Thomas Nagel’s book Mind and Cosmos has garnered a lot of attention since its publication, with the reaction being intense, heated, and of largely mixed opinion (For a nice overview and response to negative views, see here). Not surprisingly, those sympathetic to naturalism have been harsh in their assessment, and those not so sympathetic have been full of praise. My assessment is ultimately negative, but I think both the defenders and critics of Nagel’s book have relied on a set of shared assumptions that are deeply problematic and masked by ambiguity. By exposing the hidden assumptions framing the debate between “reductionism” and “non-reductionism”, I show that Nagel’s criticism of “reductionism” is sound, but this is no reason to reject physicalism, only an outdated and confused version of physicalism.
1. The Problem of Hierarchy: Two Views of Reduction
The controversial subtitle of Mind and Cosmos is “Why the Materialist Neo-Darwinian Conception of Nature Is Almost Certainly False”. Quite the claim! But what exactly is “Materialist Neo-Darwinism” and who really holds this view? More precisely, Nagel’s target for criticism is what he calls:
“A particular naturalistic Weltanshauung that postulates a hierarchical relation among the subjects of those sciences.”
I have a lot to say about this notion of a “hiearchical relation”, so bear with me! I propose Nagel’s invocation of a “hierarchy” is ambiguous between two senses of the term. First, there is a hierarchy of scale that is compositional e.g. Society is composed of individuals, which are composed of cells, which are composed of molecules, which are composed of atoms, etc. I take compositional hierarchy to be non-controversial.
The second sense of “hierarchy” is what John Heil calls a “hierarchy of being” or “hierarchy of reality”. On this view, you have lower-level “realizer bases” out of which higher-order properties “emerge”, in the sense of “strong” emergence where the whole is non-compositionally greater than the sum of the parts (whatever that means). For example, on the hierarchical view of reality, the mind “emerges”, “arises”, or is “generated” as a higher-order property “out of” the lower realizer bases, and is said to “supervene” on that lower base. Nagel is perfectly right to think this is a mysterious process, but that’s a good reason for physicalists to reject the entire hierarchical view of reality, not give up on physicalism.
The alternative picture is that reality is flat. There is one level of reality: it’s called reality! Within that level we can find hierarchies of scale and composition, but ultimately there are not two different levels or types of “being”, one physical and the other mental, with the latter “emerging” or “arising” out of the former. Nagel seems to think that physicalism is committed to this view, but ironically the problems he locates with reductionist physicalism stem from his own dualist intuitions about the existence of “higher” levels of reality. However, an ontologically serious physicalism has no room for different “levels” of reality, and thus it is not a “problem” to try and reduce the mental “level” to the physical “level” because that view of reduction relies on the problematic notion of there being a hierarchy of reality in the first place. In his book From an Ontological Point of View, John Heil recommends that
…[W]e abandon the notion that reality is hierarchical. We can accept levels of organization, levels of complexity, levels of description, and levels of explanation, without commitment to levels of reality in the sense embraced by many self-proclaimed anti-reductionist philosophers today.The upshot is a conception of the world and our representations of it that is ontologically,but not analytically,reductive.
Agreed! This result generalizes to the entirety of Nagel’s critique of physicalist reductionism. While Nagel is right to argue that reductionism is problematic, the entire problematic of reducing higher levels of reality to lower levels of reality is a product of the dualist worldview, not the physicalist one. It bears repeating that on the physicalist view, there is no problem of “reducing” the mental to the physical because setting the problem up like that buys into the traditional dualist framework of reality as hierarchically layered. In a nutshell, Nagel is criticizing a dualistic view of physicalism, and thus his critique of physicalism says more about how Nagel views the world than it does about how physicalists do.
To be fair, many physicalists have unwittingly bought into the same “levels of being” view that is problematic, but insist that the “reduction” of the highest levels to the lowest is tractable. I agree entirely with Nagel that this project of reduction is doomed to failure, but not for the same reason Nagel does. My problem with the project of reduction is that it sets up the problem wrong, whereas Nagel think it reveals the truth of dualism (or at least the falsity of physicalism).
Nagel claims that his book is
“…[N]ot just about acknowledging the limits of what is actually understood but of trying to recognize what can and cannot in principle be understood by certain existing methods.”
Having cleared up the ambiguity of hierarchies, we can now analyze Nagel’s conclusions about what can and cannot be understood by the physical sciences. If the project is to say it is impossible for the physical sciences to “reduce” the “mental level” to the “physical level” then I agree entirely. But not because I doubt the truth of physicalism. The mental level cannot be reduced to the physical level because the whole idea of there being different levels is a product of the traditional dualist worldview where reality is divided into base terrestrial materials and higher spiritual realms.
Thus, when Nagel criticizes the project of “materialist reductionism,” he kind of has a point, but it’s impossible to see this unless we clear up ambiguities about how physicalists can think about reductionism. I propose there are essentially two types of materialist reductionism: call them Reductionism and Eliminativism. Reductionism is the confused view we have been discussing, where it’s acknowledged at the start there are two levels of being, and the problem is to reduce the mental level to the physical level or provide psycho-physical “bridge laws” between the two levels. In contrast, Eliminativism says all that exists is the physical level. Rather than starting with two levels, Eliminativism starts with one level and all phenomena as well as explanations of phenomena are framed in terms of this level of reality.
As I see it, Nagel is right to criticize Reductionism, but his arguments don’t apply to Eliminativism. Unfortunately, Nagel doesn’t make this distinction clear and doesn’t engage with the alternative one-level view of reality, which is the true competitor to the traditional dualist worldview. I can imagine what Nagel would say if he heard about this distinction. He might say it’s completely counter-intuitive because it denies the reality of “first person subjective experience”. But again, this talk of “denying reality” is ambiguous between denying that psychological phenomena “live” in the higher-realm of a two-level reality or denying that it exists at all. Physicalists don’t have to deny that psychological phenomena exist; they will just say that, whatever they are, they “are” just like everything else: existing in the complex, flat ontology that is our physical universe. Thus, physicalists only deny the reality of mental phenomena if it is assumed that mental phenomena “live” in a higher plane of existence in the two-level picture of reality.
Of course, given the complexity of the universe, physicalists have no expectation that humans will be cognitively comfortable (or even capable) of accounting for everything they want to explain in terms of the lowest compositional scales (atoms, quarks, etc). Rather, we abstract to higher compositional scales to deal with complex phenomena like human civilization. But all this abstraction is abstracted within a single level of reality. The psychological necessity of parsing the world into higher and lower scales is not a metaphysical proof of separate planes of reality. In fact, a physicalist understanding of human cognition would predict that humans would not be capable of handling reams of data without compressing it with language, graphs, equations, simulations, models, etc.
Thus, the following statement by Nagel is actually a prediction of the naturalistic concept of human cognition:
“For a long time I have found the materialist account of how we and our fellow organisms came to exist hard to believe.”
Of course such a massively complicated proposition is “hard to believe”. A lot of things are hard to believe because we are forced to use our puny primate brains to understand a large, scarily complex physical universe with weird things like black holes, quarks, and gravity. Why would we expect it to be easy to imagine abiogenesis? Essentially, Nagel’s argument that not-P is: “I don’t understand P, therefore not-P.” But if P is physicalism, and physicalism predicts that humans will have a hard time understanding complicated things, our lack of understanding (especially of nonscientists) is surely not a refutation of physicalism, merely a recognition of its cognitive complexity.
2. Flubbing the Facts
Ontology aside, I do think Nagel makes some empirical gaffes exposing his ignorance of mainstream biological science, especially when discussing the state-of-the-art materialist conception of abiogenesis. He says, incredibly:
“The more details we learn about the chemical basis of life and the intricacy of the genetic code, the more unbelievable the standard historical account becomes”
If history has taught us anything, believability by non-scientists is not a good metric for the truth of scientific claims. For many non-scientists, Einsten’s theory of relativity is unbelievable, but that obviously has no bearing on its truth. Nagel continues:
“[Abiogenesis] flies in the face of common sense…
“….It is prima facie highly implausible that life as we know it is the result of a sequence of physical accidents together with the mechanism of natural selection
“…When we go back far enough, to the origin of life – of self-replicating systems capable of supporting evolution by natural selection- those actually engaged in research in the subject recognize that they are very far from even formulating a viable explanatory hypothesis of the traditional materialist kind”
It would behoove Nagel to google the phrase “hydrothermal origins of life” to find proposed models of abiogenesis that start from recent discoveries of deep-sea heat vents. A quick search on Wikipedia and you can find the “iron-sulfur world theory” of abiogenesis. There are probably other rival hydrothermal theories, and I have no idea whether any of them are true, but the point is that they seem, to me, like a “viable explanatory hypothesis of the traditional materialist kind”.
Perhaps there is an ambiguity in Nagel’s use of “formulation”. Maybe he means “nobody has formulated a complete explanation established to be almost certainly true”. This would be true, but that’s simply an argument from ignorance. You can’t infer from the fact that we haven’t yet formulated a “complete” explanation that there isn’t a complete explanation.
However, if Nagel means “nobody has even formulated a viable educated guess” about the origins of life, this is almost certainly false, unless he can show why hydrothermal models aren’t “viable” as possible explanations.
3. Good Darwinians vs Bad Darwinians
Another thing that confused me is who Nagel considers to be a “bad Darwinian materialist” and who is a “good” materialist. Right after harshly criticizing Richard Dawkins for explaining the evolution of the eye “merely as a result of chemical accident, without the operation of some other factors determining and restricting the forms of genetic variation”, Nagel gives an example in a footnote of presumably “good”, non-reductionist Darwinians, “who insist that the evidence calls for a more restricted account of the sources of variation in the genetic material.” Surprisingly, Nagel cites Stuart Kauffman has a “good” non-reductionist, saying in that same footnote “Stuart Kauffman suggests in several books that variation is not due to chance, and that principles of spontaneous self-organization play a more important role than natural selection in evolutionary history.”
This is confusing on multiple levels. Is Nagel implying that Kauffman is “on his side” merely because he denies that all variation is due to chance alone and not chance in addition to other factors like the material property of spontaneous self-organization? If big, bad “Darwinian materialism” is supposed to be the claim that chance alone is the full story, then Nagel is making a stink about a view that few biologists actually hold. It might be convenient to treat chance has the primary operator, but this is because dealing with the entire material evolution of cellular matter is absurdly complicated, and would require a ridiculous amount of computing power to simulate.
I can’t speak for Kauffman’s own ontological views, but Kauffman-esque “non-reductionism” is ambiguous between materialist Reductionism and materialist Eliminativism. It’s conceptually possible that all the “spontaneous” self-organization of matter is a property of the single level of reality that is the physical universe. This might be wildly complex, but why shouldn’t it be? It’s the universe itself we’re talking about here, not a toy model of it. But nothing about wild and strange self-organizing properties of matter is inconsistent with materialist Eliminativism. Thus, Nagel’s appeal to Kauffman has a counter-example to reductionism is ineffective as a genuine criticism of the physicalist worldview.
As a card-carrying “Darwinian materialist” who wears his naturalistic credentials proudly, I am sympathetic to the received criticisms of Nagel’s book. However, unlike other naturalists who have criticized Nagel’s book, I think Nagel’s criticism of Reductionism is sound: it is impossible to reduce the mental level to the physical level. Nagel’s critics have mistakenly limited their options to either joining Nagel in attacking Reductionism or accepting Reductionism as formulated by Nagel.
I reject both options in favor of Eliminativism. The reason why it’s impossible to reduce the mental level to the physical level is that this very way of setting up the problem belies the traditional view of reality as composed of “levels of being”, with the mental level “emerging out of” or “arising from” the physical level. For eliminativists, there is a single level: physical reality, and humans exist only on this level. But we certainly don’t talk or write this way. For good reason: we have puny primate brains with finite memory and processing power. It’d be futile to try to understand WWII in terms of Schroedinger’s equation, but this cognitive inability does not entail the falsity of quantum mechanics. Likewise with psychology.