Opponents of Divine Simplicity argue that the doctrine is incompatible with free choice in God. But is this objection coherent? The objection seems to use (unintentionally) a sleight of hand in its premises, one that implicitly assumes a real distinction between God's nature on the one hand and his intellect and will on the other. But such a real distinction is exactly what proponents of Divine Simplicity deny.
To expand on this, under Divine Simplicity, we cannot claim that God's nature in any way determines his intellect and will. His intellect is his nature. His will is his nature. In a creature (at minimum a material one) we can say that intellect and will follow from the nature of the creature. The intellect and will of a creature have dependence on the nature, which is in some way prior to them. But there is no nature prior to God's intellect and will. His intellect and will are primeval, ultimately first, and are in no way determined to what they are by any nature that is in any way prior to them. Pure Act doesn't determine God's intellect and will to immutability or certain effects. Pure Act is God, is his Intellect, is his Will. (As a short aside, the Fifth Way also demonstrates by other means that Intellect and Will must be prior to any unintelligent nature, and ultimately First and in no way determined. Aquinas brings up this line of argument in various places, such as in De Potentia and Summa Contra Gentiles.)
Given the radical priority of the Intellect and Will of God, it is absurd to reject that God's effects are self-determined. For what is self-determination but the knowingly willing of something without any external determination of the intellect and will to its effects? I can imagine here the objection "But could God's will have been different? If there is no passive potency in God as you say, it could not have been different." And this is where the sleight of hand comes into play, because (while I still acknowledge the conceptual difficulties of Divine Simplicity on this matter) this line of objection only seems to follow if the objector has some (unspoken) premise of real distinction between God's nature and his Intellect and Will such that the Intellect and Will are determined by what he is rather than self-determined (through themselves).
Returning to the conceptual difficulties, the question then seems to be whether there is any active potency in God (whether there is power in God to do other than he has), for perhaps self-determination might be granted in some sense but free choice is not. But I don't believe the objector is successful in eliminating active potency in God (I think that could only follow if God's nature determined his Intellect and Will/effects, which just begs the question against the doctrine of Divine Simplicity). God as Subsistent Being (and not just the being of this world) must be in his essence (his intellect, will, and power) all possible things. And it is not incompatible with Being that I drank orange juice instead of milk for breakfast, or that unicorns could exist, so these non-actual possibilities must be contained in God even though they are not effected. We also have the other conclusions that the road to Divine Simplicity has already provided us. That there is no privation in God (no passive potency). That God is Goodness Itself. That God is perfectly good in himself. All of which just means that God is Pure Act. That being so, and God already being perfectly fulfilled, then any effect of a thing other than God does not increase (or decrease) God's perfection, goodness, fulfillment, joy, or so on. And if that much has already been shown, then this world cannot be absolutely necessary for God to effect, nor can any other world, nor even this world more necessitated than any other. The only necessity that can be applied is what Aquinas calls the necessity of supposition. If Socrates sits, it is necessary that he is sitting. If God wills this world (which under Divine Simplicity can only be coherently considered a self-determined effect) then it is necessary that this world be. And so it is that there is active potency in God, and if God self-determined to these effects instead of those effects, that is free choice.
I do not expect this to entirely satisfy. Indeed, I'm not entirely satisfied with what I've written and feel it is incomplete. And certainly there are other areas for discussion, such as whether a difference in the cause is needed for a difference in effects for all things (such as freedom in some types of human action, or some may bring up quantum mechanics). But I do feel there is something to be had in what I've written on the ultimate primacy of the Intellect and Will (truly just the Divine Essence) in God (or as God), and that it is incoherent to speak of God as anything but self-determined in effecting what he does if one accepts the doctrine of Divine Simplicity.
