To better understand the doctrine of supposita and the role it plays in the doctrines of Trinity and Incarnation, we turn to Alfred J. Freddoso, Human Nature, Potency and the Incarnation (bolding added):
According to the Christian faith, as defined in this instance by the great Christological Councils and mirrored in centuries of liturgical practice and theological reflection, Jesus Christ is truly God and truly man. More precisely, he is a single divine person, the eternal Word, in whom are united, whole and unmixed, a divine nature and a human nature–so that he is, to quote Chalcedon, "one with the Father in his divinity and one with us in his humanity."
In expounding this doctrine medieval theologians fashioned the technical metaphysical notion of a suppositum (or hypostasis), i.e. an independently existing ultimate subject of characteristics.4 The philosophically astute will detect at once that, so understood, the concept of a suppositum is remarkably akin to that of an Aristotelian primary substance or individual(ized) nature. Indeed, had it not been for the doctrines of the Trinity and the Incarnation, medieval Christian thinkers would never have been led to assert that suppositum and substance (or: individual nature) are distinct concepts. To speak now only of the Incarnation, Christ's individual human nature, i.e. the individual composed of a body and an intellective soul and united to the Son of God, is a paradigmatic Aristotelian [primary] substance . . . .Yet, because of its metaphysical union with and dependence upon the eternal Word, this nature is not the ultimate metaphysical subject of Christ's characteristics–not even of his "purely human" characteristics. So in this one instance, known to us only by divine revelation, we have a substance which is not a suppositum, a substance which is metaphysically "sustained" by something distinct from it. To complete the picture, a person is just a suppositum with an intellectual nature, i.e. a suppositum essentially endowed with intellect and free will.
In technical medieval terminology, then, Jesus Christ is a divine suppositum or person, the Son of God, who has freely "assumed" and now "sustains" an individual human nature. What's more, this human nature is united to the divine person "hypostatically," i.e. in such a way that properties had immediately by the human nature have the Son of God as their ultimate metaphysical subject–in a manner analogous to that in which many properties had immediately by a proper part of a whole have the whole itself as their ultimate metaphysical subject.
Here are some key points that Freddoso makes:
1. Suppositum = hypostasis. The only difference is the difference between Latin and Greek.
2. Suppositum =df "an independently existing subject of characteristics."
3. A supposit is akin to an Aristotelian primary substance.
4. An Aristotelian primary substance is an individual nature, e.g., "the individual composed of a body and an intellective soul . . . ."
5. Although a supposit is akin to a primary substance, it is distinct from a primary substance.
Now what is the argument for (5)? What motivates the supposit/substance distinction? Why introduce supposits in addition to primary substances? Jesus Christ (JC) is an individual nature, a primary substance composed of a human body and a human intellective soul. But because of JC's "metaphysical union with and dependence upon the eternal Word," JC cannot be the ultimate subject of JC's characteristics, not even of his purely human characteristics. So "in this one instance" as Freddoso says, we must distinguish between suppositum and substance, between the ultimate subject of characteristics and that which it sustains, the primary substance.
On this interpretation of the Incarnation, Jesus Christ is a divine suppositum, the Second Person of the Trinity. The Second Person, God the Son, freely assumed or took on an individual human nature. Now an individual human nature is a primary substance, whence it follows that God the Son freely assumed a primary substance, a composite of body and intellective soul, and in such a way that the properties had immediately by the soul-body composite have God the Son as their ultimate metaphysical subject.
Is the doctrine of the Trinity, thus interpreted, logically coherent? If Jesus Christ is a divine suppositum, God the Son, the Second Person of the Trinity, then Jesus Christ is a necessarily existent substance. In the patois of 'possible worlds,' JC exists in all possible worlds. But the particular man he assumes, the individual human nature which is a composite of human body and human intellective soul, is not a necessarily existent primary substance, but a contingently existing one. For that man exists only in those possible worlds in which God creates a physical universe, and those worlds are only some of the possible worlds. This is because the act of creation is libertarianly free: the creation does not proceed of metaphysical necessity from the divine nature.
What we have, then, are two primary substances, one a necessary being and the other a contingent being. The first is a divine suppositum, God the Son. The second is a composite of human soul and human body. The first assumes the second in at least one possible world, the actual world. So a necessary primary substance assumes, and in assuming somehow becomes one with, a contingent primary substance.
Now it is clear that a necessary substance cannot be the same as (identical to) a contingent substance. Why not? Well, if x and y are identical, then they must share all properties. (Indiscernibility of Identicals) But the divine suppositum and the human body-soul composite substance that the divine suppositum incarnates in and as do not share all properties: the first has the modal property of being necessary, while the second does not. So they can't be identical.
If they can't be identical, then what is the relation that relates these two primary substances? What is the relation of 'assumption'? When Jesus of Nazareth walked the earth, he and the Second Person of the Trinity were in some sense one. In what sense?
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