Objection 1. It would seem that hope is not in the will as its subject. For the object of hope is an arduous good, as stated above (17, 1; I-II, 40, 1). Now the arduous is the object, not of the will, but of the irascible. Therefore hope is not in the will but in the irascible.
Objection 2. Further, where one suffices it is superfluous to add another. Now charity suffices for the perfecting of the will, which is the most perfect of the virtues. Therefore hope is not in the will.
Objection 3. Further, the one same power cannot exercise two acts at the same time; thus the intellect cannot understand many things simultaneously. Now the act of hope can be at the same time as an act of charity. Since, then, the act of charity evidently belongs to the will, it follows that the act of hope does not belong to that power: so that, therefore, hope is not in the will.
On the contrary, The soul is not apprehensive of God save as regards the mind in which is memory, intellect and will, as Augustine declares (De Trin. xiv, 3,6). Now hope is a theological virtue having God for its object. Since therefore it is neither in the memory, nor in the intellect, which belong to the cognitive faculty, it follows that it is in the will as its subject.
I answer that, As shown above (I, 87, 2), habits are known by their acts. Now the act of hope is a movement of the appetitive faculty, since its object is a good. And, since there is a twofold appetite in man, namely, the sensitive which is divided into irascible and concupiscible, and the intellective appetite, called the will, as stated in I, 82, 5, those movements which occur in the lower appetite, are with passion, while those in the higher appetite are without passion, as shown above (I, 87, 2, ad 1; I-II, 22, 3, ad 3). Now the act of the virtue of hope cannot belong to the sensitive appetite, since the good which is the principal object of this virtue, is not a sensible but a Divine good. Therefore hope resides in the higher appetite called the will, and not in the lower appetite, of which the irascible is a part.
Reply to Objection 1. The object of the irascible is an arduous sensible: whereas the object of the virtue of hope is an arduous intelligible, or rather superintelligible.
Reply to Objection 2. Charity perfects the will sufficiently with regard to one act, which is the act of loving: but another virtue is required in order to perfect it with regard to its other act, which is that of hoping.
Reply to Objection 3. The movement of hope and the movement of charity are mutually related, as was shown above (Question 17, Article 8). Hence there is no reason why both movements should not belong at the same time to the same power: even as the intellect can understand many things at the same time if they be related to one another, as stated in I, 85, 4.
Source: Summa Theologica (New Advent)