11. The Union of Divine and Human Natures in Christ

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Lecture Notes

Main Topics #

The Hypostatic Union: Definition and Nature #

• The union of divine and human natures in Christ occurs in the hypostasis (person), not in nature
• Christ possesses two complete natures (divine and human) but only one person—the divine person of the Word
• This is fundamentally distinct from having two natures that form a new composite nature
• The union is secundum hypostasim (according to hypostasis/subsistence), not according to accidental union or confused union of natures

Condemned Heresies and the Catholic Position #

• Monophysitism/Eutychianism: Confused union that destroys both natures, resulting in “something that is neither God nor man”
• Apollinarianism: Attacked for similar confusion of natures
• Nestorianism: Advocates two persons with merely effectual (moral/voluntary) union of wills
• The Catholic Position (confirmed at the Fifth Ecumenical Council): One hypostasis of God and man, neither confusing nor dividing the natures
• The Fifth Synod condemns those who follow the iniquity of Apollinaris and Eutyches or who rejoice in division like Theodorus and Nestorius

Why the Union is Not Accidental #

• Accidental Union: Would mean human nature could exist apart from the Word while still being united to it—inherently contradictory
• The Core Principle: Accidents can be present or absent without corrupting the subject, but human nature assumed by the Word cannot be separated from it without destroying the Incarnation itself
• Subsistence Requirement: The human nature does not have its own hypostasis; it subsists in the divine hypostasis of the Son
• If human nature had its own hypostasis, there would be two persons (Nestorianism), making the incarnation a mere conjunction

The Relation of Union: Real vs. Rational #

• The union itself is a relation (relatio) between divine and human natures
• This relation is real in the human nature because the human nature is created and genuinely changed by assumption
• This relation is only according to reason in God because God is immutable and unchangeable—no real change occurs in God
• Principle from Prima Pars: Every relation between God and creature is really in the creature by whose change the relation comes to be, but is not really in God, only according to reason
• Therefore, the union must be said to be something created, existing really only in the created human nature

Illustration from Philosophy: Relations and Their Natures #

• Real Relations: Have both termini existing and a real foundation (e.g., my height in relation to your height)
• Relations of Reason: Exist only in the mind’s consideration (e.g., today before tomorrow, when neither fully exists; genus and species as logically related)
• Reciprocal Relations: Seeing to seen—the relation of seer to seen is real, but the relation of seen to seer is only rational, not a real property of what is seen
• God’s relation to creatures as creator is real in creatures but only rational in God, parallel to the union of natures in Christ

Key Arguments #

Against Accidental Union (Objection 2) #

Premise: That which comes after complete being comes accidentally unless drawn into the communion of that complete being.

Application: Human nature came to the Son in time, after his eternally complete divine being. Why is this not accidental?

Response via Resurrection Analogy:

• At resurrection, body comes to pre-existing soul but not accidentally
• Why? Because body is drawn into the very same being (esse) that the soul has
• Body shares vital being through the soul; it is not joined accidentally
• Similarly, human nature is drawn into the divine being by assumption to the divine hypostasis
• The difference: unlike whiteness (which has its own distinct being from man), human nature assumes the subsistence of the divine person

Against Substance/Accident Dichotomy (Objection 3) #

Objection: “Everything is either substance or accident. Human nature does not pertain to the essence of the Son. Therefore human nature must be accidental to him.”

Resolution via Hypostasis Distinction:

• Accident is divided against substance
• Substance is said in two ways:
  1. Essence/natura—what something is
  2. Suppositum/hypostasis—the individual underlying subject
• It suffices for non-accidental union that there be union according to hypostasis, even if not according to nature
• Union secundum hypostasim is genuine substantial union without requiring oneness of nature

Against Relations of Reason Undermining Reality #

Concern: If the union is a relation of reason in God, is it therefore not real?

Response:

• Relations of reason can express genuine truths about reality
• The union is truly real—but real precisely in the human nature, not in God
• One can truly say “the divine nature is united to human nature” because the relation grounds a real fact
• The relation itself, as such, is not diminished by being in the creature; it expresses the real hypostatic assumption
• The absence of change in God does not make the assumption unreal—it makes God immutable while truly assuming human nature

Important Definitions #

Hypostasis (ὑπόστασις): An individual substance that subsists in itself, especially a rational substance; the underlying subject of predication. Often identified with suppositum in Latin usage.

Suppositum: The underlying individual subject that exists in itself and receives predication; equivalent to hypostasis.

Person (persona): Individual substance of a rational nature (following Boethius). In the Incarnation, Christ is one person—the divine person of the Word.

Subsistence (subsistentia): The condition of having complete being in oneself (not existing in another as form in matter, or as an accident in a subject).

Hypostatic Union (unio hypostatica): Union of two complete natures (divine and human) in one hypostasis/person.

Union according to Nature (secundum naturam): Would require the two natures to merge or compose into one nature (heretical).

Union according to Hypostasis (secundum hypostasim): The authentic Christian position—two natures in one person, with no confusion or mixture.

Relation of Reason (relatio rationis): A relation that exists only in the mind’s consideration, not in external reality (e.g., genus to species, today to tomorrow).

Real Relation (relatio realis): A relation grounded in real properties and existing in external reality; requires both termini to exist.

Examples & Illustrations #

The Line and Point Analogy #

• A second line drawn to an existing point becomes “terminated” at that point
• The point is not changed or composed with the line
• Similarly, human nature is “drawn to” the pre-existing divine person without the divine person being changed or becoming composite
• The endpoint of the second line already existed; it was not created by the line’s arrival

Body and Soul at Resurrection #

• When the body is reunited to the soul at resurrection, it does not come accidentally
• Why? Because it is drawn into the very same being (esse) that the soul possesses
• The soul has being not only in the body but also apart from it (because it understands)
• At resurrection, the body again shares in the being of the soul, not acquiring a new being accidentally
• This illustrates how something can come “in time” without coming accidentally

Whiteness vs. Substantial Form #

• Whiteness is accidental: the being of whiteness is other than the being of the man to which it belongs
• A man can lose whiteness without losing his being as a man
• By contrast, the soul is a substantial form: the body shares in the soul’s very being, not a merely accidental overlay
• Human nature in Christ is united substantially to the divine hypostasis, not as an accident would be

The Geometer Analogy #

• A man becomes a geometer by acquiring geometry—but “geometer” expresses an accidental being
• It is not necessary to a man’s being that he be a geometer
• This illustrates the principle that not everything coming to a subject comes substantially

The Hand as Instrument (Joined vs. Separate) #

• The hand is an instrumentum coniunctum (joined instrument) belonging to the hypostasis of the person
• A sword is a separate instrument, not part of the person’s hypostasis
• Nestorius claimed human nature was merely a separate instrument (like a sword) through which the Word acted
• The Catholic faith: human nature is a joined instrument, pertaining to the unity of the divine hypostasis
• The soul animates the body; the divine person assumes human nature into substantial union

Clothing Analogy #

• Human nature is compared to clothing in relation to the Word
• But crucially: not as a merely accidental union
• Rather, clothing is modified and conformed to the shape of the wearer
• Similarly, human nature is made better (elevated, glorified) by assumption, while the Word remains unchanged
• Augustine explains this in his Eighty-Three Questions

Questions Addressed #

Why is the union not accidental? #

• The Problem: Human nature came to the Son in time; whatever comes after complete being seems to come accidentally
• The Solution: The human nature is not merely added to the Son as an accident; it is drawn into the communion of his being by assumption to his hypostasis
• The Parallel: As the body at resurrection shares the being of the soul without coming accidentally, human nature shares the being of the divine person

How can a relation in God be merely rational without undermining the reality of the union? #

• The Principle: God’s relations to creatures are always real in the creature but only rational in God (due to God’s immutability)
• The Application: The relation of union exists really in the human nature (which is created and assumes a new relation to the Word) but is only according to reason in God (who undergoes no change)
• The Truth Preserved: We can truly and genuinely say “The Word is united to human nature” because the relation grounds the real assumption and hypostatic presence

Is the union itself created or uncreated? #

• The Objections: (1) Nothing in God can be created; the union is in God. (2) The end of union (the divine hypostasis) is uncreated, so the union should be uncreated.
• The Resolution: The union, as a relation, is real in the created human nature and therefore is something created
• The Principle: The union is not in God really but only according to reason; what is really in God is uncreated, but what is really in the creature (the human nature) is created
• The Distinction: The divine hypostasis is uncreated, but the union as the relation between natures, existing in the created human nature, is created

Notable Quotes #

“Since in many ways unity can be understood… those who follow the iniquity of Apollinaris and Eutyches… speak of confused union… But Theodorus and Nestorius… introduce a union of wills. But the holy Church of God, rejecting the impiety of both… confessed the union of the word to the flesh according to composition [i.e., hypostasis], according to the subsistence.” — Fifth Ecumenical Council

“If someone attempts to introduce into the mystery of Christ two subsistences or two persons, such a one is accursed.” — Fifth Ecumenical Council (paraphrased by Berquist)

“As Damascene says in the third book, it is not necessary that in every way and with no defect to assimilate what examples or likenesses [are]. For what is in every way alike would be the same thing and not an example.” — St. John of Damascus

“That which comes after complete being comes accidentally unless it be drawn into the communion of that complete being.” — St. Thomas Aquinas

“The body comes to the soul already existing… it does not come accidentally. Why? Because it is taken to the very same being.” — St. Thomas Aquinas (resurrection analogy)

“It suffices to this that there be a union according to hypostasis, even though it is not made a union according to nature.” — St. Thomas Aquinas

“Wisely and slow, they stumble that run fast.” — Shakespeare, Romeo and Juliet (cited by Berquist in connection with Aristotle’s method in Metaphysics V)

“The most common mistake is from mixing up the senses of the word.” — Aristotle (referenced by Berquist)

“As much as they can, we’re trying to understand… these mysteries… with St. Thomas as a teacher.” — Second Vatican Council (cited by Berquist)