It seems to be an open question whether distinctively neural properties are essential to the instantiation of mental properties. One can buy into reductive physicalism and reject neural reduction bases in favor of chemical or thermodynamic reduction bases, just to name a few. Perhaps, then, systems that have no distinctively neural propertiesâ€”no brains and no neuronsâ€”nonetheless have certain chemical or thermodynamic profiles that suffice for mentality. Perhaps. But I doubt it. I hope that I may be forgiven for being so brief about this, but I think there are three reasons (at least) for thinking that the physical reduction of the mental should be a neural reduction.
The first reason for believing in neural reduction is that no non-controversial examples of entities that implement consciousness or cognition exist without brains, or at least, neural networks. It is uncontroversial that alert human adults have mental states. It is also uncontroversial that they have brains. Things are much more contested for the brainless. While it is arguable that my laptop has mental properties, it is also arguable that it does not. It is also arguable, by the way, whether or not my laptop has neural properties. I do, after all, run neural network simulations on it (Mandik 2003). How many properties are literally shared by the simulation and the simulated? We need not settle this now. What is clear is that thereâ€™s controversy about the mental prowess of the brainless. And while some brain-havers may lack mentality (who knows what to say about the vegetative patients? (Begely 2006) ), there are no uncontroversial confirmations of mind-havers lacking brains. Letâ€™s talk about the mind-having brain-havers a bit more under the heading of â€œreason #2 for thinking that the physical reduction bases of mentality will be neural.â€?
The second reason for believing in neural reduction is that there is no reason to doubt that that it is in virtue of their brains (or their brains plus something else) that creatures like us implement consciousness or cognition. Putting the parenthetical â€œplus something elseâ€? to the side for the moment, let us entertain briefly how unpromising non-neurocentric theories have been. Mental properties are had by organisms either in virtue of the whole organism or part and it is easy to see that it canâ€™t be the whole organism. Amputees retain their mentality, and while my appendectomy doesnâ€™t exactly count as an amputation, comparing its relative effect to oneâ€™s mentality to the potential effects of a lobotomy is like comparing nothing to something. That the seat of our soul is some proper part of us is old news, but the appendix never had a chance and the Aristotelian coronary hypothesis was rejected long ago. So much of what we know about where drugs need to go to go to work and what brain injuries impair what mental functions has tipped the scales pretty clearly in favor of neruo-centrism. But, must it be merely neuro-centric? Canâ€™t it be exhaustively neural? Here we have to pause to consider various embodied, embedded, and externalist proposals for including the body and even chunks of the environment of the organism as part of the supervenience base of the organismâ€™s mental properties. There are a couple of things to say about this. The first is that none of it removes the brain from the center of the story. The second is to echo Fodorâ€™s (1989) suggestion that we individuate neural properties widely.
The third reason for believing in neural reduction is that no reductive research program has been as productive as neurocentric ones. One might even be so bold as to suggest that non-neurocentric reductionists have no research program at all. There have been, in recent decades, three major proposals that have been physicalistic without reducing mentality, a la behaviorism, to the behavior of whole organisms: classic computationalism, connectionism, and (certain versions of) dynamic systems theory. Classicism got wedded, in many peopleâ€™s minds, to non-reductive physicalism, largely due to the influence of Fodor (1974) and Putnam (1967). Dynamic systems theory included proposals of a specifically neural character, (e.g. Freeman 1991) while others looked like warmed-over behaviorism (van Gelder, 1995). Either way, dynamic systems theory was confronted with some devastating objections (see Glymour (1997), Grush, (1997) and Eliasmith (2001) for a taste). The main point here, though, is not any knock-down refutations of non-neurocentric research programs. The point here is that neurocentric research programs have been massively productive both in theory and in application.
Update: Futher discussion of this post on Paul Baxter’s blog [link].