Objection 1. It would seem that in Christ there were two wills as regards the reason. For Damascene says (De Fide Orth. ii, 22) that there is a double will in man, viz. the natural will which is called thelesis, and the rational will which is called boulesis. Now Christ in His human nature had whatever belongs to the perfection of human nature. Hence both the foregoing wills were in Christ.
Objection 2. Further, the appetitive power is diversified in man by the difference of the apprehensive power, and hence according to the difference of sense and intellect is the difference of sensitive and intellective appetite in man. But in the same way as regards man's apprehension, we hold the difference of reason and intellect; both of which were in Christ. Therefore there was a double will in Him, one intellectual and the other rational.
Objection 3. Further, some [Hugh of St. Victor, De Quat. Volunt. Christ.] ascribe to Christ "a will of piety," which can only be on the part of reason. Therefore in Christ on the part of reason there are several wills.
On the contrary, In every order there is one first mover. But the will is the first mover in the genus of human acts. Therefore in one man there is only one will, properly speaking, which is the will of reason. But Christ is one man. Therefore in Christ there is only one human will.
I answer that, As stated above (1, ad 3), the will is sometimes taken for the power, and sometimes for the act. Hence if the will is taken for the act, it is necessary to place two wills, i.e. two species of acts of the will in Christ on the part of the reason. For the will, as was said in I-II, 8, 2,3, regards both the end and the means; and is affected differently towards both. For towards the end it is borne simply and absolutely, as towards what is good in itself; but towards the means it is borne under a certain relation, as the goodness of the means depends on something else. Hence the act of the will, inasmuch as it is drawn to anything desired of itself, as health, which act is called by Damascene thelesis--i.e. simple will, and by the masters "will as nature," is different from the act of the will as it is drawn to anything that is desired only in order to something else, as to take medicine; and this act of the will Damascene calls boulesis--i.e. counseling will, and the masters, "will as reason." But this diversity of acts does not diversify the power, since both acts regard the one common ratio of the object, which is goodness. Hence we must say that if we are speaking of the power of the will, in Christ there is but one human will, essentially so called and not by participation; but if we are speaking of the will as an act, we thus distinguish in Christ a will as nature, which is called thelesis, and a will as reason, which is called boulesis.
Reply to Objection 1. These two wills do not diversify the power but only the act, as we have said.
Reply to Objection 2. The intellect and the reason are not distinct powers, as was said in I, 79, 8.
Reply to Objection 3. The "will of piety" would not seem to be distinct from the will considered as nature, inasmuch as it shrinks from another's evil, absolutely considered.
Source: Summa Theologica (New Advent)