Simply put, compatibilism recognizes the principle that everything that happens is caused in a more or less deterministic fashion but still contends that there is a place for a robust notion of free will and moral responsibility, which seems to require it. Nahmias begins his critique of full determinism (at least as is presented in the context of the findings of neuroscience) by arguing that the definition of free will assumed makes free will implausible on the face of things:
Scientists’ arguments that free will is an illusion typically begin by assuming that free will, by definition, requires an immaterial soul or non-physical mind, and they take neuroscience to provide evidence that our minds are physical.... We should be wary of defining things out of existence. Define Earth as the planet at the center of the universe and it turns out there is no Earth.Nahmias's point is well taken here. It is easy to slide from a common concept of free will (i.e., a belief some thing in us that is considered essential to who we are is in control of what we do) to the idea that such a thing is nonphysical and somehow interacts with the physical world. Fail to distinguish between the two and anyone who cannot stomach dualism will quickly concede to a full deterministic view of human agency. The connection between free will and dualism has a long philosophical history, but if the two can be successfully disentangled, then there may be room for a physicalist version of free will.
He then moves on to discuss in rough outline a physicalist understanding of the neural machinery that produces free will:
Our brains are the most complexly organized things in the known universe, just the sort of thing that could eventually make sense of why each of us is unique, why we are conscious creatures and why humans have abilities to comprehend, converse, and create that go well beyond the precursors of these abilities in other animals. Neuroscientific discoveries over the next century will uncover how consciousness and thinking work the way they do because our complex brains work the way they do.There is an element of hand waving here: the brain is supercomplicated, so we'll figure this all out as neuroscience progresses. Nahmias might be right, but the fact that discoveries may be made in the future that flesh out the physical mechanisms for genuine free will is not evidence at this time that there is some coherent version of a robust concept of a physicalist free will. Neuroscience needs to not only show that the will is the product of neural mechanisms, but that it is genuinely free (i.e., that it is possible for us to do otherwise than we have done in any given circumstance).
He follows with his own definition of free will and its relationship to responsibility:
Many philosophers, including me, understand free will as a set of capacities for imagining future courses of action, deliberating about one’s reasons for choosing them, planning one’s actions in light of this deliberation and controlling actions in the face of competing desires. We act of our own free will to the extent that we have the opportunity to exercise these capacities, without unreasonable external or internal pressure. We are responsible for our actions roughly to the extent that we possess these capacities and we have opportunities to exercise them.This seems to be a fairly standard iteration of free will among compatibilists. Free will is construed as a type of probability calculation that aims at maximizing the achievement of our desires (which are presumably hierarchically organized), guiding action based expected outcomes of different possible courses of action.
But the question I have here is whether this is an account of free will or merely an elaborate modeling of a deterministic calculation that gives rise to the illusion of free will. Where in this model is choice? If we say that the agent chooses among different options, is such a choice uncaused or caused? If it is caused (either by something internal to the agent or external), then could the agent have chosen otherwise? In other words, given the set of desires possessed by an agent and the set of choices available to him, is it possible that he could choose a different course of action? If so, what would explain that choice? Whatever explanation we give will seemingly rely on some type of causal account of the decision being determined by the set of facts prior to the choice being different. If this is the case, then the ability to do otherwise is not provided by the model. The idea seems to be that given a different history and a different set of aims and options, an agent might choose differently. But the issue of free will addresses a different question altogether: given the exact same history, aims, and options is it possible for an agent to choose otherwise than she does? This model doesn't appear to address that core question of freedom at all.
Nahmias argues that the interpretation of the evidence of neuroscience as demonstrating determinism--as many people interpret the studies of Benjamin Libet, for example--assumes just the kind of definition of free will he is combating. In fact, he argues, the evidence is just what we would expect if we view free will as the product of physical processes in the brain. We shouldn't expect the brain to make a conscious decision without any activity neural activity preceding it at all. That would be to expect a miracle.
He concludes that neuroscience will mean the death of free will only if
it somehow demonstrated that conscious deliberation and rational self-control did not really exist or that they worked in a sheltered corner of the brain that has no influence on our actions. But neither of these possibilities is likely.But this is to constrain the sphere of questions we want answered when asking about free will far too narrowly for the version of free will it posits to be palatable. Unless we're willing to give up on the idea of the ability to do otherwise, Nahmias's model fails to provide what we want from a theory of freedom. Ultimately, if it fails to give us a genuine ability to do otherwise, this model collapses into determinism, albeit one of a fairly complex variety.