God does not have accidents, says Augustine, and virtually every other theologian since.  It’s the corollary of God’s simplicity: He always is what He is, nothing added or taken away.  God cannot lost any attribute without losing His being as God.

But then along comes the incarnation.  God the Son takes a body.  What shall we call that?  It is not of God’s essence, since He did not always have a body and since He essentially has no body.  The Son’s body certainly seems to meet the definition of an accident.  As S. Marc Cohen succinctly puts it, “for a non-substance F to inhere in a substance x is for F to belong accidentally to x.”  That’s what the body (x) does in the Son (F).

And the body of the Son also fulfills other criteria of accidents.  Cohen again, explaining the asymmetry in the mutual dependence of substance and accident (whose mutuality Aristotle never explores):

“one might suppose that substances depend on non-substances in just the same way. For a substance can no more exist without any accidents than an accident can exist without belonging to any substance. Aristotle never discusses this ‘reverse’ dependence – he neither asserts nor denies it – but it is clear that he thinks that in some way the ontological dependence of non-substances on substances is asymmetrical. One possible account of the asymmetry is this.Since non-substances are accidental to the substances they inhere in, a particular substance can exist without the particular accidents that inhere in it. That is not to say that the substance might be lacking in accidents altogether, but only that it is capable of possessing different accidents from the ones it actually has. A particular accident, on the other hand, is ontologically dependent on the particular substance that it inheres in; it could not exist without that particular substance.”

That asymmetry certainly exists in Christ: It is the Son, not the body, that is capable of existing without the accident of the body.  The body cannot exist without the Son.  This is the an/enhypostatic relation of the Word and the human nature.

How can we maintain God’s simplicity and still affirm an incarnation?  One option is to say that while the Son assumes the body, it does not “inhere” in Him in the way an accident does.  It is not, strictly, His own.  That’s not orthodox, and it is a great credit to the classical theologians that they spotted the error in that, and rejected it when they rejected Nestorian and other sorts of Christological heresies.  The majority option has been to hold the two side-by-side with all their tensions: God is simple, and yet He takes on the flesh, which is accidental to the Son.  God is simple “in Himself,” but still capable of taking on flesh.  There’s nothing wrong with tension and perhaps the best we can do.

The other option, it seems, is to challenge the framework of discussion, that is, the substance/accident framework into which simplicity is often made to fit.  That will no doubt leave us with plenty of tension, but perhaps we can relocate the tension in a fruitful way.

Perhaps the way to go is to work from the puzzle of the incarnation back to the Trinity, and to recognize that in the Triune God attributes that are typically “accidental” are rendered substantial.  Arius and many others had a hard time seeing “Father” as a strict name for God, since Fatherhood seems to be an accident, dependent on the existence of the Son.  Again, the classical Trinitarian theologians spotted the error there, and for all the difficulties it raised, rightly insisted that the Father is eternally Father precisely because He eternally has a Son.  The “accident” of Father-Son relations is, in the Trinity, ontological and absolute.  Trinitarian doctrine seems to cut through the substance/accident contrast; by virtue of Trinitarian theology, we do not make the incarnation essential to God, but we do see (through a glass darkly) how God might be incarnate-able.

We want to affirm what the classic doctrine of simplicity gets at: God is God, and never less than fully Himself.  God cannot be God without being wholly Himself, wholly righteous, good, just, love.  But we should affirm this in a Trinitarian framework: God is wholly and completely Himself precisely as Father, Son, and Spirit.  God is simple in being the Father who begets the Son in the Spirit.  We can run all the attributes of God through that grid.  God is good in that He is the Father who is not stingy but eternally begets the Son through His Spirit, and in that He is Son who eternally honors the Father in the Spirit.  Is the Son an “accident” of the Father?  No: As Athanasius says, the Son is “proper” to the Father’s substance.  But is the Son simply another way to say “Father”? No: The Son is begotten by the Father, and “toward” the Father (John 1:1-3).  The Son is not a mutable “attachment” external to the Father and “inhering” in Him; at the same time, He is not identical to the Father.  Again, this seems to blur the edges of substance/accident distinctions.

I suspect too that the substance/accident distinction might be overcome with a Jenson-like narrative construal of the Trinity.