“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.”