I know class hasn't really gotten off and running yet, but lately I have been doing undergraduate research on the Nestorian heresy and the supposed heretic Nestorius and I have certainly been ruminating upon a key question thereof, namely, that of the hypostatic union. The hypostatic union, for those in this class who aren't armchair theologians like myself is the union of Christ's two usia or essences into one prosopon or person. The key question thereof is primarily the question of by what mechanism a human nature and a divine nature may be united into one person. A second question would be on how those natures interact.
To start off with, one must first engage in the study of the philosophy of lanugage, more specifically, the philosophy of its usage by the theologians and Hellenized Christians of the early Church. In their worldview, which is heavily influenced by neo-Platonism each existing animal, object, thing or person including man has their own substance or essence (usia) and from this essence is derived life or existence. The usia, which is invisible, is what the object is in itself, in its innermost being, apart from being perceived. Each usia has a distinct nature, (physis), i.e., the totality of qualities, features, attributes, and peculiarities (both positive and negative) which give it its individual stamp or character. every nature is founded upon its own usia; there is not nature without an usia; and usia without a nature. Thus usia and nature are correlative terms, each of which implies and requires the other. But neither the usia nor the nature is fully present effective without a third equally indispensable element, the prosopon. None of the three can be separated from the other two, nor can the usia and the nature be recognized externally apart from the prosopon which reveals them. No ordinary entity or individual being has more than one each of these three components, nor does any one of the three have more than one each of the other two.
Nestorius stressed the Christological point that God the Word and the human nature of Christ were never mixed. These two were "alien to one another." (as per Nestorius's Bazaar of Heracleides) In the same breathe, he further explained that these two things, the manhood (usia) of Jesus and the usia of God, were joined together in the prosopon (one prosopon of both natures). These concepts are presented in the most explicit terms. Nestorius describes that the union of the two natures are in the one prosopon of Jesus Christ, and denies that it should be described as a union of prosopa. He maintains through out the discussion a firm belief in the God-Man which is a union of the divine Logos and the separate individual man Jesus from the moment of conception.
It is crucial that we understand the concept of prosopon as Nestorius presented it. Prosopon was understood in two senses. The first sense is a more general one. It may be called the external appearance of a thing which has substantive reality and distinct qualities. Prosopon in this sense is another aspect of physis or usia. The second sense which he understood prosopon was in the same way we understand the word "person." When we think of a name we think of a person. When we think of the name Jesus Christ or Joshua the Messiah (Yeshua ha Meshiach) we think of a person. In ordinary usage we do not separate a name from a person. After we have understood these two senses of the term prosopon Nestorius introduces a unifying factor between God the Logos and that person who the disciples saw. He uses a Latin term - communicatio idiomatum - which simply means a transfer of attributes. So God the Logos (understood in the first sense of divine nature) became the prosopon of Jesus Christ's human nature.
Thus is the sum of my research so far and I hope to continue my thoughts and research upon the topic and be able to represent at least a reasonable theory as to how human and divine natures may be combined into the person of Christ.