D. Stephen Long (The Perfectly Simple Triune God) claims that the doctrine of divine simplicity is designed to answer a question of Trinitarian theology, specifically, “How do we speak well of the mystery of the Holy Trinity?” Simplicity has been used to answer other questions: “What can be known of God by reason independent of faith or nature independent of grace?” and “How does God predestine all things to God’s own glory?” Those questions displace simplicity from its proper locus and, Long thinks, create more problems than they solve.

One implication of this overall argument is that simplicity isn't a piece of natural theology that can be established without recourse to Scripture. Long devotes a long early section to the first 43 questions of the Summa theologiae, in an effort to follow Thomas on his “journey” toward the knowledge of God, a journey marked by Thomas's declaration “This one is God” (ST 11, 3). Long denies that Thomas begins with rational theology, proofs accessible to all, or a theology that is extra-revelational. It is a single journey, and Thomas depends on revelation from the outset.

Thomas does not say that “human reason can investigate de deo uno but not de deo trino, nor that faith provides knowledge of de deo trino but not de deo uno. Nor has he divided his treatise into two: de deo uno and de deo trino. He refers to the entire treatise as de Deo and has explicitly stated that even that aspect of de Deo that human reason can investigate will require ‘divine revelation.' . . . Thomas not only denies any sharp distinction between de deo uno and de deo trino based on reason and faith, but he tells us that knowledge of God’s essence requires revelation as much as knowledge of God’s persons will.” For Thomas, and certainly for Long, “the Triune persons reveal what simplicity means so that it can be applied to God, who is known to have real distinctions, which at first glance appears to deny simplicity.”

Long acknowledges that Rahner identified a serious problem when he complained about the division of the treatise de deo uno from that de deo trino. That division can make it seem that everything that matters about God is said in the treatise on the One God, before we ever talk about the trinity; it can make metaphysical properties “more determinative for the doctrine of God than ‘salvation history'”; and the metaphysical properties can misname God. All very real and damaging errors, but they don't apply to Thomas: “We do not need to read divine simplicity, perfection, eternity, etc. as ‘metaphysical properties' if we understand the treatise de Deo as integrated.”

On Long's reading, then, Thomas is not the prince of ontotheologians, or a sign of the fall of Trinitarian theology. His Trinitarian theology is part of what Robert Jenson calls the “evangelization of metaphysics,” the internal transformation of the inherited metaphysical tradition by the gospel. Long provides a number of illustrations of this phenomenon, but one will suffice.

When Thomas begins to discuss Trinitarian theology directly, he focuses on the processions of the Son and Spirit. Procession, he says “signifies motion without (extra).” But that raises an obvious objection, and Thomas poses it: How can that which does not move (immutability) and does not contain an end (infinity) be signified by motion toward something? Internal processions in God also seem to violate simplicity, since a procession would suggest that there is diversity within God, on the assumption that “proceeding is diverse from that which it proceeds” (Long's wording). In short, it seem that “Everything [Thomas has] accomplished to this point moves in the direction of rejecting divine processions.”

In fact, though, Thomas has been preparing for precisely this discussion of processions, and has laid out a framework within which processions within God do not undermine the confession that God is one. Thomas notes that divine processions are to be rejected if they refer to a movement ad aliquid extra. But they don't; we can speak well of divine processions if we speak of them if we observe the grammar of simplicity, unity, and eternity. As Long summarizes, “Only if God is perfectly simple is it intelligible to posit procession ‘in' God without mixing error with the use of such a term. The ‘movement' internal to God is not from one place to another. God is infinite. It is not from one substance to a different one; God is simple. It is not from something lesser to something more, or vice versa; God is perfect. It is not from a beginning to an end; God is eternal. It is an immutable movement of ‘internal' processions, which he will, o on to argue, are only two - the processions of the Son and the Spirit.”

When used to describe Trinitarian persons within one God, “simplicity” doesn't exclude processions; it doesn't imply a blank and static divine essence. Simplicity ensures that the processions do not constitute a splitting-up of divine essence. Simplicity is not “an ‘attribute' or ‘property' of God known by reason alone; it is what allows theologians to identify the persons as the essence of God without positing four essences, or making creation a fourth divine hypostasis.”

Long's treatment of Thomas is compelling and presents an attractive Thomistic Trinitarianism similar (as Long acknowledges) to that found in Gilles Emery's Trinitarian Theology of St. Thomas Aquinas. Yet I'm left with the suspicion that there's a great deal more work to do here.

For example, Long writes: “God’s essence as simple, perfect, immutable, infinite, and eternal is one, and that one essence is revealed in three persons. Each person is the essence. The Father is the essence. The Son is the essence, and the Spirit is the essence. The Father, Son, and Spirit are also the essence.” Agreed. But it would seem that on the premise of simplicity each of these statements implies, conversely, that the essence is identical to the Person. When you say, “God is just,” by the principle of simplicity you're identifying the essence of God with the perfection of justice, because God simply is His perfections. If the Father is the essence and God is simple, then it would seem to follow in saying “the Father is God” you're identifying the essence of God as Fatherhood. But that's an error, since the essence isn't the Father alone but the Father of the Son whom He eternally begets by the Spirit. That may suggest that simplicity is being stretched even further than Long suggests. On the other hand, perhaps it's a mistake to apply simplicity to the essence-Person relation as opposed to the essence-perfection relation. If that's the case, then much of Long's argument (or Thomas's) is off-kilter, because Long says that simplicity applies especially to Trinitarian processions.

Another example: Long rejects the later Reformed use of simplicity as an argument for predestination and the divine decree. Long acknowledges that Thomas mounts an analogous argument: “Et sicut suum intelligere est suum esse, ita suum velle (‘Just as his understanding is his esse, so also is his will') (ST I 19.1). God’s knowledge and will are God’s esse; they do not divide up into composite parts, even if knowledge and will are not the same operation. For this reason, God wills who God is necessarily, but God wills all other things only with a conditional necessity that does not deny God’s freedom.” He thinks that Thomas goes too far in teasing out the import of this claim, locating predestination in God and using simplicity to describe the God-world relation rather than the essence of God Himself. But it's hard to see how Thomas could have avoided this shift. Even granting the distinction that Thomas makes between necessary and conditional will, it cannot be the case that there is any temporal interval between them. God didn't decide to create or to create this particular world at some point in eternity past; He didn't come to will or know this. Or, this: He knew all that He would do in the world, and even when we distinguish knowledge and will we cannot separate them: God wills to know all that will come to pass.

Now, that's all unobjectionable, until we say that will and knowledge are identical to essence. Saying that God's will concerning created things has a “conditional necessity” is a verbal dodge, and it doesn't seem to avoid what Long wants to avoid. We seem to be left with the implication that the conditional will is identical to the essence of God. It's not clear that the absolute/conditional distinction is enough to protect God's freedom in creating.

Long's book is rich and rewarding, a careful and judicious contribution to contemporary Trinitarian theology. But even those who affirm simplicity (as I do) may well conclude that things are not so, well, simple.