Some philosophical works are valuable because of the problems they formulate and because they pose the questions that really matter with greater precision than we're used to, even though their approach to those problems and questions leaves much to be desired by way of argumentative rigor. Nagel's work here is of that sort, I think. He himself has the humility and self-awareness to explicitly recognize this. I commend his effort to defend the value of doing this sort of problem-clarifying work in an intellectual milieu which has become a minefield of controversy and ideological turf-wars.
The thing is that, to my knowledge at least, there is still no consensus as to whether there is an explanatory gap, where the explanatory gap refers to the idea that we cannot in principle explain, without remainder, the structure of conscious experience by referring solely to data derived from even an ideally-completed third-person, scientific account of mind (which we could arrive at by adding up the evidence from neuroscience, evolutionary theory, psychology, and cognitive science). Moreover, there is no consensus concerning just how much of an issue this explanatory gap really poses for scientific naturalism. Can scientific naturalism address it by making localized changes to its general conceptual framework? Or is the explanatory gap the critical anomaly that reveals that we need to rethink our general approach to explaining nature as a whole? Nagel (boldly) argues for the latter claim. Needless to say, that’s not going to win him many allies. The ideological climate in contemporary Analytic philosophy is such that I suspect that most readers are not likely to find his founding assumptions - never mind his overall conclusions - even remotely plausible.
Nagel argues that we need an account of the structure of nature that reveals how, given what we know about the fundamental constituents of the natural world, consciousness, rationality, value, meaning, and knowledge itself must necessarily arise as a result of natural processes. And yet, Nagel argues that current scientific naturalism can at best explain how these properties, like rationality, are merely accidental by-products of natural processes. Given how scientific naturalism explains both nature and mind, it seems unintelligible that rational minds should appear that are capable of responding to values and of disclosing the structure of nature through science. So, Nagel reasons that we need to rethink nature, as well as our dominant patterns of explanation, if we are to make sense of how nature could give rise to minds.
Moreover, he argues that rational self-understanding can reveal that moral and rational agency are essential features of the mind that any complete explanation must take into account. Thus, Nagel makes a very unpopular move here (again, one not likely to win him many fans in a scientistic intellectual climate), by arguing that it’s not just science that must inform philosophy, but also the other way around, such that our most rigorous philosophical self-understanding should constrain and guide scientific inquiry. A priori, rational, philosophical reflection can provide data that science must then seek to explain. Thus, by clarifying what he takes to be the proper target for explanation, Nagel thinks he can establish that our current scientific naturalist paradigm is set on a course that could never reach that target.
Most interestingly for me, he seems to argue that a key constraint on any explanation is that it should enable us to explain the possibility of explanation itself. What explains the explainer? What must nature be like if it is to make possible the emergence of an explanation of nature? How do you get meanings, values and rational structures out of brute facts about objects and their causal relations?
The key, initiating move in Nagel’s work is a very unpopular one, and one which primarily rationalists and idealists have made, historically: namely, it’s the move of starting metaphysical speculation with self-knowledge. The idea is that the adequacy of our current naturalist metaphysical theory, in general, can be measured in part by the extent to which it can explain the data revealed by a complete theory of human nature. In other words, for Nagel, the human mind is a corner of nature that exhibits rather bizarre and paradoxical properties - such as a responsiveness to objective values. Moreover, these properties cannot be explained as mere extensions and complexifications of the properties that other natural processes exhibit. And, if nature generated such strange abominations as we are, then our general theory of nature must surely include fundamental properties that don’t make it seem absurd that we exist. Our best philosophical account of the properties of human nature thus gives us enough reason to rethink the inventory of fundamental natural properties and principles.
Ultimately, Nagel argues that far from thinking of the explanatory gap as just a localized side-issue within a specialized sub-branch of philosophy (namely, philosophy of mind), we should instead recognize that the solution to this problem enjoins us to rethink nature itself. Thus, the problem of explaining consciousness and rationality is, for him, a symptom of a much more general deficiency in our entire understanding of what nature is. If he’s right, scientific naturalism lacks the conceptual and methodological resources required to explain what nature must be like if it is to generate beings capable of comprehending it. It lacks the resources to explain how it is that being can become intelligible to begin with. It also lacks the resources needed to explain what nature must be like if it is to be a bearer of value. It’s thus a view fundamentally incapable of adequate philosophical self-understanding.
Unsurprisingly, if you are suspicious of Nagel’s founding argumentative move - namely, of starting with self-knowledge/philosophical anthropology, and trying to use that to identify constraints for your general theory of being - you’re unlikely to think much of Nagel’s overall conclusion.
So maybe we need an enriched, teleological account of causation, Nagel argues. That might solve the multi-headed explanatory gap described above. A Neo-Aristotelian picture of nature, which recognizes not just mechanical causation but also teleological causes (and so can presumably ground values in the causal structure of the natural process), might save us and solve the problem of explaining how nature can generate not just creatures capable of digestion and reproduction, but also creatures capable of knowledge, meaning, value, rationality, and consciousness.
There have been people who explored that sort of move: see Terrence Deacon’s work, Incomplete Nature, which argues that what we ultimately need to do in order to close the explanatory gap and explain the place of mind and value in nature is to explain how a teleological level of causality, at the level of mind and life, can emerge out of mere mechanical causality at the level of fundamental physics. Deacon provides a much more sophisticated argument than Nagel does, and he ultimately shows just how much argumentative work we’d need in order to explore the overlooked teleological alternative rather crudely sketched in Nagel’s work here. Hence, it is all the more surprising that Nagel doesn’t engage with his work.
However, I have my doubts that by “going teleological” we’ll solve the explanatory gap that Nagel says we still haven’t solved (for all our complacent efforts to find ways to safely ignore it, or define it away - see the heroic efforts of Dan Dennett and the Churchlands for a shining example of that strategy). And this brings me to another notable omission of Nagel: he makes no mention at all of the entire post-Kantian tradition’s sustained critiques of scientific naturalism. Philosophers like Kant, Cassirer, and Husserl have long since argued that reason is not the kind of thing that you can explain by getting behind it, precisely because it is the ever-presupposed foundation that we stand on in making any intelligible knowledge claims about anything whatsoever.
To be fair, Nagel briefly touches on this line of argument in passing, in his section on “Cognition” There, he argues that any attempt to provide a naturalistic, evolutionary explanation of reason (e.g. by trying to ground the validity of rational inference by citing its reliability as a fitness-enhancing instrument in the lives of our ancestors) will inevitably presuppose, rather than explain, reason. Nagel argues that all explanation must bottom out in truths that we take to be evident in themselves. When we endeavour to explain reason “from outside,” like evolutionary accounts of rationality do, we simply presuppose the activity of reason which gives us access to self-evident truths. Nagel thus ultimately defends Descartes’ basic insight that reason, as the faculty that alone gives us immediate access to truths grasped as “valid in themselves”, is its own only possible arbiter, and can thus only be explained in its own terms, rather than being something we could explain from a third-person perspective that tries to ground it in external physical facts that we take to be more fundamental. In other words, by trying to explain reason naturalistically, we simply presuppose it in rendering the naturalistic explanation intelligible. Thus, reason cannot be explained naturalistically without circularity. I have a feeling that a lot of the readers of this work (judging by the reviews on here) simply missed the force of some of these more subtle and interesting arguments proposed by Nagel.
However, while Nagel touches on what to me seems to be the crux of the issue (i.e. why we have an explanatory gap at all to begin with), he doesn’t have the conceptual resources needed to fully explore possible solutions to the explanatory gap. This is where a dialogue between Analytic and Continental (esp. post-Kantian) philosophical traditions would be really helpful. Nagel, working on the explanatory gap by using primarily the resources of Analytic philosophy of mind, can barely hint at the general problem of constitution, as Kant, Cassirer, and Husserl called it - which is the problem of identifying the necessary conditions for the possibility of rendering objectivity as such intelligible to begin with. In my view, once you pose that problem - which scientific naturalism doesn’t even have the resources to pose, as Husserl argued in the Cartesian Meditations - you begin to fall down the endless rabbit hole that lets you fathom just how deep the explanatory gap really is, as well as understanding just how thorough-going a conceptual challenge it poses to our scientific naturalist worldview.
I suspect that a great deal of the (by now predictably) vitriolic response to this book is powered by the fact that, as a culture, we secretly suspect that all critiques that seek to expose the conceptual limits of naturalism are powered by hidden theological motives (despite the author's protestations to the contrary), or by a desire to protect some special, heart-warming, life-preserving personal fiction. Any line of argument that tries to reveal the in-principle conceptual limitations of this dominant pattern of explanation, such as Nagel explores here, has become anathema.
It is thus no longer surprising to me that Nagerl’s work has come under such attack for simply posing anew what is still, I think, the greatest challenge for scientific naturalism. He accurately identified the issues, and nicely explained why they’re issues. He also usefully showed how at the heart of this debate lie some deep-seated intuitions about what we're willing to take to be a basic truth. What do we take to be more fundamental? Do we have idealist intuitions, such that we think that physical facts aren’t self-explanatory, but that their intelligibility must instead be explained by reference to actual or possible experience? Or do we have physicalist intuitions, such that we think that experiential facts must be explained in terms of more fundamental physical (behavioural, evolutionary, neurophysiological) facts? Your basic intuitions about what you take the order of explanatory primacy to be here affects how you’ll approach the explanatory gap debate, and I am not sure that this fundamental theoretical decision can be determined through purely rational means (in this, my gut feeling seems borne out by the vitriolic pissing contest between these two sides, where both sides seem stuck attacking each other in the same rut, never gaining traction because at bottom the whole shitshow is a contest of competing intuitions).
I think that Nagel is right to suggest that you cannot adequately explain how consciousness can enable the rational disclosure of the world by just adding more of the same kind of epicyclical postulates to the inventory of the natural world, as conceived in the standard physicalist worldview. Accounting for how a chunk of the universe can turn itself back upon itself to take itself as an object of knowledge (and as subject to values that exist independently of it!) requires -something other- than adding in more of the same, and just filling in some blanks that are remaining here and there in an overall picture that is otherwise structurally sound.
Instead, I think that Nagel is right to argue that explaining this requires a creative (and heretical) rethinking of the pattern of explanation we’re all used to taking for granted. We’re so used to just mistaking this scientific naturalist picture with nature that we’ve forgotten the need to critique the limitations of this picture - and to remember that it really is just a revisable picture, whose SOLE value is its capacity to explain our experience - which, as Nagel points out, it is woefully inadequate at. How it is even possible for nature to become intelligible through the medium of experience seems, on this world picture, like an absurd little miracle.
Unfortunately, though, Nagel simply lacked the conceptual tools needed to do more than just pose the problem anew - albeit eloquently and fairly accurately, I think. I also think that the teleological move is probably a dead-end, particularly given the fact that that’s not the only option on the table (see the bit above about engaging with the post-Kantian tradition).
Ultimately, your intuitions, as a reader, about what counts as an answer to the following questions will determine whether you will take Nagel’s book to be a liberating breath of fresh air, a brave work exploring an overlooked (as well as repressed and much demonized) alternative in an increasingly dogmatic intellectual climate, or else a deluded crypto-theological, or at least crypto-dualist piece of tripe:
Can we know, through purely rational reflection and in advance of any experimental work, i) what the essential properties of our minds are, and ii) that these essential properties, like rationality, cannot be explicated without making reference to the first-person perspective that reveals us not merely as objects bound by causal relations, but rather as subjects of knowledge?
If your answer to both questions is yes, then Nagel’s argument will have some force for you. If the answer is no, then your foundational assumptions are likely to be so far skewed away from Nagel’s as to make his arguments seem to have hardly more than dishwatery consistency. You are unlikely to be persuaded by his argument that he has identified in-principle limitations for a scientific naturalist explanation of the place of consciousness, rationality, and value in the natural world. You’re going to leave open the possibility that by going down the same road it’s currently headed on - and by stacking up some exotic new species of fact - scientific naturalism in one of its myriad permutations will explain consciousness, rationality and value without remainder. If, in contrast, his in-principle arguments seem plausible, and you’re open to philosophical reflection informing our scientific self-understanding, you might at least find his line of argument interesting.