Objection 1. It would seem that human acts are not specified by their end. For the end is an extrinsic cause. But everything is specified by an intrinsic principle. Therefore human acts are not specified by their end.
Objection 2. Further, that which gives a thing its species should exist before it. But the end comes into existence afterwards. Therefore a human act does not derive its species from the end.
Objection 3. Further, one thing cannot be in more than one species. But one and the same act may happen to be ordained to various ends. Therefore the end does not give the species to human acts.
On the contrary, Augustine says (De Mor. Eccl. et Manich. ii, 13): "According as their end is worthy of blame or praise so are our deeds worthy of blame or praise."
I answer that Each thing receives its species in respect of an act and not in respect of potentiality; wherefore things composed of matter and form are established in their respective species by their own forms. And this is also to be observed in proper movements. For since movements are, in a way, divided into action and passion, each of these receives its species from an act; action indeed from the act which is the principle of acting, and passion from the act which is the terminus of the movement. Wherefore heating, as an action, is nothing else than a certain movement proceeding from heat, while heating as a passion is nothing else than a movement towards heat: and it is the definition that shows the specific nature. And either way, human acts, whether they be considered as actions, or as passions, receive their species from the end. For human acts can be considered in both ways, since man moves himself, and is moved by himself. Now it has been stated above (Article 1) that acts are called human, inasmuch as they proceed from a deliberate will. Now the object of the will is the good and the end. And hence it is clear that the principle of human acts, in so far as they are human, is the end. In like manner it is their terminus: for the human act terminates at that which the will intends as the end; thus in natural agents the form of the thing generated is conformed to the form of the generator. And since, as Ambrose says (Prolog. super Luc.) "morality is said properly of man," moral acts properly speaking receive their species from the end, for moral acts are the same as human acts.
Reply to Objection 1. The end is not altogether extrinsic to the act, because it is related to the act as principle or terminus; and thus it just this that is essential to an act, viz. to proceed from something, considered as action, and to proceed towards something, considered as passion.
Reply to Objection 2. The end, in so far as it pre-exists in the intention, pertains to the will, as stated above (01, ad 1). And it is thus that it gives the species to the human or moral act.
Reply to Objection 3. One and the same act, in so far as it proceeds once from the agent, is ordained to but one proximate end, from which it has its species: but it can be ordained to several remote ends, of which one is the end of the other. It is possible, however, that an act which is one in respect of its natural species, be ordained to several ends of the will: thus this act "to kill a man," which is but one act in respect of its natural species, can be ordained, as to an end, to the safeguarding of justice, and to the satisfying of anger: the result being that there would be several acts in different species of morality: since in one way there will be an act of virtue, in another, an act of vice. For a movement does not receive its species from that which is its terminus accidentally, but only from that which is its "per se" terminus. Now moral ends are accidental to a natural thing, and conversely the relation to a natural end is accidental to morality. Consequently there is no reason why acts which are the same considered in their natural species, should not be diverse, considered in their moral species, and conversely.
Source: Summa Theologica (New Advent)