Nagel’s chapter on consciousness is a concise and critical survey of a literature that is both vast and fascinating. He further extends his survey to other mental phenomena, including reason and value, that he also finds recalcitrant to materialism. (Nagel concludes that the existence of objective moral truths is incompatible with materialist evolutionary theory; because he is sure that moral truths exist, he again concludes that evolutionary theory is incomplete.)
Nagel concedes that many philosophers do not share his skepticism about the plausibility of reducing mind to matter. And I can assure readers that most scientists don’t. I, however, share Nagel’s sense of mystery here. Brains and neurons obviously have everything to do with consciousness but how such mere objects can give rise to the eerily different phenomenon of subjective experience seems utterly incomprehensible.
Despite this, I can’t go so far as to conclude that mind poses some insurmountable barrier to materialism. There are two reasons. The first is, frankly, more a sociological observation than an actual argument. Science has, since the seventeenth century, proved remarkably adept at incorporating initially alien ideas (like electromagnetic fields) into its thinking. Yet most people, apparently including Nagel, find the resulting science sufficiently materialist. The unusual way in which physicists understand the weirdness of quantum mechanics might be especially instructive as a crude template for how the consciousness story could play out.
Physicists describe quantum mechanics by writing equations. The fact that no one, including them, can quite intuit the meaning of these equations is often deemed beside the point. The solution is the equation. One can imagine a similar course for consciousness research: the solution is X, whether you can intuit X or not. Indeed the fact that you can’t intuit X might say more about you than it does about consciousness.
And this brings me to the second reason. For there might be perfectly good reasons why you can’t imagine a solution to the problem of consciousness. As the philosopher Colin McGinn has emphasized, your very inability to imagine a solution might reflect your cognitive limitations as an evolved creature. The point is that we have no reason to believe that we, as organisms whose brains are evolved and finite, can fathom the answer to every question that we can ask. All other species have cognitive limitations, why not us? So even if matter does give rise to mind, we might not be able to understand how.
To McGinn, then, the mysteriousness of consciousness may not be so much a challenge to neo-Darwinism as a result of it. Nagel obviously draws the opposite conclusion. But the availability of both conclusions gives pause.
Perhaps a bit of definitional clarity would help. Or at least a gesture in that direction, a modification, if you like, of the US Supreme Court Justice Potter Stewart’s oft-repeated observation about pornography: we might not be able to define consciousness, but we know it when we experience it. I propose that consciousness can be defined as a particular state of awareness, characterised by a curious recursiveness in which individuals are not only aware, but aware that they are aware. By this conception, many animals are aware but not strictly conscious. My two German shepherd dogs, for example, are exquisitely aware of and responsive to just about everything around them — more so, in many cases, than me. I know, however, that I am conscious because I am aware of my own internal mental state, sometimes even paradoxically aware of that about which I am unaware.
I know I am conscious, and I also know that my dogs are aware. But are they conscious? Frankly (and speaking now as an animal-loving human observer, rather than as a scientist), I have little doubt that they are conscious, and that my cats are, too, and my horse as well … although as a biologist, I can’t prove it. A more satisfying stance, therefore — empathically, as well as ethically — is to give in to common sense and stipulate that different animal species possess differing degrees of consciousness. This might be more intellectually satisfying as well, since postulating a continuum of consciousness is consistent with the fundamental evolutionary insight of cross-species research: organic continuity. Most likely, consciousness ranges across a spectrum rather than being a special state that only humans experience.
...One possibility — a biological null hypothesis if you like — is that consciousness hasn’t been selected for at all. Maybe it is just a nonadaptive by-product of having brains bigger than is strictly necessary for bossing our bodies around. A single molecule of water, for example, isn’t wet. Neither are two, or, presumably, a few thousand, or even a million. But with enough of them, we get wetness — not because wetness is adaptively favoured over, say, dryness by the evolutionary process, but simply as an unavoidable physical consequence of piling up enough H2O molecules. Could consciousness be like that? Accumulate enough neurons — perhaps because they permit its possessor to integrate numerous sensory inputs and generate complex, variable behaviour — wire them up and, hey presto, they’re conscious?
The marketing department informs me that at least 33% of the regular readership here report being "deeply interested" in this very topic, so, having nothing substantial to add to the discussion here beyond some sagacious chin-stroking, some judicious brow-furrowing, and some well-timed "Hmmm's", I will recommend both of these fine essays for your perusal and go about my merry way.