To state my bias upfront, I’m a physicalist (or perhaps more accurately a ‘computationalist’, which is kinda the same thing plus the assumption that it’s impossible to realize physical processes that are capable of hypercomputation); thus, I believe that the cause of consciousness is the activity of the brain, and nothing else. To me, any different position amounts essentially to defeatism – saying that we can’t explain it now, and never will – which I don’t think can ever be a well justified stance; just because I can’t explain, say, the emergence of qualia from physical processes, doesn’t mean it’s inexplicable.
My reason for this position is mainly the problem of interaction – that if there is some sort of mind/body duality, that is, if mind and body are of fundamentally different substance, there seems to be no way for one to interact with the other. This is an old problem; Descartes, the architect of the modern dualist view, was well aware of it and had to weasel himself out of it by positing a special gateway, located within the pineal gland, through which the mind could influence the body, i.e. its physical state and motion.
There’s also the idea that mind and body are in a sense ‘synchronized’ – i.e. that while there is no actual interaction between the two, they evolve ‘in parallel’. Picture two clocks, one on your desk, the other in the hallway. You’ll notice that whenever the clock on your desk shows the full hour, you hear chimes from the hallway indicating the same time. However, there is no interaction between both clocks – they’re just synchronized. A similar kind of synchronization then might exist between the mind and the body, with both realms firmly separated. This explains why, when we think ‘grab the pot of coffee on the desk’, our body actually executes this action, without running into the trouble of having to explain the interaction taking place in order to inform the body of the mind’s will.
However, this runs into a problem – since everything we ever see, think, do, or more generally, experience, is, in fact, mental, there would be no difference to the realm of the mind if the realm of the body did not exist at all! We would still feel, think, do and experience the same things ‘in here’ whether or not the body ‘out there’ actually acted correspondingly. This kind of dualism thus abolishes itself.
So the only viable option, to me, seems to be monism – the idea that, whatever it may be, there is only one kind of substance, one kind of stuff for stuff to consist of, in the world. In particular, both mind and brain must consist of the same kind of stuff.
To this end, then, one might conduct a thought experiment. The behaviour of one neuron – roughly, how its outputs are correlated with its inputs – is relatively well understood, to a point that it seems feasible to construct something like an ‘artificial neuron’. If the behaviour of this artificial neuron can be made to be absolutely indistinguishable from the behaviour of a real neuron (and I see no reason why that shouldn’t be the case), one could imagine swapping this neuron in for a real one in one’s brain. There would then be no observable difference in the functioning of this brain, and, since brain and mind are of the same stuff, correspondingly no difference in the functioning of one’s mind – i.e. one wouldn’t notice any difference. The same goes for exchanging a second neuron. And a third. And so on, until, at some point, the whole brain has been replaced by artificial neurons (of course, there’s more to the brain’s functioning than just the neurons, but for the sake of simplicity, I’ll just pretend there wasn’t; at any rate, there does not seem any reason to assume that other parts could not similarly be duplicated and replaced). (I call this the ‘brain of Theseus’-experiment.)
So if, indeed, at no step there occurred a catastrophic loss of consciousness (for which there doesn’t seem to be any justification), then, after having my whole brain replaced, to me, I’d still be me – i.e. subjectively, there wouldn’t be any difference. But then, this must mean that the rejection of dualism – or the acceptance of monism – implies that there is no ‘mysterious element’ to consciousness; and conversely, that those claiming for there to be a fundamental mystery to consciousness are really dualists in disguise, whether or not they consider themselves to be. Thus, everybody that claims that, say, the apparent existence of qualia precludes an explanation of consciousness as arising from fundamental non-conscious, physical processes has to concern themselves with the contradiction inherent in proposing the existence of fundamentally distinct, yet interacting substances.
This has so far been a wholly negative account of the causes of consciousness, which has its roots in the fact that, while I think I have a good idea of what consciousness is not, my thoughts are rather fuzzy on the subject of what, then, consciousness is. But I think perhaps its central phenomenon is its reflexivity – as Douglas Hofstadter put it, ‘a self is a pattern perceived by a self’. So if I had to put my finger on a sloganized ‘cause for consciousness’, this self-reflexivity would probably be what I’d point to.
I think Dennett is right in his criticism of the thought experiments leading to the idea of qualia. How an (apparently) irreducibly subjective account might result from processes in which there is no room for a subject is hard to imagine, and it is easy to present the matter in a way such that it seems impossible to imagine – but I’m not sure that’s quite as clear cut as it is often made out to be.
The classic example is, of course, Mary the colour scientist. I believe that one reason it seems so convincing is the mismatch in bandwidth between visual and textual modes of information gathering: Our visual system gathers tons of bits effortlessly, while reading – implicitly the way Mary avails herself of ‘all the physical information about seeing colours’ – conveys relatively little information in an instant. Indeed, one could posit that the way information is processed in the brain depends on the speed that information is presented to it; then, it would indeed be possible for Mary to ‘read’ all about colour, yet learning something new about colour upon seeing it for the first time, without this having any deleterious consequences to physicalist explanations. But I don’t think even this is necessary.
Tale a variation on Mary: Marv, who is, through unknown means, confined to a two-dimensional world. One could argue analogously that there is no way for him to imagine his experiencing a three-dimensional world, as there is no way for a linear combination of two dimensional vectors – which is all he has in his repository to build representations of the world from – to be three dimensional. However, this conclusion we know to be wrong – it is hard, though not impossible, to teach yourself to visualize four dimensions – at least well enough to conceivably not be confronted with something ‘fundamentally new’ when suddenly exposed to a four-dimensional reality. (French mathematician Étienne Ghys has a page where you can learn to do visualizations of four dimensions.)
The difference between Mary and Marv is, I think, only one of quantity, not of quality. While it is just barely possible to imagine that Marv might succeed in visualizing the third dimension, the problem is just so much more intractable in Mary’s case that we are easily persuaded to call it impossible. But if Marv can succeed, Mary just might, too, though we perhaps can’t begin to imagine how.
The thought experiment of Mary succeeds due to the trick of presenting as a sharp divide what is actually a far more fuzzy gradation – generating new experiences from old ones. If the way you need to go to produce a new experience is short, we do not think of it as difficult – you can, say, often predict how a dish you make for the first time will taste, or you can imagine a scenery from a prediction in a book well enough in order to recognize it when confronted with it in actuality. These are easy cases – you directly possess the building blocks necessary to ‘assemble’ the ‘new’ experience. It gets harder in Marv’s case, where the building blocks you possess – two dimensions – don’t suffice to generate a three-dimensional manifold. And in Mary’s case, it seems that there does not exist any way to reassemble the known to create or infer the unknown – colours can only be thought of in terms of colours. But of course, when it comes right down to the fundamentals, everything in the mind is composed of the same fundamental building blocks – information, encoded in neuron firings. It’s very hard to create the ‘what it is like’ of colour vision from these building blocks; but there is no reason to think it should be impossible.