Those things then are united 'in nature,' which combine to constitute the integrity of some specific type, as soul and body are united to constitute the specific type of 'animal.' Once a specific type is set up in its integrity, no foreign element can be united with it in unity of nature without the breaking up of that specific type.* But what is not of the integrity of the specific type is readily found in some individual contained under the species, as whiteness and clothedness in Socrates or Plato.* All such non-specific attributes are said to be united 'in unity of suppositum,' or in the case of rational beings, 'in unity of person,' with the individual.
Now some have reckoned the union of God and man in Christ to be after the manner of things united 'in unity of nature.' Thus Arius and Apollinaris and Eutyches. But that is quite an impossibility. For the nature of the Word is a sovereignly perfect whole from all eternity, incapable of alteration or change: nothing foreign to the divine nature, -- no human nature, nor any element of human nature, -- can possibly come to thrust itself into that unity.* Others saw the impossibility of this position, and turned aside in the contrary direction. Whatever is added to any nature without belonging to the integrity of the same, may be reckoned to be either an accident, as whiteness and music, or to stand in an accidental relation to the subject, as a ring, a dress, a house. Considering then that human nature is added to the Word of God without belonging to the integrity of His nature, these [Nestorians] thought that the union of this supperadded human nature with the Word was merely accidental. Manifestly, it could not be in the Word as an accident, for God is not susceptible of accidents; and besides human nature itself stands in the category of substance, and cannot be an accident of anything. The alternative which they embraced was to conclude that the human nature stood in an accidental relation with the Word. Nestorius then laid it down that the human nature stood to the word in the relation of a temple to the Deity whose temple it was; and that union with human nature meant a mere indwelling of the Word in that nature. And because a temple has its individuality apart from him that dwells in it, and the individuality proper to human nature is personality, it followed that the personality of the human nature was one, and the personality of the Word another; and thus the Word and the Man were two persons: all which conclusion has been set aside by our previous arguments.
We must therefore lay it down that the union of the Word with the Man was such, that neither was one nature compounded out of two; nor was the union of the Word with human nature like the union of a substance with something exterior to it and standing in an accidental relation to it, like the relation of a man to his garment and his house: but the Word must be considered to subsist in human nature as in a nature made properly its own, so that that Body is truly the Body of the Word of God, and that Soul the Soul of the Word of God, and the Word of God truly is man. And though such union cannot be perfectly explained by mortal man, still we will endeavour, according to our capacity and ability, to say something towards the building up of faith and the defence of this mystery of faith against unbelievers.
In all creation there is nothing so like this union as the union of soul and body. So the Athanasian Creed has it: "As the rational soul and flesh is one man, so God and man is one Christ."* But whereas the rational soul is united with the body, (a) as form with matter, (b) as chief agent with instrument (B. II, Chapp. LVI, LVII ); this comparison cannot hold in respect of the former mode of union, for so we should be brought round to the [Eutychian] conclusion, that of God and man there was made one nature. We must take the point of the comparison then to be the union of soul with body as of agent with instrument. And with this the sayings of some ancient Doctors agree, who have laid it down that the human nature in Christ is an instrument of His divinity, as the body is an instrument of the soul.* The body and its parts, as instruments of the soul, come in a different category from exterior instruments. This axe is not my own proper instrument as is this hand. With this axe many men may work: but this hand is set aside for the proper activity of this soul. Therefore the hand is a tool conjoined with and proper to him that works with it: but the axe is an instrument extrinsic to the workman and common to many hands. Thus then we may take it to be with the union of God and man. All men stand to God as instruments wherewith He works: For he it is that worketh in us to will and accomplish on behalf of the good will (Phil. ii, 13). But other men stand to God as extrinsic and separate instruments. God moves them, not merely to activities proper to Himself, but to activities common to all rational nature, such as understanding truth, loving goodness, and working justice. But human nature has been taken up in Christ to work as an instrument proper to God alone, such works as cleansing of sins, illumination of the mind by grace, and introduction to everlasting life. The human nature therefore of Christ stands to God as an instrument proper and conjoined, as the hand to the soul.
The aforesaid examples however are not alleged as though a perfect likeness were to be looked for in them. We must understand how easy it was for the Word of God to unite Himself with human nature in a union far more sublime and intimate than that of the soul with any 'proper instrument.'
4.39 : The Doctrine of Catholic Faith concerning the Incarnation
4.40, 49 : Objections against the Faith of the Incarnation, with Replies