides being conducive to health.
We must therefore say that when the external action derives goodness or malice from its relation to the end only, then there is but one and the same goodness of the act of the will which of itself regards the end, and of the external action, which regards the end through the medium of the act of the will. But when the external action has goodness or malice of itself, i.e. in regard to its matter and circumstances, then the goodness of the external action is distinct from the goodness of the will in regarding the end; yet so that the goodness of the end passes into the external action, and the goodness of the matter and circumstances passes into the act of the will, as stated above (1,2).
Reply to Objection 1. This argument proves that the internal and external actions are different in the physical order: yet distinct as they are in that respect, they combine to form one thing in the moral order, as stated above (Question 17, Article 4).
Reply to Objection 2. As stated in Ethic. vi, 12, a moral virtue is ordained to the act of that virtue, which act is the end, as it were, of that virtue; whereas prudence, which is in the reason, is ordained to things directed to the end. For this reason various virtues are necessary. But right reason in regard to the very end of a virtue has no other goodness than the goodness of that virtue, in so far as the goodness of the reason is participated in each virtue.
Reply to Objection 3. When a thing is derived by one thing from another, as from a univocal efficient cause, then it is not the same in both: thus when a hot thing heats, the heat of the heater is distinct from the heat of the thing heated, although it be the same specifically. But when a thing is derived from one thing from another, according to analogy or proportion, then it is one and the same in both: thus the healthiness which is in medicine or urine is derived from the healthiness of the animal's body; nor is health as applied to urine and medicine, distinct from health as applied to the body of an animal, of which health medicine is the cause, and urine the sign. It is in this way that the goodness of the external action is derived from the goodness of the will, and vice versa; viz. according to the order of one to the other.
Source: Summa Theologica (New Advent)