Note that compatibilists and libertarians might both agree that it needs to be possible that one would have "done otherwise"; they differ in how strongly they take this requirement. Compatibilists might hold this to be an epistemic requirement; it's possible for what we know about this agent (who might be ourselves) to be substantially the same, though he acts otherwise. This is because all of the differences were unobservable. The libertarian thinks that one must be able to act otherwise even if the history of the universe were exactly the same in all of its details, known and unknown to those assessing the action.

A compatibilist hylomorphic dualism seems to be possible, but I think Thomistic free will is incompatibilist. I do think that this requires rejecting the causal closure of the physical, and appealing to the hylomorphic account does not seem to get around this. Thus there is a spookiness to Thomistic free will that would upset naturalists.

Thomists typically don't trumpet this commitment, which is understandable but unfortunate. Somewhere John Haldane was discussing the causal closure of the physical and its relation to physicalist theories, and he made a comment about how he does not commit himself to it; unfortunately I don't recall where it was. Another important article on this theme is Anscombe's "Causality and Determination". She writes:

Ever since Kant it has been a familiar claim among philosophers, that one can  believe in both physical determinism and 'ethical' freedom. The reconciliations have always seemed to me to be either so much gobbledegook, or to make the alleged freedom of action quite unreal. My actions are mostly physical movements; if these physical movements are physically predetermined by processes which I do not control, then my freedom is perfectly illusory. The truth of physical indeterminism is thus indispensable if we are to make anything of  the claim to freedom. But certainly it is insufficient. The physically undetermined is not thereby 'free'. For freedom at least involves the power of acting according to an idea, and no such thing  is ascribed to whatever is the subject  (what would be the relevant subject?) of unpredetermination in indeterministic physics.

She has some further comments in the paper on statistical laws that might make the rejection of causal closure slightly more palatable.

Another paper, which I haven't gotten around to reading, is John J. Davenport's "Aquinas's Teleological Libertarianism," collected in Analytical Thomism: Traditions in Dialogue, eds. Craig Paterson and Matthew S. Pugh. See also the chapter "Freedom: Action, intellect and will" in Eleonore Stump's Aquinas; she also determines that Aquinas's account is a form of libertarianism.

The positive Thomistic case, I think, has to differ from standard arguments for libertarianism. Man is determined to seek beatitude, but no particular action is sufficient for bringing it about. Actions are seen as attractive under the universal aspect of the good, and particular goods do not necessitate the will. Davenport writes, "It has long been observed by scholars that free will on Aquinas's account  is incompatible with natural necessitation (or causal determinism), since he holds that nothing outside the agent's will can be an efficient cause of his volitions."

bulldog91 wrote:

If this is the case, then it seems like the interaction problem is just as bad for (this version of) hylemorphic dualism as it is for substance dualism, since the mind must be able to exert some degree of EFFICIENT causal control over the material operations of the brain in order for our actions to result from anything more than the operation of physical/biological/neural laws. Is this a fair statement?

Probably yes. (I suppose whether the interaction problem is "just as bad" for hylomorphic dualism depends on what one thinks makes the interaction problem bad at all. If what makes it bad is the rejection of the causal closure of the physical, then yes, I think Thomists would reject that. Perhaps one could argue it is less mysterious than substance dualism in other respects.) I would be interested if anyone disagrees.

Perhaps one could argue that, though something like quantum indeterminism is insufficient for free will, it might be part of the Thomist account. I am not sure whether such an account, consistent with the laws of physics, is possible.

Dennis wrote:

People have argued that things like quantum indeterminism is insufficient for free will.

And surely quantum indeterminism is insufficient for free will.

Dennis wrote:

They've usually moved on to attack the whole notion of necessity in causation from there.

This arguably is one way to fudge one's way out of the distastefulness of denying causal closure. This is Anscombe's point: the notion of causation need not include necessity. The "laws of physics" are often understood as being deterministic above the quantum level, but so what? Nancy Cartwright thinks that the laws of physics are false; they state what contributions different factors make but are not true descriptions of how bodies move. If the laws of physics have this role, then perhaps denying causal closure does not even violate the laws of physics.

Dennis wrote:

What does Dr. Cuypers have to say here?

You mean Stefaan E. Cuypers, who authored "Thomistic Agent-Causalism" in Haldane's Mind, Metaphysics and Value in the Thomistic and Analytic Traditions? He denies that one can adapt a true picture of human action to standard event-causalism and then spells out some aspects of the Thomistic picture, focusing on distinguishing between determination to the ultimate good and contingent direction to particular apparent goods, as well as on God's distal causation of the will and the will's proximate self-causation.

But he doesn't have a whole lot to say about determinism and how one resolves the freedom of the human with apparent microstructural determination.

iwpoe wrote:

"Thus there is a spookiness to Thomistic free will that would upset naturalists."

Well, there is the whole God thing... So much the worse for naturalists.

That's right. God does play a role not just in Thomistic metaphysics but a pretty direct role in Thomistic philosophy of action.

I mention the unnaturalistic spookiness, though, particularly because Thomists often write as though they neatly avoid the liabilities of substance dualism.* That is in some respects true, but naturalists find violation of causal closure really unacceptable. So far as I can tell, Thomists don't regularly own up to this feature of their theories. Haldane is an exception, though he mentions it in passing and so far as I know does not give an extended analysis. Anscombe is kind of an exception, but even her treatment is (as usual) very guarded.

* bulldog91's initial post was surely motivated by this tension. There is a lot of Thomistic rhetoric about Thomism's establishing of immaterialism without the interaction problem. But does it do this? Thomists are really reticent to write about topics like microstructural determinism.

bulldog91 wrote:

"
Why is this relevant to the discussion? There are good reasons to believe in the causal closure of the physical, at least when it comes to the day-to-day operation of the physical stuff inside our skulls: in particular, we have observed lots of neurons, and they all behave according to predictable (though often very complex) laws. This remains the case regardless of whether or not God exists. God might violate the causal closure of the physical in other ways (e.g. miracles), but that is quite independent of the question of whether the day-to-day operations of our brains involve the immaterial nudging of neurons (as interactionist/libertarian free will requires). Once you've established that God exists, you've established that *in principle* the causal closure of the physical is false, but then you still have to empirically show in any particular case that causal closure has been violated (for instance: through observing a miracle. Or, in this case, through finding a neuron whose activity tracks our free choices but that violates known physical laws). 

1) interactive dualism does not necessarily entail the "nudging" of neurons. For instance, if a non physical mind were to interact via the physical world via wave function collapse (a proposed opening for interaction, I recognize there are problems with this view), there would be no "nudging" one possible situation would simply be made actual that was not determined by physical laws.

2) libertarian free will may be preserved without interactionism. One such way is here:
http://alexanderpruss.blogspot.com/2014/11/the-traveling-minds-interpretation-of.html?m=1

Another way is be adopting a kind of libertarianism in which God is the cause of our free choices (see Matthews Grant's article in Faith and Philosophy "can a libertarian hold god causes our free choices." In this manner, God could cause the entire history of the physical universe and all of free choices. The laws of physics may be deterministic or not. Either way, God can cause both physical and mental events without causing it to be the case that one has an effect on the other (note: Grant's position is not this, I only point out that in adopting his theory of action, one could hold this view). It is similar to Lebniz's pre established harmony