Having read Maximus and things about Maximus lately, I've been careful to listen for any important distinctions between hypostasis and persons, but as far as Saint Thomas is concerned, person only specifies the nature of the hypostasis--he repeats this several times in this question. At one point, he distinguishes individual substance from hypostasis which are not commonly distinguished, but hypostasis adds the notion of completion. It seems that St Thomas is calling the humanity of Christ an individual substance, but not a hypostasis since it is in the Word that it exists. In 2.6, Thomas makes the interesting claim that the hypostasis is "midway" between the nature and the accidents, so that substantial union or accidental union are extremes of which the hypostatic union is a mean. This is interesting. Hypostasis is a mean at least in that it shares something with the accounts of nature and accident: both nature and hypostasis share the name "substance" and are not in something, whereas hypostasis is closer to accidents than nature in that hypostasis is the subject of accidents.
And so there is then the question of Christ's esse. When Thomas talks about whether the union of the Incarnation is created, he says that it is created because it begins to be. So there is created esse in the humanity of Christ. And yet it is the hypostasis that exists completely, and later on (17.2, though I didn't look closely) where Thomas will say that Christ has only one esse and not two. So then which will it be? The esse of the Word is uncreated and the same with the Father and the Spirit, no? Yet the humanity and union itself is created, so does not Christ have created esse? I suppose I will find out in due time... Another note, St Thomas talks about "personal esse" (Christ's union) and how this cannot be merited, unlike habitual grace (like we have in our soul) which can be merited in a certain way. One more note: In 2.7.ad3, to-be-created pertains more to esse than to relation; here esse refers to the union/nature whereas relation indicates the person. More familiar example: that I am created has to do with my esse and not my relation (neither to my parents nor even to God, since that relation is logically consequent to my esse).
In 2.9, there is a rare case in the Summa of Thomas replying to the Sed contra and disputing the authority of St Augustine! Augustine claims that humanity is more in the Son than the Son is in the Father. St Thomas says he is wrong, and yet says the humanity of Christ is united to the Son in a hypostasis (they are the same person) whereas the Father and the Son are not the same person, but wholly one according to nature and power. I'm inclined to agree with St Thomas, and yet there's something that makes me want to consider it further (and perhaps read Augustine in context). Earlier in the Summa, Thomas says the "Father" is said per prius of the person and per posterius of the Godhead. So would the unity of the person somehow be greater than that of the Godhead? (Remember, God is absolutely one; the only distinction within God is relative--real, but relative.)
At this point, I am looking forward to question 17 which contains only two articles: Is Christ one or two? And does he have only one esse? I also want to write something clear and consistent (and true) about the distinction between in re and in ratione. This distinction is very important in understanding God's tri-unity, and does not stop being important in considering the incarnation. It is surprising to read how created the Incarnation is, and yet how thoroughly divine. His treatment of Nestorius position is helpful as well, since he gleans a lot of the positive things in his teaching. He even calls one position in the Sentences worse than Nestorius. Oh my.
