The third chapter of Mind & Cosmos is maybe the most critical of the entire book. If the phenomenon of consciousness resists strict reduction to physical and purely chemical laws, then we lack a comprehensive and detailed clarification of how it might have appeared in living organisms over the course of evolution.
Nagel starts noting that reductive materialism has failed to account for subjective aspects of mental content. As a long history of unsuccessful attempts since Descartes to present time shows, subjective experiences cannot be reduced to quantitative descriptions of the type developed by physical sciences. Phenomenal qualities such as smell, color, sound, etc., resist a straight rendering in terms of physical laws (Nagel discards one after another a certain amount of proposals which rely on reduction, such as conceptual behaviorism, functionalism, psycho-physical identity theories and eliminative materialism). Nonetheless, it has been widely hailed in the last few decades that an evolutionary account of the nature and origins of life on earth is meant to represent a suitable connection between physics and biology capable of elucidate the experience of the mental, along with its place in nature, within a complete reductionist scheme. However, Nagel assumes that if psychophysical reductionism proves to be untenable, then evolutionary accounts of the development of consciousness should be queried as well; the decisive point seems to be that such evolutionary accounts have to make clear how consciousness has emerged into certain living organisms as one of their constitutive features, not only as a fortuitous bonus or an epiphenomenon.
And to complete the link with physics, the explanation has to suppose that there is a nonnegligible probability that some sequence of steps, starting from nonliving matter and depending on purely physical mechanisms, could eventually have resulted in a replicating molecule capable of all this, embodying a precise code billions of characters long, together with the ribosomes that translate that code into proteins. It is not enough to say, “Something had to happen, so why not this?” I find the confidence among the scientific establishment that the whole scenario will yield to a purely chemical explanation hard to understand, except as a manifestation of an axiomatic commitment to reductive materialism (49).
As far as we know, a reply to that intricate problem has not been given yet. Nagel points out two necessary conditions for a theory To be capable of explaining consciousness as a non-accidental feature of the world. It needs to evince (1) “why specific organisms have the conscious life they have” and (2) “why conscious organisms arose in the history of life on earth” (50-51), in other words, we need an “a-historical constitutive account” of how certain biological organisms are also mental, and a “historical account” of how they arose in the universe. According to Nagel, a constitutive view can be reductive or emergent, and an historical reconstruction can be either causal, teleological or intentional.
Nagel dedicates careful attention to the distinction between “reductive” and “emergent” answers to the constitutive problem. “Reductive” In this context is meant to be an account which explicates the mental content of organisms entirely in terms of the properties of their basic constituents, while an “emergent” account relies on the possibility to link mental aspects to the higher-order physical functioning of neural systems (specifically, by somehow relating mental states to higher brain activities). Both solutions, however, has to confront with serious drawbacks. If mental character, or consciousness, is to be present in the basic elements of organisms, it follows that elementary matter has both physical and non-physical properties (we might call “panpsychism” this kind of monist conception); on the other hand, the explanation of the appearance of “emergent” mental states on the basis of complex physical (brain) structures seem like magic in so far as consciousness has to be conceived as something completely new in the natural order.
As far as historical considerations are concerned, we have to deal with three alternatives. Leaving aside “intentional” renderings of natural order which implies at some point a sort of divine intervention, the causal explanation seems to have more credits then the teleological one. Both in its emergent and reductive forms, however, causal reconstruction of evolutionary development of consciousness has to deal with a lack of intelligibility. For example, monist panpsychism might explain, on the assumption that consciousness experience has causal implications, how visual capacities along with their related phenomenal mental aspects were built in the development of certain living beings in a way that the presence of those features enhanced the fitness of resulting organisms.
But that would not explain why such structures formed in the first place. Even if the possibility of a visual system is somehow already implied by the properties of the basic elements, how can a nonmaterialist monism help to explain its appearance in actuality, over geological time? How could the same active principles that accounts for action and perception in a fully formed organism also account for the original formation of organisms and the generation of viable mutations over evolutionary history? These questions are analogous to those that can be posed with respect to a purely materialistic reductive evolutionary theory, and they seem just as hard for a nonnaturalistic theory (64)
Teleological laws might be a feasible explanation of how living beings had evolved over time following not fully deterministic principles of change; however, teleology has to do with outcomes endowed with value, and it is difficult to understand value apart from the purposes of organisms who aim at it. Moreover, it might be impossible to grasp universal teleological laws from our limited point of view of earthly creatures. “The teleological option is in many ways obscure” concluded Nagel (67). Though consciously in need of a clear response to intelligibility and likelihood problems, Nagel does not end up in skepticism. Philosophy has to do with the uncertain, and has its most fundamental aim in forging patterns out of current knowledge instead of giving wholly definitive answers.
Philosophy cannot generate such explanations; it can only point out the gaping lack of them, and the obstacles to constructing them out of presently available materials. But in contrast to classical dualism, I suggest that we should not renounce the aim of finding an integrated naturalistic explanation of a new kind. (68-69)