Objection 1. It would seem that the goodness of the will does not depend on the object. For the will cannot be directed otherwise than to what is good: since "evil is outside the scope of the will," as Dionysius says (Div. Nom. iv). If therefore the goodness of the will depended on the object, it would follow that every act of the will is good, and none bad.
Objection 2. Further, good is first of all in the end: wherefore the goodness of the end, as such, does not depend on any other. But, according to the Philosopher (Ethic. vi, 5), "goodness of action is the end, but goodness of making is never the end": because the latter is always ordained to the thing made, as to its end. Therefore the goodness of the act of the will does not depend on any object.
Objection 3. Further, such as a thing is, such does it make a thing to be. But the object of the will is good, by reason of the goodness of nature. Therefore it cannot give moral goodness to the will. Therefore the moral goodness of the will does not depend on the object.
On the contrary, the Philosopher says (Ethic. v, 1) that justice is that habit "from which men wish for just things": and accordingly, virtue is a habit from which men wish for good things. But a good will is one which is in accordance with virtue. Therefore the goodness of the will is from the fact that a man wills that which is good.
I answer that, Good and evil are essential differences of the act of the will. Because good and evil of themselves regard the will; just as truth and falsehood regard reason; the act of which is divided essentially by the difference of truth and falsehood, for as much as an opinion is said to be true or false. Consequently good and evil will are acts differing in species. Now the specific difference in acts is according to objects, as stated above (Question 18, Article 5). Therefore good and evil in the acts of the will is derived properly from the objects.
Reply to Objection 1. The will is not always directed to what is truly good, but sometimes to the apparent good; which has indeed some measure of good, but not of a good that is simply suitable to be desired. Hence it is that the act of the will is not always good, but sometimes evil.
Reply to Objection 2. Although an action can, in a certain way, be man's last end; nevertheless such action is not an act of the will, as stated above (1, 1, ad 2).
Reply to Objection 3. Good is presented to the will as its object by the reason: and in so far as it is in accord with reason, it enters the moral order, and causes moral goodness in the act of the will: because the reason is the principle of human and moral acts, as stated above (Question 18, Article 5).
Source: Summa Theologica (New Advent)