Among the band of philosophers who hold that free will is supernatural, one of the reigning ideas is called agent causation. This hypothesis states that volitional acts are a special category of event, one that is caused not by any other event but – in some deeply mysterious way – by the agent itself. Philosopher Roderick Chisholm describes this as follows:
If we are responsible… then we have a prerogative which some would attribute only to God: each of us, when we act, is a prime mover unmoved. In doing what we do, we cause certain events to happen, and nothing—or no one—causes us to cause those events to happen.
The consequence of agent causation is that free will is not a process but some sort of irreducible substance: one that spontaneously originates acts and decisions, unconnected to the causal chain that binds together all other causes and effects. The usual apologetic corollary is that even God cannot intervene in or influence this process short of destroying free will altogether. It’s plain that this is just the religious doctrine of the soul, the supernatural “ghost in the machine”, portrayed in technical philosophers’ language.
Agent causation depicts human free will as a binary state – a quantity which can either be present or not present, but which has no internal structure and cannot be subdivided. However, this is obviously false, which makes this entire view unsustainable. Free will is not a mathematical point; free will is a complex bundle of contingent desires, habits, and predispositions, which can be added to, altered or removed.
You can determine this by empirical studies of human behavior. There are countless things that human beings could do that we do not do and do not feel any desire to do. On the other hand, the vast majority of us do experience desires to have sex with an attractive partner, to consume foods high in fat and sugar, or to form tight emotional bonds with parents and relatives. Human free will, then, is not just an irreducible point source that bubbles up actions at random; it operates within a defined set of parameters, giving rise to a predictable variety of behaviors (anthropologists call them cultural universals).
You can also determine it by the evidence of the human brain: it’s well known that certain, specific kinds of brain damage alter desires and behavior in predictable ways. Dementias such as Alzheimer’s disease often cause loss of interest in religion, while epileptic seizures in the temporal lobes can induce religious experiences. Damage to the frontal lobes leaves people unable to control their behavior or ignore sudden impulses; injuries localized to the left hemisphere often cause depression, while injuries to the right can leave people constantly and inappropriately euphoric. And of course, drugs and intoxicants also have reliable, predictable effects on behavior.
If free will was an irreducible, nonphysical substance, producing actions free from external causation, then we should not see brain damage affect desires or behavior – much less change them in the predictable ways that neurologists observe. That we do see this shows beyond a reasonable doubt that it is false that “nothing… causes us” to make the decisions we do. Our decisions manifestly are caused.
Of course, there are other varieties of supernatural dualism that are not as clumsy as agent causation. But what these other varieties have in common is that they must give up the line in the sand. They cannot declare, as agent causation does, that we are purely supernatural beings whose decisions ultimately arise from the soul and nowhere else. Instead, these other dualisms must acknowledge that we are, at least in part, material beings, and that changes to the physical composition of body and brain can affect and alter our selves. Whether they realize it or not, advocates of these beliefs are drawing closer to atheism, as they implicitly grant that we are not spirits whose choices arrive from outside the world, but physical beings whose acts are an inextricable part of the fabric of cause and effect.