It's commonly assumed -- at least by me -- that the brain is the seat of the mind. But now I wonder whether there's actually any principled basis on which to draw a strict delineation between the brain and other organs (e.g. the eye and optic nerve, etc.), insisting that our minds do not extend beyond the former. I'd previously supposed that the idea of a 'brain in a vat', or a body transplant, shows that the brain is all that intrinsically matters for mentality. But now I'm not so sure that this works after all.
On the standard picture, the brain is basically a computational device. It takes 'input' from various sensory nerves in our body, performs computations on this data (which amounts to 'thinking'), and then outputs behavioural instructions for the body to perform. At least, that's how I think of it. I'm just assuming this is 'standard'. Anyway, this picture seems to make the body rather superfluous: you could replace it with anything else that gives the same 'inputs' to the brain, and reacts appropriately to the resulting 'output'. Hence the possibility of Matrix-like illusions involving "brains in vats", where the body is replaced by a complicated computer simulation feeding input to our brains, resulting in mental lives indistinguishable from our own. This possibility suggests that the brain is all that matters for mentality. (Or so I assumed.)
But then, couldn't the same sort of "replacement" occur to portions of the brain itself? Suppose a small portion of my brain was removed, and replaced with a functional equivalent. That is, the replacement part would feed the exact same inputs as before into the various neurons that it's connected to. It would react exactly as my original brain-part had: taking in information from neighbouring neurons, running the appropriate computation, and then returning the appropriate result. If parts of my brain were replaced in such a way by these functionally identical "artificial neurons", I would never notice the difference. My mental life would be unchanged. So, by the same reasoning as above, it seems we are led to conclude that brain-parts are inessential to our minds, in exactly the same way that body parts are.
Of course, if you just remove a brain portion without replacement, then my resulting cognition will be completely different. But the same is true of my body (say if you remove my eyes), or even the external world -- take away my calculator and I won't be nearly so good at solving math problems!
So indispensibility or the possibility of 'replacement' cannot be what delineates which physical parts are involved in mentality. The brain in a vat is a red herring, for we can replace even more than that; we could have a "frontal cortex in a vat", or even a single neuron in a vat, but that doesn't mean that the rest of the brain is non-mental. What this example shows us is that the mind can extend beyond what's in the vat. In case of your neuron-in-a-vat, the single neuron certainly does not exhaustively comprise your mind. More plausibly, your mind also includes whatever has 'replaced' the rest of your brain -- perhaps part of the 'vat' architecture. But then, what's stopping us from saying the same thing in the original BIV scenario? The computers that have replaced our body (and even the external environment) might now be part of our minds.
Am I missing something here? If mentality is computation, and it doesn't matter how the computation is physically realized, then it seems arbitrary to restrict the mind to the brain -- or even the body, for that matter, as Clark & Chalmers argue in 'The Extended Mind'. They argue that our dispositional/'standing' beliefs consist in information stored in any source that we rely upon and access easily and regularly, e.g. a notebook carried around by an Alzheimer's patient, and not just internal memory. This could, in principle, even extend to other people, yielding the intriguing idea that an inseparable couple's minds might to some degree overlap! I'll quote a bit from C&C's fascinating conclusion:
In each of these cases, the major burden of the coupling between agents is carried by language. Without language, we might be much more akin to discrete Cartesian "inner" minds, in which high-level cognition relies largely on internal resources. But the advent of language has allowed us to spread this burden into the world. Language, thus construed, is not a mirror of our inner states but a complement to them. It serves as a tool whose role is to extend cognition in ways that on-board devices cannot. Indeed, it may be that the intellectual explosion in recent evolutionary time is due as much to this linguistically-enabled extension of cognition as to any independent development in our inner cognitive resources.
What, finally, of the self? Does the extended mind imply an extended self? It seems so. Most of us already accept that the self outstrips the boundaries of consciousness; my dispositional beliefs, for example, constitute in some deep sense part of who I am. If so, then these boundaries may also fall beyond the skin. The information in Otto's notebook, for example, is a central part of his identity as a cognitive agent. What this comes to is that Otto himself is best regarded as an extended system, a coupling of biological organism and external resources. To consistently resist this conclusion, we would have to shrink the self into a mere bundle of occurrent states, severely threatening its deep psychological continuity. Far better to take the broader view, and see agents themselves as spread into the world.
As with any reconception of ourselves, this view will have significant consequences. There are obvious consequences for philosophical views of the mind and for the methodology of research in cognitive science, but there will also be effects in the moral and social domains. It may be, for example, that in some cases interfering with someone's environment will have the same moral significance as interfering with their person. And if the view is taken seriously, certain forms of social activity might be reconceived as less akin to communication and action, and as more akin to thought. In any case, once the hegemony of skin and skull is usurped, we may be able to see ourselves more truly as creatures of the world.
Despite their other radical suggestions, C&C conservatively assume that consciousness is purely in-the-head. But again, what is the principled basis for such a boundary? Perhaps if consciousness was seen as a fundamentally biological or neurological process, essentially arising from neural interactions, then we could get this result. (Though it would seem to imply - implausibly, I think - that replacing each of my neurons with exact artificial replicas would rob me of my consciousness.) But on cognitive theories of consciousness (ala Dennett), extended cognition would seem to straightforwardly imply the possibility of extended consciousness. Maybe someone will figure out a way to use this to test the theories one day...