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I would like to introduce you to your zombie twin. Far from the crass Hollywood representations of zombies, your twin looks and behaves precisely as you do. In fact, it is an exact replica of you down to the finest subatomic detail and, from the outside at least, is utterly indistinguishable from you.
Right now your zombie twin is sitting down and reading this very article. All this is taking place in another zombie universe, a possible world that is an exact replica of this universe, also down to the finest subatomic detail. As you stare at these very words, your zombie twin is doing the same. Right now.
However, there is one crucial difference between you and your zombie twin: where you gaze upon this page, experiencing sensations of light and colour, your zombie twin lacks such sensations. When you see a red flower, you have a characteristically red experience, but it does not. When you are pricked by a pin, you recoil in pain. When your zombie is pricked, it too recoils, but it feels no pain.
Your zombie twin might be physically, functionally and behaviourally identical to you, but it lacks a rich inner life of conscious experience. Its phenomenal world is blank. All the lights are on, as they say…
As you pause to reflect on the possibility of such a world containing a zombie twin, your zombie twin does the same. And if you believe such worlds can exist, then so too does your zombie twin.
Much of this rides on the conceivability of such zombies. If they could exist – even if only in another zombie universe – then this may have dramatic ramifications on our understanding of consciousness. In fact, if zombies are even remotely possible, then it implies that there is more to consciousness than just physical processes, such as the buzz and pop of neurons.
Those neurons that buzz and pop might explain why we behave the way we do, and this activity might correlate precisely to the phenomenal experiences we have. But if zombies possess the same buzzing and popping neurons but lack the corresponding experiences, that suggests the neural activity alone is insufficient to explain what causes those experiences to happen.
In short: no amount of talk about wavelengths of light, neurons firing and brain states can explain why red looks red rather than green, blue or even blank – there is an explanatory gap between descriptions on the physical level and the conscious phenomena that accompany those physical goings on.
How, then, does a physical thing like a brain produce conscious experience? And why that particular conscious experience rather than another? As David Chalmers has famously suggested, these questions constitute the “Hard Problem” of consciousness.
The “Easy Problem,” on the other hand, concerns a far simpler set of questions. How does the brain processes and react to environmental stimuli? How does it integrate information? How does it produce behaviour? Which brain states correlate with which phenomenal experiences? According to Chalmers, all we need is a couple of centuries of hard work and we should answer all these questions. The Hard Problem, however, may forever elude explanation.
That doesn’t mean people haven’t tried. In the past few decades, a slew of cognitive scientists and neuroscientists have offered detailed pictures of how the brain produces conscious experience, and have argued that once these pictures are complete there might be no Hard Problem left to explain.
We can now image the brain in unprecedented detail, both spatially and temporally, and hone in on the specific regions that are activated when we experience something like redness or pain. We can see how the brain integrates multiple streams of information –from the outside world, from the body and from within the brain itself – to synthesise a complex multi-layered picture that gives the impression of a unified consciousness.
We can pinpoint the moment someone makes a decision to act, and the moment later when they become aware of that decision. We can trick our consciousness and have it project on to inanimate objects, or manipulate it through surgery or chemical stimulation.
At the root, all these processes and all these explanations of consciousness are physical. There is no call for non-physical things, no ethereal or ephemeral substance that needs to be added to bring the world to light.
However, this may not be enough to dissolve the Hard Problem. Even many neuroscientists admit there is something of a gulf between brain activity and experience, and that knowledge of someone’s brain states will never be sufficient to gain knowledge of their phenomenal world. After all, your zombie twin has read about the same studies taking place in the zombie world by zombie scientists with precisely the same results.
The unnerving question remains: are all these explanations only of how certain physical processes correlate with phenomenal experience? If so, where do they explain why that brain state produces that experience rather than another? The Hard Problem persists.
Ultimately, given the way it is defined, the Hard Problem may never be solved by science. Instead it might be up to philosophy to tackle it. One of the avenues of attack is to deny there is a problem in the first place, namely by denying the plausibility of zombies. Your zombie twin will no doubt be shocked by this approach. Or would it?
Consider: if zombies are possible, then so too is the zombie philosopher writing a book about consciousness and defending the existence of a Hard Problem. What would be the motivation of such a missive? Clearly, it cannot be consciousness, which the zombie philosopher lacks. Brain activity in response to environmental stimulation might explain the articulation of limbs and the physical activity that puts pen to page. But if our behaviour can be explained without reference to consciousness, what causal role does consciousness play in directing our behaviour? If zombies are possible, then the answer might well be: none.
If that’s the case, then why do we have consciousness in the first place? If consciousness is merely along for the ride, piggy-backing on our neural activity – if it’s just an epiphenomenon– then what’s the point of red things being red? And why would a zombie write about such things at all?
Another approach is to deny that there is any mysterious qualitative character of experience – or qualia – to explain. Qualia are nothing more than a “string of judgements made possible by language and culture”, says Daniel C. Dennett.
He refers to the pervasive illusion that there appears to be a central core to our consciousness, almost as if there were an observer residing within our head, viewing a projection of everything we experience – the “Cartesian theatre”– even though no such homunculus exists. We are desperately prone to such illusions, much as we are prone to the illusion that redness is something more than the interaction of certain wavelengths of light with our visual system. However, others have responded that this simply dodges the Hard Problem rather than tackling it head on.
And if zombies are plausible, and consciousness is something in addition to all the physical stuff in the universe, then what is it? Chalmers offers a kind of neo-dualism, where qualia are nonphysical properties, and more recently a brand of panpsychism, whereby even subatomic particles can have an element of consciousness. But invoking new kinds of mysterious non-physical properties is an unsatisfying solution for many. Isn’t that just like bridging the explanatory gap with unicorns?
Few would deny that consciousness is a deeply mysterious puzzle. But the question remains whether it is also a problem. And if you agree, so too does your zombie twin.
Tim Dean is currently researching the implications of evolution on moral philosophy and exploring the insights that evolution can yield on this strangest of human capacities.