“The good or evil of an action, as of other things, depends on its fullness of being or its lack of that fullness. Now the first thing that belongs to the fullness 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… 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’.”
ST I-II.Q 18. A 2.