When Christ was conceived, exactly what did He take from Mary? Was one of her eggs fertilized, or was a fertilized egg implanted, and if so, then how could He be human? What of her human nature was passed on to Him, and what was not?
This union of the two natures in one Person has been for centuries called a hypostatic union, that is, a union in the Divine Hypostasis. What is an hypostasis? The definition of Boethius is classic: rationalis naturae individua substantia (P. L., LXIV, 1343), a complete whole whose nature is rational. This book is a complete whole; its nature is not rational; it is not an hypostasis. An hypostasis is a complete rational individual. St. Thomas defines hypostasis as substantia cum ultimo complemento (III:2:3, ad 2um), a substance in its entirety. Hypostasis superadds to the notion of rational substance this idea of entirety; nor does the idea of rational nature include this notion of entirety. Human nature is the principle of human activities; but only an hypostasis, a person, can exercise these activities. The Schoolmen discuss the question whether the hypostasis has anything more of reality than human nature. To understand the discussion, one must needs be versed in scholastic Philosophy. Be the case as it may in the matter of human nature that is not united with the Divine, the human nature that is hypostatically united with the Divine, that is, the human nature that the Divine Hypostasis or Person assumes to Itself, has certainly more of reality united to it than the human nature of Christ would have were it not hypostatically united in the Word. The Divine Logos identified with Divine nature (Hypostatic Union) means then that the Divine Hypostasis (or Person, or Word, or Logos) appropriates to Itself human nature, and takes in every respect the place of the human person. In this way, the human nature of Christ, though not a human person, loses nothing of the perfection of the perfect man; for the Divine Person supplies the place of the human.
It is to be remembered that, when the Word took Flesh, there was no change in the Word; all the change was in the Flesh. At the moment of conception, in the womb of the Blessed Mother, through the forcefulness of God’s activity, not only was the human soul of Christ created but the Word assumed the man that was conceived. When God created the world, the world was changed, that is. it passed from the state of nonentity to the state of existence; and there was no change in the Logos or Creative Word of God the Father. Nor was there change in that Logos when it began to terminate the human nature. A new relation ensued, to be sure; but this new relation implied in the Logos no new reality, no real change; all new reality, all real change, was in the human nature. Anyone who wishes to go into this very intricate question of the manner of the Hypostatic Union of the two natures in the one Divine Personality, may with great profit read St. Thomas (III:4:2); Scotus (in III, Dist. i); (De Incarnatione, Disp. II, sec. 3); Gregory, of Valentia (in III, D. i, q. 4). Any modern text book on theology will give various opinions in regard to the way of the union of the Person assuming with the nature assumed…
