The Incarnation of the Word is like to this union, hence called hypostatic or personal union. The second person of the Trinity united himself to the entire human nature, constituted of body and soul, in a manner so close and intimate as to be _himself_ the subsistence of the human nature; the latter never enjoying a subsistence of its own, because, contemporaneously to the very first instant of its existence, it became the internal, the immediate, and the most intimate organ of the Word of God, and subsisted of the subsistence of the Word, so that it never bore the attribution and solidarity of those actions which have an immediate origin in human nature, but the attribution and solidarity, and, consequently, the moral worth, of those actions belonged to the personality of the Word, according to the axiom that _Actiones sunt suppositorum_.
Hence the union between the Word of God and his human nature was not a moral union, which always implies the distinct individuality and personality of the two terms united, as Nestorius thought, and many would-be Christians of the present day seem to hold.
Nestorius was ready to grant that the union between the Word and human nature was as high and intimate as possible, so far as moral union can permit; but never would he concede that it was any higher than simple moral union, which kept whole and entire the two individualities united. Consequently, he admitted two persons and two individualities in Christ--the _Word_ of God, and the man called Christ. From which theory it follows that our Lord was a mere man--a saint, if you will, the highest of all saints, yet simply a man.
Catholic doctrine, on the contrary, teaching that the union of the Word and the human nature was personal, inasmuch as the divine person of the Word was the subsistence in which his human nature subsisted, teaches consequently, at the same time, that in Christ there is one person, one individuality--the divine personality of the Word; that therefore Christ, the new individual, is God, being the second divine person, in which both his divine and human nature subsist. Nor was the human nature of this new individual so absorbed by the divine personality as to cease to be a substance, as Eutyches affirmed, who upheld, it would seem, a fusion and a mixture of the two natures altogether inconceivable and absurd.