Objection 1. It would seem that the good or evil of an action is not derived from its object. For the object of any action is a thing. But "evil is not in things, but in the sinner's use of them," as Augustine says (De Doctr. Christ. iii, 12). Therefore the good or evil of a human action is not derived from their object.
Objection 2. Further, the object is compared to the action as its matter. But the goodness of a thing is not from its matter, but rather from the form, which is an act. Therefore good and evil in actions is not derived from their object.
Objection 3. Further, the object of an active power is compared to the action as effect to cause. But the goodness of a cause does not depend on its effect; rather is it the reverse. Therefore good or evil in actions is not derived from their object.
On the contrary, It is written (Hosea 9:10): "They became abominable as those things which they loved." Now man becomes abominable to God on account of the malice of his action. Therefore the malice of his action is according to the evil objects that man loves. And the same applies to the goodness of his action.
I answer that, as stated above (Article 1) the good or evil of an action, as of other things, depends on its fulness of being or its lack of that fulness. Now the first thing that belongs to the fulness of being seems to be that which gives a thing its species. And just as a natural thing has its species from its form, so an action has its species from its object, as movement from its term. And therefore just as the primary goodness of a natural thing is derived from its form, which gives it its species, so the primary goodness of a moral action is derived from its suitable object: hence some call such an action "good in its genus"; for instance, "to make use of what is one's own." And just as, in natural things, the primary evil is when a generated thing does not realize its specific form (for instance, if instead of a man, something else be generated); so the primary evil in moral actions is that which is from the object, for instance, "to take what belongs to another." And this action is said to be "evil in its genus," genus here standing for species, just as we apply the term "mankind" to the whole human species.
Reply to Objection 1. Although external things are good in themselves, nevertheless they have not always a due proportion to this or that action. And so, inasmuch as they are considered as objects of such actions, they have not the quality of goodness.
Reply to Objection 2. The object is not the matter "of which" (a thing is made), but the matter "about which" (something is done); and stands in relation to the act as its form, as it were, through giving it its species.
Reply to Objection 3. The object of the human action is not always the object of an active power. For the appetitive power is, in a way, passive; in so far as it is moved by the appetible object; and yet it is a principle of human actions. Nor again have the objects of the active powers always the nature of an effect, but only when they are already transformed: thus food when transformed is the effect of the nutritive power; whereas food before being transformed stands in relation to the nutritive power as the matter about which it exercises its operation. Now since the object is in some way the effect of the active power, it follows that it is the term of its action, and consequently that it gives it its form and species, since movement derives its species from its term. Moreover, although the goodness of an action is not caused by the goodness of its effect, yet an action is said to be good from the fact that it can produce a good effect. Consequently the very proportion of an action to its effect is the measure of its goodness.
Source: Summa Theologica (New Advent)