Objection 1. It would seem that in Christ there is but one operation of the Godhead and the Manhood. For Dionysius says (Div. Nom. ii): "The most loving operation of God is made manifest to us by the supersubstantial Word having taken flesh integrally and truly, and having operated and suffered whatsoever befits His human and Divine operation." But he here mentions only one human and Divine operation, which is written in Greek theandrike, i.e. God-manlike. Hence it seems that there is but one composite operation in Christ.
Objection 2. Further, there is but one operation of the principal and instrumental agent. Now the human nature in Christ was the instrument of the Divine, as was said above (7, 1, ad 3; 8, 1, ad 1;k1 18, 1, ad 2). Hence the operations of the Divine and human natures in Christ are the same.
Objection 3. Further, since in Christ there are two natures in one hypostasis or person, whatever pertains to the hypostasis or person is one and the same. But operation pertains to the hypostasis or person, for it is only a subsisting suppositum that operates; hence, according to the Philosopher (Metaph. i, 1), acts belong to singulars. Hence in Christ there is only one operation of the Godhead and the Manhood.
Objection 4. Further, as being belongs to a subsisting hypostasis, so also does operation. But on account of the unity of hypostasis there is only one operation of the Godhead and the (17, 2). Hence, on account of the same unity, there is one operation in Christ.
Objection 5. Further, as being belongs to a sub-operated there is one operation. But the same thing was operated by the Godhead and the Manhood, as the healing of the lepers or the raising of the dead. Hence it seems that in Christ there is but one operation of the Godhead and the Manhood.
On the contrary, Ambrose says (De Fide ii, 8): "How can the same operation spring from different powers? Cannot the lesser operate as the greater? And can there be one operation where there are different substances?"
I answer that, As was said above (Question 18, Article 1), the aforesaid heretics who placed one will in Christ placed one operation in Christ. Now in order better to understand their erroneous opinion, we must bear in mind that wherever there are several mutually ordained agents, the inferior is moved by the superior, as in man the body is moved by the soul and the lower powers by the reason. And thus the actions and movements of the inferior principle are things operated rather than operations. Now what pertains to the highest principle is properly the operation; thus we say of man that to walk, which belongs to the feet, and to touch, which belongs to the hand, are things operated by the man--one of which is operated by the soul through the feet, the other through the hands. And because it is the same soul that operates in both cases, there is only one indifferent operation, on the part of the thing operating, which is the first moving principle; but difference is found on the part of what is operated. Now, as in a mere man the body is moved by the soul, and the sensitive by the rational appetite, so in the Lord Jesus Christ the human nature is moved and ruled by the Divine. Hence they said that there is one indifferent operation on the part of the Godhead operating, but divers things operated, inasmuch as the Godhead of Christ did one thing by Itself, as to uphold all things by the word of His power--and another thing by His human nature, as to walk in body. Hence the Sixth Council [Third Council of Constantinople, Act. 10] quotes the words of Severus the heretic, who said: "What things were done and wrought by the one Christ, differ greatly; for some are becoming to God, and some are human, as to walk bodily on the earth is indeed human, but to give hale steps to sickly limbs, wholly unable to walk on the ground, is becoming to God. Yet one, i.e. Incarnate Word, wrought one and the other--neither was this from one nature, and that from another; nor can we justly affirm that because there are distinct things operated there are therefore two operating natures and forms."
But herein they were deceived, for what is moved by another has a twofold action--one which it has from its own form--the other, which it has inasmuch as it is moved by another; thus the operation of an axe of itself is to cleave; but inasmuch as it is moved by the craftsman, its operation is to make benches. Hence the operation which belongs to a thing by its form is proper to it, nor does it belong to the mover, except in so far as he makes use of this kind of thing for his work: thus to heat is the proper operation of fire, but not of a smith, except in so far as he makes use of fire for heating iron. But the operation which belongs to the thing, as moved by another, is not distinct from the operation of the mover; thus to make a bench is not the work of the axe independently of the workman. Hence, wheresoever the mover and the moved have different forms or operative faculties, there must the operation of the mover and the proper operation of the moved be distinct; although the moved shares in the operation of the mover, and the mover makes use of the operation of the moved, and, consequently, each acts in communion with the other.
Therefore in Christ the human nature has its proper form and power whereby it acts; and so has the Divine. Hence the human nature has its proper operation distinct from the Divine, and conversely. Nevertheless, the Divine Nature makes use of the operation of the human nature, as of the operation of its instrument; and in the same way the human nature shares in the operation of the Divine Nature, as an instrument shares in the operation of the principal agent. And this is what Pope Leo says (Ep. ad Flavian. xxviii): "Both forms" (i.e. both the Divine and the human nature in Christ) "do what is proper to each in union with the other, i.e. the Word operates what belongs to the Word, and the flesh carries out what belongs to flesh."
But if there were only one operation of the Godhead and manhood in Christ, it would be necessary to say either that the human nature had not its proper form and power (for this could not possibly be said of the Divine), whence it would follow that in Christ there was only the Divine operation; or it would be necessary to say that from the Divine and human power there was made up one power. Now both of these are impossible. For by the first the human nature in Christ is supposed to be imperfect; and by the second a confusion of the natures is supposed. Hence it is with reason that the Sixth Council (Act. 18) condemned this opinion, and decreed as follows: "We confess two natural, indivisible, unconvertible, unconfused, and inseparable operations in the same Lord Jesus Christ our true God"; i.e. the Divine operation and the human operation.
Reply to Objection 1. Dionysius places in Christ a theandric, i.e. a God-manlike or Divino-human, operation not by any confusion of the operations or powers of both natures, but inasmuch as His Divine operation employs the human, and His human operation shares in the power of the Divine. Hence, as he says in a certain epistle (Ad Caium iv), "what is of man He works beyond man; and this is shown by the Virgin conceiving supernaturally and by the unstable waters bearing up the weight of bodily feet." Now it is clear that to be begotten belongs to human nature, and likewise to walk; yet both were in Christ supernaturally. So, too, He wrought Divine things humanly, as when He healed the leper with a touch. Hence in the same epistle he adds: "He performed Divine works not as God does, and human works not as man does, but, God having been made man, by a new operation of God and man."
Now, that he understood two operations in Christ, one of the Divine and the other of the human nature, is clear from what he says, Div. Nom. ii: "Whatever pertains to His human operation the Father and the Holy Ghost no-wise share in, except, as one might say, by their most gracious and merciful will," i.e. inasmuch as the Father and the Holy Ghost in their mercy wished Christ to do and to suffer human things. And he adds: "He is truly the unchangeable God, and God's Word by the sublime and unspeakable operation of God, which, being made man for us, He wrought." Hence it is clear that the human operation, in which the Father and the Holy Ghost do not share, except by Their merciful consent, is distinct from His operation, as the Word of God, wherein the Father and the Holy Ghost share.
Reply to Objection 2. The instrument is said to act through being moved by the principal agent; and yet, besides this, it can have its proper operation through its own form, as stated above of fire. And hence the action of the instrument as instrument is not distinct from the action of the principal agent; yet it may have another operation, inasmuch as it is a thing. Hence the operation of Christ's human nature, as the instrument of the Godhead, is not distinct from the operation of the Godhead; for the salvation wherewith the manhood of Christ saves us and that wherewith His Godhead saves us are not distinct; nevertheless, the human nature in Christ, inasmuch as it is a certain nature, has a proper operation distinct from the Divine, as stated above.
Reply to Objection 3. To operate belongs to a subsisting hypostasis; in accordance, however, with the form and nature from which the operation receives its species. Hence from the diversity of forms or natures spring the divers species of operations, but from the unity of hypostasis springs the numerical unity as regards the operation of the species: thus fire has two operations specifically different, namely, to illuminate and to heat, from the difference of light and heat, and yet the illumination of the fire that illuminates at one and the same time is numerically one. So, likewise, in Christ there are necessarily two specifically different operations by reason of His two natures; nevertheless, each of the operations at one and the same time is numerically one, as one walking and one healing.
Reply to Objection 4. Being and operation belong to the person by reason of the nature; yet in a different manner. For being belongs to the very constitution of the person, and in this respect it has the nature of a term; consequently, unity of person requires unity of the complete and personal being. But operation is an effect of the person by reason of a form or nature. Hence plurality of operations is not incompatible with personal unity.
Reply to Objection 5. The proper work of the Divine operation is different from the proper work of the human operation. Thus to heal a leper is a proper work of the Divine operation, but to touch him is the proper work of the human operation. Now both these operations concur in one work, inasmuch as one nature acts in union with the other.
Source: Summa Theologica (New Advent)