Certain actions are called human, inasmuch as they are voluntary, as stated above (Question 1, Article 1). Now, in a voluntary action, there is a twofold action, viz. the interior action of the will, and the external action: and each of these actions has its object. The end is properly the object of the interior act of the will: while the object of the external action, is that on which the action is brought to bear. Therefore just as the external action takes its species from the object on which it bears; so the interior act of the will takes its species from the end, as from its own proper object.
Now that which is on the part of the will is formal in regard to that which is on the part of the external action: because the will uses the limbs to act as instruments; nor have external actions any measure of morality, save in so far as they are voluntary. Consequently the species of a human act is considered formally with regard to the end, but materially with regard to the object of the external action. Hence the Philosopher says (Ethic. v, 2) that “he who steals that he may commit adultery, is strictly speaking, more adulterer than thief.”