Objection 1. It would seem that in Christ there are several human operations. For Christ as man communicates with plants by His nutritive soul, with the brutes by His sensitive soul, and with the angels by His intellective soul, even as other men do. Now the operations of a plant as plant and of an animal as animal are different. Therefore Christ as man has several operations.
Objection 2. Further, powers and habits are distinguished by their acts. Now in Christ's soul there were divers powers and habits; therefore also divers operations.
Objection 3. Further, instruments ought to be proportioned to their operations. Now the human body has divers members of different form, and consequently fitted to divers operations. Therefore in Christ there are divers operations in the human nature.
On the contrary, As Damascene says (De Fide Orth. iii, 15), "operation is consequent upon the nature." But in Christ there is only one human nature. Therefore in Christ there is only one human operation.
I answer that, Since it is by his reason that man is what he is; that operation is called human simply, which proceeds from the reason through the will, which is the rational appetite. Now if there is any operation in man which does not proceed from the reason and the will, it is not simply a human operation, but belongs to man by reason of some part of human nature--sometimes by reason of the nature of elementary bodies, as to be borne downwards--sometimes by reason of the force of the vegetative soul, as to be nourished, and to grow--sometimes by reason of the sensitive part, as to see and hear, to imagine and remember, to desire and to be angry. Now between these operations there is a difference. For the operations of the sensitive soul are to some extent obedient to reason, and consequently they are somewhat rational and human inasmuch as they obey reason, as is clear from the Philosopher (Ethic. i, 13). But the operations that spring from the vegetative soul, or from the nature of elemental bodies, are not subject to reason; consequently they are nowise rational; nor simply human, but only as regards a part of human nature. Now it was said (1) that when a subordinate agent acts by its own form, the operations of the inferior and of the superior agent are distinct; but when the inferior agent acts only as moved by the superior agent, then the operation of the superior and the inferior agent is one.
And hence in every mere man the operations of the elemental body and of the vegetative soul are distinct from the will's operation, which is properly human; so likewise the operations of the sensitive soul inasmuch as it is not moved by reason; but inasmuch as it is moved by reason, the operations of the sensitive and the rational part are the same. Now there is but one operation of the rational part if we consider the principle of the operation, which is the reason and the will; but the operations are many if we consider their relationship to various objects. And there were some who called this a diversity of things operated rather than of operations, judging the unity of the operation solely from the operative principle. And it is in this respect that we are now considering the unity and plurality of operations in Christ.
Hence in every mere man there is but one operation, which is properly called human; but besides this there are in a mere man certain other operations, which are not strictly human, as was said above. But in the Man Jesus Christ there was no motion of the sensitive part which was not ordered by reason. Even the natural and bodily operations pertained in some respects to His will, inasmuch as it was His will "that His flesh should do and suffer what belonged to it," as stated above (Question 18, Article 5). Much more, therefore, is there one operation in Christ, than in any other man whatsoever.
Reply to Objection 1. The operations of the sensitive and nutritive parts are not strictly human, as stated above; yet in Christ these operations were more human than in others.
Reply to Objection 2. Powers and habits are diversified by comparison with their objects. Hence in this way the diversity of operations corresponds to the divers powers and habits, as likewise to the divers objects. Now we do not wish to exclude this diversity of operations from Christ's humanity, nor that which springs from a diversity of time, but only that which regards the first active principle, as was said above.
(St. Thomas gives no reply to Objection 3; some codices add: Hence may be gathered the reply to the third objection.)
Source: Summa Theologica (New Advent)