Objection 1. It would seem that the will is not the subject of virtue. Because no habit is required for that which belongs to a power by reason of its very nature. But since the will is in the reason, it is of the very essence of the will, according to the Philosopher (De Anima iii, text. 42), to tend to that which is good, according to reason. And to this good every virtue is ordered, since everything naturally desires its own proper good; for virtue, as Tully says in his Rhetoric, is a "habit like a second nature in accord with reason." Therefore the will is not the subject of virtue.
Objection 2. Further, every virtue is either intellectual or moral (Ethic. i, 13; ii, 1). But intellectual virtue is subjected in the intellect and reason, and not in the will: while moral virtue is subjected in the irascible and concupiscible powers which are rational by participation. Therefore no virtue is subjected in the will.
Objection 3. Further, all human acts, to which virtues are ordained, are voluntary. If therefore there be a virtue in the will in respect of some human acts, in like manner there will be a virtue in the will in respect of all human acts. Either, therefore, there will be no virtue in any other power, or there will be two virtues ordained to the same act, which seems unreasonable. Therefore the will cannot be the subject of virtue.
On the contrary, Greater perfection is required in the mover than in the moved. But the will moves the irascible and concupiscible powers. Much more therefore should there be virtue in the will than in the irascible and concupiscible powers.
I answer that, Since the habit perfects the power in reference to act, then does the power need a habit perfecting it unto doing well, which habit is a virtue, when the power's own proper nature does not suffice for the purpose.
Now the proper nature of a power is seen in its relation to its object. Since, therefore, as we have said above (Question 19, Article 3), the object of the will is the good of reason proportionate to the will, in respect of this the will does not need a virtue perfecting it. But if man's will is confronted with a good that exceeds its capacity, whether as regards the whole human species, such as Divine good, which transcends the limits of human nature, or as regards the individual, such as the good of one's neighbor, then does the will need virtue. And therefore such virtues as those which direct man's affections to God or to his neighbor are subjected in the will, as charity, justice, and such like.
Reply to Objection 1. This objection is true of those virtues which are ordained to the willer's own good; such as temperance and fortitude, which are concerned with the human passions, and the like, as is clear from what we have said (35, 6).
Reply to Objection 2. Not only the irascible and concupiscible powers are rational by participation but "the appetitive power altogether," i.e. in its entirety (Ethic. i, 13). Now the will is included in the appetitive power. And therefore whatever virtue is in the will must be a moral virtue, unless it be theological, as we shall see later on (62, 3).
Reply to Objection 3. Some virtues are directed to the good of moderated passion, which is the proper good of this or that man: and in these cases there is no need for virtue in the will, for the nature of the power suffices for the purpose, as we have said. This need exists only in the case of virtues which are directed to some extrinsic good.
Source: Summa Theologica (New Advent)