Stump speculates about a natural dualism that is not an extreme Cartesian dualism - one that depends on a metaphysical mental substance - but one that is an emergent property at the system level. She argues that Aquinas' dualism, which takes the soul to be the form of the body, can accept the characterization of the mental as correlated with the physical because the mind is implemented in configurations of the physical.

But this may imply that the mental state is the causal outcome of the physical state, so Stump develops her own version of the standard two-part argument against free will.

Libertarianism and causal determination

Libertarian free will is sometimes characterized in this way:

(L) an act of will, such as a decision, is free only if
[1] "the decision [is] not ... causally determined, and ...
[2] the agent could have avoided making it."

This characterization of the mental state as causally brought about by a series of neural firings does not presuppose any particular sort of correlation between the mental and the physical. It is compatible with either type-type or token-token identity theories, but it does not presuppose either of them. By saying that mental states are correlated with neural sequences, I mean to make only a vague association between mental states and neural sequences, compatible with various theories of relations between mind and brain. Those who think that the mental is identical to the physical can suppose that the mental states and the neural sequences are correlated because the mental states are the neural sequences. Non-reductive materialists can take the correlation as some version of emergence or supervenience. As far as that goes, even dualists such as Aquinas, who takes the soul to be the form of the body, can accept this characterization of the mental as correlated with the physical in virtue of the mind's being implemented in configurations of the physical. Only the most extreme versions of Cartesian substance dualism will reject it. On an extreme version of Cartesian dualism — which Descartes himself may have held — some mental states, such as thinking and willing, go on only in the immaterial soul and are not mirrored by or correlated with brain processes.

On any theory of the mind, including Aquinas's, that sees a stronger tie between mind and body than extreme Cartesian dualism, there will be some sort of correlation (up to and including identity) between mental processes and brain processes. On such non-Cartesian theories, however exactly we interpret the correlation between a physical state and a mental state, the mental state is a causal outcome of physical states.

If (L1) is right, however, only those mental acts which are not so much as correlated with patterns of neural firings can count as free. But then libertarianism could be held only by extreme Cartesian dualists. And, clearly, there are committed libertarians who reject any form of Cartesian dualism. So (L1) is also too strong; to avoid making libertarianism a theory only extreme Cartesian dualists can hold, it needs to be revised. Because libertarians identify themselves at least in part by their opposition to compatibilism, libertarianism does need to rule out as non-free mental or bodily acts that are causally determined by something outside the agent. The claim that a free act is the outcome of a causal chain which originates in some cause external to the agent's own intellect and will is incompatible with libertarian free will. But the mere claim that a free act is the outcome of a causal chain is not.

A more reasonable version of the relevant necessary condition for libertarianism is therefore this:

(L1') a decision is free only if it is not the outcome of a causal chain that originates in a cause outside the agent.

At this point someone may object. If we bring contemporary theories of the nature of the mind into the discussion of Aquinas's theory of free will, the objector will argue, then (unless we accept extreme Cartesian dualism) no volition can be free in the sense specified in (Ll'). So, insofar as Aquinas's account centrally includes (L1'), it can be shown to be false. For ease of discussion we can put the objector's point in contemporary terms by taking the physical states with which the mental is correlated to be neural states. Then the objector's point can be formulated this way:

(O)
(1) there are no uncaused neural events, and
(2) the chain of causation eventuating in a human action will be traceable ultimately to something outside the agent.

If correct, this objection, which presupposes that the mental is embedded in a complete causal nexus governing the whole realm of the physical, is fatal to Aquinas's theory of free will since on Aquinas's theory nothing outside the agent exercises efficient causality on the will. So the objection is worth considering in some detail.

The objector will perhaps meet little opposition regarding (O1), the claim that all neural events are caused. Is he also right in (O2), the claim that the chain of causation for neural events will lead outside the agent? Are all brain processes causally determined, ultimately, by something outside the agent? We might suppose that they would have to be.

But the objector's way of looking at things, which will perhaps seem obviously right to many people, rests on philosophical convictions that include both reductionism and determinism, its well as the assumption of causal completeness at the microlevel. Although reductionism comes in many forms, its different forms share a common attitude: all the sciences are reducible to physics, and all scientific explanation is ultimately formulable solely in terms of the microstructural. But this attitude discounts the importance of form, as Aquinas thinks of it (or levels of organization, as contemporary philosophers of biology tend to say). It also discounts the causal efficacy things have in virtue of their form or level of organization. (This feature of reductionism perhaps helps explain why it has come under special attack in philosophy of biology. Biological function is frequently a feature of the way in which the microstructural components of something are organized, rather than of the intrinsic properties of the microcomponents themselves.)

Aquinas among the libertarians

Of course, (L1') is not sufficient, on Aquinas's account or on the views of contemporary libertarians, for libertarian free will. What else is to be added, if it is not some version of PAP?

What is sufficient for libertarian free will, then, on Aquinas's account, is that the ultimate source of an action be the agent's own will and cognitive faculties. Since this condition entails (L1'), we can reformulate the characterization of libertarianism in this way:

(L') an act is free if and only if the ultimate cause of that act is the agent's own will and intellect.

Some contemporary philosophers share Aquinas's basic intuition about the nature of freedom. For example, John Martin Fischer's account of moral responsibility is like Aquinas's account of free actions in this respect: Fischer thinks that moral responsibility is a function of an agent's reasons-responsive mechanism. And one way of understanding the point of contention between compatibilists and libertarians of any sort is in terms of this question:

(Q) Is it possible for the mind (or the brain) to be a reasons-responsive mechanism if the only candidates for the origin of mental events (or neural events) are random accidents or causes outside the agent?

Part of what makes it hard to adjudicate between compatibilists and libertarians here is that we are so far from understanding how the brain (or the mind implemented in matter) can be a reasons-responsive mechanism at all, on anybody's theory of mind. Except for extreme Cartesian dualists, most contemporary philosophers suppose that the brain does constitute a reasons-responsive mechanism, but it is hard to see how a biological organ such as the brain can respond to reasons or process information. Neurobiologists are in no position to give anything other than promissory notes on this subject, and the best philosophical attempts are ultimately unpersuasive even if ingenious. But unless we understand how a biological organ such as the brain can be an information-processor or a reasons-responsive mechanism, we will not be able to give a neurobiological account of our cognitive functioning which successfully adjudicates the different answers to (Q) given by compatibilists and libertarians, such as Aquinas.

Looked at from the point of view of philosophy, compatibilism appears to be a sort of codicil to reductionism and determinism. If all macrophenomena are reducible to microstructural phenomena and If' there is a complete causal story to be told at the microlevel, then if we as macroscopic agents are free with respect to any of our actions, that freedom has to be not only compatible with but in fact just a function of the complete causal story at the microlevel. On the other hand, if the metaphysical attitudes of Aquinas are right, compatibilism is an unnecessary concession, an attempt to preserve what we commonly believe about our control over our actions in the face of a philosophically mistaken and scientifically premature commitment to reductionism and determinism.