Objection 1. It would seem that there was contrariety of wills in Christ. For contrariety of wills regards contrariety of objects, as contrariety of movements springs from contrariety of termini, as is plain from the Philosopher (Phys. v, text. 49, seq.). Now Christ in His different wills wished contrary things. For in His Divine will He wished for death, from which He shrank in His human will, hence Athanasius says [De Incarnat. et Cont. Arianos, written against Apollinarius]: "When Christ says 'Father, if it be possible, let this chalice pass from Me; yet not My will, but Thine be done,' and again, 'The spirit indeed is willing, but the flesh weak,' He denotes two wills--the human, which through the weakness of the flesh shrank from the passion--and His Divine will eager for the passion." Hence there was contrariety of wills in Christ.
Objection 2. Further, it is written (Galatians 5:17) that "the flesh lusteth against the spirit, and the spirit against the flesh." Now when the spirit desires one thing, and the flesh another, there is contrariety of wills. But this was in Christ; for by the will of charity which the Holy Spirit was causing in His mind, He willed the passion, according to Isaiah 53:7: "He was offered because it was His own will," yet in His flesh He shrank from the passion. Therefore there was contrariety of wills in Him.
Objection 3. Further, it is written (Luke 22:43) that "being in an agony, He prayed the longer." Now agony seems to imply a certain struggle [Greek, agonia] in a soul drawn to contrary things. Hence it seems that there was contrariety of will in Christ.
On the contrary, In the decisions of the Sixth Council [Third Council of Constantinople, Act. 18] it is said: "We confess two natural wills, not in opposition, as evil-minded heretics assert, but following His human will, and neither withstanding nor striving against, but rather being subject to, His Divine and omnipotent will."
I answer that, Contrariety can exist only where there is opposition in the same and as regards the same. For if the diversity exists as regards diverse things, and in diverse subjects, this would not suffice for the nature of contrariety, nor even for the nature of contradiction, e.g. if a man were well formed or healthy as regards his hand, but not as regards his foot. Hence for there to be contrariety of wills in anyone it is necessary, first, that the diversity of wills should regard the same. For if the will of one regards the doing of something with reference to some universal reason, and the will of another regards the not doing the same with reference to some particular reason, there is not complete contrariety of will, e.g. when a judge wishes a brigand to be hanged for the good of the commonwealth, and one of the latter's kindred wishes him not to be hanged on account of a private love, there is no contrariety of wills; unless, indeed, the desire of the private good went so far as to wish to hinder the public good for the private good--in that case the opposition of wills would regard the same.
Secondly, for contrariety of wills it is necessary that it should be in the same will. For if a man wishes one thing with his rational appetite, and wishes another thing with his sensitive appetite, there is no contrariety, unless the sensitive appetite so far prevailed as to change or at least keep back the rational appetite; for in this case something of the contrary movement of the sensitive appetite would reach the rational will.
And hence it must be said that although the natural and the sensitive will in Christ wished what the Divine will did not wish, yet there was no contrariety of wills in Him. First, because neither the natural will nor the will of sensuality rejected the reason for which the Divine will and the will of the human reason in Christ wished the passion. For the absolute will of Christ wished the salvation of the human race, although it did not pertain to it to will this for the sake of something further; but the movement of sensuality could nowise extend so far. Secondly, because neither the Divine will nor the will of reason in Christ was impeded or retarded by the natural will or the appetite of sensuality. So, too, on the other hand, neither the Divine will nor the will of reason in Christ shrank from or retarded the movement of the natural human will and the movement of the sensuality in Christ. For it pleased Christ, in His Divine will, and in His will of reason, that His natural will and will of sensuality should be moved according to the order of their nature. Hence it is clear that in Christ there was no opposition or contrariety of wills.
Reply to Objection 1. The fact of any will in Christ willing something else than did the Divine will, proceeded from the Divine will, by whose permission the human nature in Christ was moved by its proper movements, as Damascene says (De Fide Orth. ii, 15,18,19).
Reply to Objection 2. In us the desires of the spirit are impeded or retarded by the desires of the flesh: this did not occur in Christ. Hence in Christ there was no contrariety of flesh and spirit, as in us.
Reply to Objection 3. The agony in Christ was not in the rational soul, in as far as it implies a struggle in the will arising from a diversity of motives, as when anyone, on his reason considering one, wishes one thing, and on its considering another, wishes the contrary. For this springs from the weakness of the reason, which is unable to judge which is the best simply. Now this did not occur in Christ, since by His reason He judged it best that the Divine will regarding the salvation of the human race should be fulfilled by His passion. Nevertheless, there was an agony in Christ as regards the sensitive part, inasmuch as it implied a dread of coming trial, as Damascene says (De Fide Orth. ii, 15; iii, 18,23).
Source: Summa Theologica (New Advent)