Saint Thomas Aquinas on Virtues
Moral virtues are settled dispositions (good habits) of various appetitive powers which incline and allow their possessors to make good moral choices. Well, what does that mean?, you may ask. Well, it means that, unlike the sense powers (sight for example), the appetitive powers do not necessarily tend toward their proper object. Actually, in one sense they do; the appetite for tasty things always tends toward them. But they also have a natural disposition to be directed by reason (which disposition has been impaired, but not altogether lost, by original sin). This natural aptitude to incline toward their proper objects under the direction of reason which the appetites have allows and requires them to be perfected by the virtues.
The sensitive powers can be considered in two ways: first, according as they act from natural instinct: secondly, according as they act at the command of reason. According as they act from natural instinct, they are ordained to one thing, even as nature is; but according as they act at the command of reason, they can be ordained to various things. And thus there can be habits in them, by which they are well or ill disposed in regard to something.ST I-II, 50, 3
The powers of the nutritive part have not an inborn aptitude to obey the command of reason, and therefore there are no habits in them. But the sensitive powers have an inborn aptitude to obey the command of reason; and therefore habits can be in them: for in so far as they obey reason, in a certain sense they are said to be rational, as stated in Ethic. i, 13.Ibid., Reply to Objection 1
Thus, in one sense, the appetites don’t necessarily tend toward their objects insofar as such objects are suited to human nature; tasty things are desired in and of themselves, regardless of the amount that is suited to being a healthy human. Now for Aquinas, every human act tends something good, either truly so or only an apparent good ( ST I-II, 8, 1). However, an action is moral only insofar as it tends toward the good that is appropriate to human nature.
We must therefore say that every action has goodness, in so far as it has being; whereas it is lacking in goodness, in so far as it is lacking in something that is due to its fullness of being; and thus it is said to be evil: for instance if it lacks the quantity determined by reason, or its due place, or something of the kind.ST I-II, 18, 1.
Since it is reason which discovers what things, and to what extent, and in which circumstances, the objects of the appetitive powers are suited to human nature, reason trains, as it were, the various passions to desire their objects in an appropriate amount, and in appropriate circumstances. This training produces in these powers a disposition to desire what reason has discovered they ought to desire; in the passion for food, by being well trained by reason to desire only the amount of food appropriate, there develops a settled (i.e. not easily changed) disposition to desire only the amount of food appropriate, etc. This settled disposition would be called (a species of) the virtue of temperance. It works the same with other appetitive powers, e.g. the desire for retaliation and for self preservation, when disposed to retaliate only when appropriate and to flee danger only when appropriate, have the virtues that comprise fortitude or courage.
There are, then, four Cardinal Virtues, in the four principal powers used in moral actions: Prudence in the practical intellect, Justice in the rational appetite (will), Fortitude (Courage) in the irascible appetite, and Temperance in the concupiscible appetite. ( ST I-II, 61, 2)
It should be noted that the various appetitive powers are trained by reason to act well only by being made to perform these acts that are in fact cases of acting well; various actions of the virtues, performed by those who do not have the virtues, under the instruction of those who do (in order to tell the non-virtuous what actions are virtuous), leads to the virtues developing in them. So it is by resisting the temptation to eat chocolate, when you know that your not supposed to (even though you really do want to eat it) on many occasions that leads to developing the virtue of not wanting to eat chocolate when your not supposed to, which is part of temperance ( ST I-II, 63, 2).