Objection 1. It would seem that in Christ there are not two wills, one Divine, the other human. For the will is the first mover and first commander in whoever wills. But in Christ the first mover and commander was the Divine will, since in Christ everything human was moved by the Divine will. Hence it seems that in Christ there was only one will, viz. the Divine.
Objection 2. Further, an instrument is not moved by its own will but by the will of its mover. Now the human nature of Christ was the instrument of His Godhead. Hence the human nature of Christ was not moved by its own will, but by the Divine will.
Objection 3. Further, that alone is multiplied in Christ which belongs to the nature. But the will does not seem to pertain to nature: for natural things are of necessity; whereas what is voluntary is not of necessity. Therefore there is but one will in Christ.
Objection 4. Further, Damascene says (De Fide Orth. iii, 14) that "to will in this or that way belongs not to our nature but to our intellect," i.e. our personal intellect. But every will is this or that will, since there is nothing in a genus which is not at the same time in some one of its species. Therefore all will belongs to the person. But in Christ there was and is but one person. Therefore in Christ there is only one will.
On the contrary, our Lord says (Luke 22:42): "Father, if Thou wilt, remove this chalice from Me. But yet not My will but Thine be done." And Ambrose, quoting this to the Emperor Gratian (De Fide ii, 7) says: "As He assumed my will, He assumed my sorrow;" and on Luke 22:42 he says: "His will, He refers to the Man--the Father's, to the Godhead. For the will of man is temporal, and the will of the Godhead eternal."
I answer that, Some placed only one will in Christ; but they seem to have had different motives for holding this. For Apollinaris did not hold an intellectual soul in Christ, but maintained that the Word was in place of the soul, or even in place of the intellect. Hence since "the will is in the reason," as the Philosopher says (De Anima iii, 9), it followed that in Christ there was no human will; and thus there was only one will in Him. So, too, Eutyches and all who held one composite nature in Christ were forced to place one will in Him. Nestorius, too, who maintained that the union of God and man was one of affection and will, held only one will in Christ. But later on, Macarius, Patriarch of Antioch, Cyrus of Alexandria, and Sergius of Constantinople and some of their followers, held that there is one will in Christ, although they held that in Christ there are two natures united in a hypostasis; because they believed that Christ's human nature never moved with its own motion, but only inasmuch as it was moved by the Godhead, as is plain from the synodical letter of Pope Agatho [Third Council of Constantinople, Act. 4].
And hence in the sixth Council held at Constantinople [Act. 18] it was decreed that it must be said that there are two wills in Christ, in the following passage: "In accordance with what the Prophets of old taught us concerning Christ, and as He taught us Himself, and the Symbol of the Holy Fathers has handed down to us, we confess two natural wills in Him and two natural operations." And this much it was necessary to say. For it is manifest that the Son of God assumed a perfect human nature, as was shown above (5; 9, 1). Now the will pertains to the perfection of human nature, being one of its natural powers, even as the intellect, as was stated in I, Q79,80. Hence we must say that the Son of God assumed a human will, together with human nature. Now by the assumption of human nature the Son of God suffered no diminution of what pertains to His Divine Nature, to which it belongs to have a will, as was said in the I, 19, 1. Hence it must be said that there are two wills in Christ, i.e. one human, the other Divine.
Reply to Objection 1. Whatever was in the human nature of Christ was moved at the bidding of the Divine will; yet it does not follow that in Christ there was no movement of the will proper to human nature, for the good wills of other saints are moved by God's will, "Who worketh" in them "both to will and to accomplish," as is written Philippians 2:13. For although the will cannot be inwardly moved by any creature, yet it can be moved inwardly by God, as was said in I, 105, 4. And thus, too, Christ by His human will followed the Divine will according to Psalm 39:9; "That I should do Thy will, O my God, I have desired it." Hence Augustine says (Contra Maxim. ii, 20): "Where the Son says to the Father, 'Not what I will, but what Thou willest,' what do you gain by adding your own words and saying 'He shows that His will was truly subject to His Father,' as if we denied that man's will ought to be subject to God's will?"
Reply to Objection 2. It is proper to an instrument to be moved by the principal agent, yet diversely, according to the property of its nature. For an inanimate instrument, as an axe or a saw, is moved by the craftsman with only a corporeal movement; but an instrument animated by a sensitive soul is moved by the sensitive appetite, as a horse by its rider; and an instrument animated with a rational soul is moved by its will, as by the command of his lord the servant is moved to act, the servant being like an animate instrument, as the Philosopher says (Polit. i, 2,4; Ethic. viii, 11). And hence it was in this manner that the human nature of Christ was the instrument of the Godhead, and was moved by its own will.
Reply to Objection 3. The power of the will is natural, and necessarily follows upon the nature; but the movement or act of this power--which is also called will--is sometimes natural and necessary, e.g. with respect to beatitude; and sometimes springs from free-will and is neither necessary nor natural, as is plain from what has been stated in I-II, 10, 1,2 [Cf. I, 82, 2]. And yet even reason itself, which is the principle of this movement, is natural. Hence besides the Divine will it is necessary to place in Christ a human will, not merely as a natural power, or a natural movement, but even as a rational movement.
Reply to Objection 4. When we say "to will in a certain way," we signify a determinate mode of willing. Now a determinate mode regards the thing of which it is the mode. Hence since the will pertains to the nature, "to will in a certain way" belongs to the nature, not indeed considered absolutely, but as it is in the hypostasis. Hence the human will of Christ had a determinate mode from the fact of being in a Divine hypostasis, i.e. it was always moved in accordance with the bidding of the Divine will.
Source: Summa Theologica (New Advent)