I answer that, Every action derives its species from its object, as stated above (Article 2). Hence it follows that a difference of object causes a difference of species in actions. Now, it must be observed that a difference of objects causes a difference of species in actions, according as the latter are referred to one active principle, which does not cause a difference in actions, according as they are referred to another active principle. Because nothing accidental constitutes a species, but only that which is essential; and a difference of object may be essential in reference to one active principle, and accidental in reference to another. Thus to know color and to know sound, differ essentially in reference to sense, but not in reference to the intellect.
Now in human actions, good and evil are predicated in reference to the reason; because as Dionysius says (Div. Nom. iv), “the good of man is to be in accordance with reason,” and evil is “to be against reason.” For that is good for a thing which suits it in regard to its form; and evil, that which is against the order of its form. It is therefore evident that the difference of good and evil considered in reference to the object is an essential difference in relation to reason; that is to say, according as the object is suitable or unsuitable to reason. Now certain actions are called human or moral, inasmuch as they proceed from the reason. Consequently it is evident that good and evil diversify the species in human actions; since essential differences cause a difference of species.
When we wish to consider the good or evil present in human actions, St. Thomas is saying, we should consider these actions precisely as something done by a human being, a kind of doing, rather than simply as certain effects in the world, which would be to consider them as a kind of making. The basic question is not, “Does this have good or bad results?”, but “Is this a good or bad thing to do?”, although the answer to the former question will have some bearing on the answer to the latter.
I have pointed out elsewhere the need to consider the objections and replies in order to understand the truth regarding a disputed question; the body is typically insufficient. This is particularly true in this article, where we have the following objections and replies:
Objection 1. It would seem that good and evil in moral actions do not make a difference of species. For the existence of good and evil in actions is in conformity with their existence in things, as stated above (Article 1). But good and evil do not make a specific difference in things; for a good man is specifically the same as a bad man. Therefore neither do they make a specific difference in actions.
Objection 2. Further, since evil is a privation, it is a non-being. But non-being cannot be a difference, according to the Philosopher (Metaph. iii, 3). Since therefore the difference constitutes the species, it seems that an action is not constituted in a species through being evil. Consequently good and evil do not diversify the species of human actions.
Objection 3. Further, acts that differ in species produce different effects. But the same specific effect results from a good and from an evil action: thus a man is born of adulterous or of lawful wedlock. Therefore good and evil actions do not differ in species.
Objection 4. Further, actions are sometimes said to be good or bad from a circumstance, as stated above (Article 3). But since a circumstance is an accident, it does not give an action its species. Therefore human actions do not differ in species on account of their goodness or malice.
Reply to Objection 1. Even in natural things, good and evil, inasmuch as something is according to nature, and something against nature, diversify the natural species; for a dead body and a living body are not of the same species. In like manner, good, inasmuch as it is in accord with reason, and evil, inasmuch as it is against reason, diversify the moral species.
Reply to Objection 2. Evil implies privation, not absolute, but affecting some potentiality. For an action is said to be evil in its species, not because it has no object at all; but because it has an object in disaccord with reason, for instance, to appropriate another’s property. Wherefore in so far as the object is something positive, it can constitute the species of an evil act.
Reply to Objection 3. The conjugal act and adultery, as compared to reason, differ specifically and have effects specifically different; because the other deserves praise and reward, the other, blame and punishment. But as compared to the generative power, they do not differ in species; and thus they have one specific effect.
Reply to Objection 4. A circumstance is sometimes taken as the essential difference of the object, as compared to reason; and then it can specify a moral act. And it must needs be so whenever a circumstance transforms an action from good to evil; for a circumstance would not make an action evil, except through being repugnant to reason.
The common theme here is that a moral action is not simply some being considered in itself, but in comparison with the good as proposed by human reason. As we saw earlier, moral obligation arises from the suitability or unsuitability of certain actions in relation to the human good, sought as an end. Stealing and adultery are bad, according to St. Thomas, because they an unreasonable way of seeking the human good; that is to say, they are harmful to human life rather than helpful. In contrast, giving alms is good, because it promotes human welfare.