Objection 1. It would seem that the will is not the subject of charity. For charity is a kind of love. Now, according to the Philosopher (Topic. ii, 3) love is in the concupiscible part. Therefore charity is also in the concupiscible and not in the will.
Objection 2. Further, charity is the foremost of the virtues, as stated above (Question 23, Article 6). But the reason is the subject of virtue. Therefore it seems that charity is in the reason and not in the will.
Objection 3. Further, charity extends to all human acts, according to 1 Corinthians 16:14: "Let all your things be done in charity." Now the principle of human acts is the free-will. Therefore it seems that charity is chiefly in the free-will as its subject and not in the will.
On the contrary, The object of charity is the good, which is also the object of the will. Therefore charity is in the will as its subject.
I answer that, Since, as stated in I, 80, 2, the appetite is twofold, namely the sensitive, and the intellective which is called the will, the object of each is the good, but in different ways: for the object of the sensitive appetite is a good apprehended by sense, whereas the object of the intellective appetite or will is good under the universal aspect of good, according as it can be apprehended by the intellect. Now the object of charity is not a sensible good, but the Divine good which is known by the intellect alone. Therefore the subject of charity is not the sensitive, but the intellective appetite, i.e. the will.
Reply to Objection 1. The concupiscible is a part of the sensitive, not of the intellective appetite, as proved in I, 81, 2: wherefore the love which is in the concupiscible, is the love of sensible good: nor can the concupiscible reach to the Divine good which is an intelligible good; the will alone can. Consequently the concupiscible cannot be the subject of charity.
Reply to Objection 2. According to the Philosopher (De Anima iii, 9), the will also is in the reason: wherefore charity is not excluded from the reason through being in the will. Yet charity is regulated, not by the reason, as human virtues are, but by God's wisdom, and transcends the rule of human reason, according to Ephesians 3:19: "The charity of Christ, which surpasseth all knowledge." Hence it is not in the reason, either as its subject, like prudence is, or as its rule, like justice and temperance are, but only by a certain kinship of the will to the reason.
Reply to Objection 3. As stated in I, 83, 4, the free-will is not a distinct power from the will. Yet charity is not in the will considered as free-will, the act of which is to choose. For choice is of things directed to the end, whereas the will is of the end itself (Ethic. iii, 2). Hence charity, whose object is the last end, should be described as residing in the will rather than in the free-will.
Source: Summa Theologica (New Advent)