“In the beginning was the Word, and the Word was with God, and the Word was God… And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth.” — JOHN 1:1.14
Some people claim that Christ cannot be truly God and truly man at the same time. I disagree. And I will try to explain how this is possible, from a philosophical point of view. My starting point will be the doctrinal statements of Chalcedon and of the Athanasian Creed. In Chalcedon we profess:
We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person and one Subsistence, not parted or divided into two persons, but one and the same Son, and only begotten, God the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
And in the Athanasian Creed we say that Christ, truly God and truly man, is “with a rational soul and human flesh.” And further we profess that He is one “not by his divinity being turned into flesh, but by God’s taking humanity to himself. He is one, certainly not by the blending of his essence, but by the unity of his person.” But some people disagree with this. They claim that the person is the soul, or the union of body and soul, which would mean that Christ’s soul (or soul-body union) would constitute a person on its own. Thomas D. Senor writes that “this claim — that in addition to taking on a body, God the Son took on a human mind — brings with a rather serious philosophical difficulty. For in the standard case (and, as seen likely, in every other case), a human body and mind combination composes a human person.”
J.P. Moreland and Scott B. Rae writes: “You are essentially your soul—same soul, same person; different soul, different person.” I disagree. The person is of course closely connected to the soul, as it is connected to its nature. And the human soul, writes L.W. Geddes, “belongs to the nature as a part of it, and is therefore not a person, even when existing separately.”
Before I go on, I will take a look at the teaching on the Incarnation given by J.P. Moreland and William Lane Craig. They try to give an heresy-free reinterpretation of the thoughts of Apollinarius (d. ca. 390), and writes that he “correctly discerned … that if we are to avoid a duality of persons in Christ, the man Jesus of Nazareth and the divine Logos must share some common constituent which unites their two individual natures.” They also ask, “How does one have to complete natures without two persons?” And takes a look at the Church’s condemnation of Monotheletism, and wonders “how one can have two separate wills and operations without two persons.” The fault of Moreland, Craig Senor and Apollinarius is two-sided.
Firstly, they use a confusing concept, namely the concept of ‘individual nature.’ A nature isn’t ‘individual,’ but ‘universal.’ I believe that the universals exists in the particular things, and not independent of these.
Secondly, they believe that the essential properties of human nature — including soul and body — are sufficient for personhood. I believe that these properties are necessary, but not sufficient, for personhood. In his tract, A Treatise Against Eutyches and Nestorius, Medieval philosopher Boethius defined Nature as “the specific property of any substance” and defined a person as “the individual substance of a rational nature.” (chapter 4) The person is a substance that carries, has and owns his properties. Human nature is a property with certain properties under itself, both essential — properties needed to exist (reason, mind, will, etc.) — and accidental — properties that can change (weight, height, shape, etc.) A substance owns its properties.
It seems to me that a person — the individual, rational substance — is ontologically prior to its properties, including its nature. These properties are universal and exist in — and not independent of — the substance that owns them. Boethius writes:
You must consider that all I have said so far has been for the purpose of marking the difference between Nature and Person… The exact terms, which should be applied in each case, must be left to the decision of ecclesiastical usage. For the time being let that distinction between Nature and Person hold which I have affirmed, viz. that Nature is the specific property of any substance, and Person the individual substance of a rational nature. Nestorius affirmed that in Christ Person was twofold, being led astray by the false notion that Person may be applied to every nature. For on this assumption, understanding that there were in Christ two natures, he declared that there were likewise two persons. (chapter 4)
Thus we say that Christ is one person whom possesses the essential (and accidental) properties of His natures, divine and human. Boethius, therefore, tells us that Christ “is God and man, being God by nature, man by assumption. And in Him nature becomes double and substance double because he is God-man, and One Person since the same is man and God. This is the middle way between two heresies, just as virtues also hold a middle place.” (chapter 7)
Thus Boethius shows us that both heresies really destroy salvation. Because neither one is the assumption of humanity. In one case (Monophysitism), the human nature isn’t present at all. In the other case (Nestorianism), nothing is really assumed, the Incarnation being reduced to a close personal relationship with God, and not the real assumption of humanity. It seems to me that the classical, orthodox teaching on the Incarnation isn’t just more dogmatically correct, but certainly more philosophical correct.
When the Word assumed humanity, He assumed all the essential properties of human nature, including their proper organs, body and soul. He did this to give us a part of what is His divine life. “For in him the whole fullness of deity dwells bodily, and you have been filled in him, who is the head of all rule and authority.” (Col. 2:9-10) “For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich.” (2. Cor. 8:9)
God became man to recreate humanity in God’s Image, life humanity up from sin, and take the transformed humanity back to Heaven, to give us the possibility of becoming “partakers of the divine nature” (2. Pet. 1:4). Or, as Robert D. Crouse writes:
Perhaps the finest statement of the broadest implications of Chalcedon would be St. Thomas’ great pronouncement, that “grace does not destroy nature, but perfects it”.
Notes & references:
 This of course does not mean that it’s true, but that it is logically possible.
 Senor, Thomas D. (2007) “The Compositional Account of the Incarnation.” Faith and Philosophy 24:1, p. 53 (pp. 52-71)
 Moreland, J.P. & Rae, Scott B. (2000) Body & Soul: Human Nature & the Crisis in Ethics. InterVarsity Press, p. 180
 I have much respect for these two, but disagree with their views on the Incarnation.
 Moreland, J.P. & Craig, William Lane (2003) Philosophical Foundations for a Christian Worldview. InterVarsity Press, p. 608
 Ibid, pp. 601.602