In this article, I want to examine the issues presented by the problem of consciousness when examined from a naturalistic or physicalist point of view. What I present in this article is probably not a knock-down argument proving the computational theory of consciousness (Strong AI) within this framework. Rather it shows that the alternatives are rather problematic.
When we assume a naturalist viewpoint, what do we mean when we say that consciousness exists?
From this viewpoint, all real phenomena that exist in the world are the result of physical interactions.
So one interpretation is that consciousness has a physical substance, perhaps some as yet undiscovered form of matter. This view dates back at least to Democritus, the ancient Greek philosopher who first proposed the concept of atoms. He believed that fire was composed of fire atoms, that water was composed of water atoms. He also believed in soul atoms.
There is certainly much about fundamental physics that we have yet to learn. There are substances which remain undiscovered, for example the nature of dark matter. The idea that consciousness might have something to do with some as yet undiscovered form of matter such as this is similar to concepts explored in fictional works such as the His Dark Materials trilogy and popular non-fiction works such as the Tao of Physics.
Yet if this were the case, then such matter would be quite fundamentally different from any other kind of matter physics has ever postulated. Elementary particles in physics can only be sensibly described in terms of physical law -- mathematical relations they have to each other and certain predictable physical behaviours they exhibit.
Gravitons transmit gravity. Photons transmit light. Electrons transmit electricity. We might want to posit a "psychon" that confers consciousness in much the same way.
Gravity, light and electricity can all be understood according to the mathematical relations that form the physical laws of the universe. Consciousness, on the other hand, is an intuitive concept we share but which we cannot adequately define -- not even in words and certainly not in terms of simple mathematical relations.
Furthermore, even if we did discover the psychon, we could never be sure that it was responsible for consciousness. If we detected psychons in the brains of humans but not other animals, would that show that only humans were conscious or that only human brains have these particles? If we detected them in all animal brains but not in computers would that prove that computers were not conscious or would it simply show that psychons have something to do with organic brains?
So perhaps we reject the idea of a psychon. There might be other mysterious applications of existing physical laws at work, particularly quantum mechanics. This view is held by people such as Roger Penrose, who with Stuart Hameroff has proposed that nano-scale structures within cells called micro-tubules might be small enough to utilise quantum physics to affect brain function.
Penrose's motivation arises from Godel's incompleteness theorem, which he takes to imply that it is impossible for a system of algorithms to produce human mathematics (a not-uncontroversial interpretation to say the least!) and so the human mind cannot be computational. Introducing quantum weirdness is supposed to somehow allow us to transcend the limits of algorithmic processing and allow us to achieve human ingenuity.
Even if he's right, however, quantum physics would therefore be something our brains exploit in order to have adaptive intelligent behaviour. It's much less clear how it can resolve the problem of why we have genuine subjective experience, qualia and so on.
Once again, it's hard to see how quantum physics, which is defined in terms of mathematical relations and functions, can be used to explain something as nebulous and ill-defined as the intuitive concept of consciousness.
There may be other hypotheses supporting the concept of consciousness as arising from basic physical principles, but they must all be subject to this criticism. Basic physical laws are mathematical. Consciousness is not defined in terms of mathematics. Therefore, it seems unlikely that we can account for consciousness in terms of fundamental physics.
But there are plenty of things which do exist which are not defined in terms of fundamental physics, but which emerge from complex systems, built layer upon layer upon foundations which can be reduced to physical law only after much analysis.
Life is a good example. Before science was able to grapple with the underlying mechanisms supporting life, a leading theory to explain it involved the positing of a substance called élan vital (vital impetus). It was supposed that this substance was responsible for life, and that it might even be possible to bring non-living things to life if the substance could be isolated.
Like phlogiston and aether, élan vital does not exist. Life is not explained as a direct consequence of fundamental physics, but as an indirect and emergent phenomenon arising from incredibly complex biological systems designed by evolution upon layers biological, material, chemical, atomic and physical.
So there is no smallest physical unit of life. There is no clear distinction between living and non-living. Life is not a phenomenon which can be defined mathematically, and so it is not present at the level of basic interaction between fundamental particles.
And yet it would seem perverse to claim that this implies that life does not exist at all. Rather, we recognise that many of the things which exist in the universe do so not as fundamental physical objects but as useful labels describing the high-level emergent phenomena we observe arising from the physical interactions taking place in complex systems.
All phenomena which exist in the universe which cannot be sensibly studied at the level of fundamental physics are of this sort. Universities, market forces, evolutionary pressures, fashion trends, and even individual people are such entities. Consciousness is no different.
Some day we might find an alien planet that has reproducing and evolving organisms. We would call this life (but not as we know it!), even if its chemistry had next to nothing in common with terrestrial biological chemistry. Like other such emergent phenomena, life is not tied to specific details of its low-level implementation.
Consciousness is probably no different. But if this is so, the relationship between consciousness and fundamental physics is indirect. As such, it seems unreasonable to insist that consciousness has any crucial dependence on particular physical structures within the brain. Just as we can take an XBox game and change the details of the low level implementation so it will work on a Playstation, so can we do the same for any high-level emergent phenomenon.
There seems to be no reason in principle why we could not find consciousness supported by a physical substrate entirely different from a human brain. If a computer program had software structures analogous to our physical neurons and neurotransmitters, and if that program appeared to exhibit all the signs of consciousness we recognise in ourselves, there is no reason to suppose that there would be any fundamental qualitative difference between our conscious experiences.
If consciousness is emergent, then the physical details don't matter.