Is Neutral Monism Possible?
My aims here are limited, as befits the topic. I will make some remarks about the proper formulation of neutral monism with a view to demonstrating its obscurity, not to say infeasibility. The thought is that we should seek a level of description of reality that is neutral between the mental and the physical so as to make progress on the mind-body problem. Putting aside the (very real) question of how to define “mental” and “physical”, we can ask what is meant by “neutral” here: what does it mean to say that a type of description, or type of conceptualization, is neutral? The word usually means something like “non-committal” or “impartial”—not favoring one thing or party over another. But whatever it is that unites the mental and the physical could not be neutral in this sense; on the contrary, it must be fully committed—in both directions at once. For it must express the essence of both the mental and the physical simultaneously: it must, in a word, reduce the mental and the physical to some third conceptual category. Neutral monism must be a committed monism—not at all neutral about the nature of the mental and the physical. It is easy to be neutral (non-committal) about the nature of the mental and the physical; it is much harder to provide a positive account of them. The doctrine known as neutral monism is really best described as all-encompassing monism or unifying monism. If you believed the mental and the physical could be unified using the concepts of causality or information, you would be a neutral monist in the intended sense, but you would certainly not be neutral about the nature of the mental and the physical. What is true is that the unifying monistic theory can’t simply use existing mental or physical concepts to capture the nature of the mental and the physical—that would deliver either idealism or materialism—but it does have to commit itself on the nature of both things. Russell’s brand of neutral monism did precisely that by identifying sense data as the neutral stuff: but of course it clearly favored the mental in its construction of reality as a whole, and is really a form of idealism. So what kind of description are we looking for that can unify the two domains without biasing the theory to one side or the other?
We might think we have something ready to hand, viz. what is called topic-neutral language. Discussions of the mind-body problem regularly invoke that category of expression, which is thought to be shared by both the mental and the physical. It includes logical language, mathematical language, temporal language, and language for causal relations, abstract structure, and modality. The idea is that such language is not confined to mental or physical discourse but crops up univocally in both. All well and good, but it is a bold man that claims such language can provide what the neutral monist seeks: this looks like a conspicuously exiguous basis on which to build a grand theory that unifies the mental and the physical. The language isn’t biased towards one or other side of the divide, but it is hopelessly weak as a putative reduction of the mental or the physical. So the existence of topic-neutral language is no comfort to a would-be neutral monist; it doesn’t encourage the idea that we might be able to contrive the kind of unifying description abstractly indicated. So far, then, we have nothing with which to fill out the conceptual terrain gestured at by the neutral monist. We are left at a high level of abstraction with no indication of how we are to produce the kind of theory we are looking for. The theory appears to be more of a wan hope than a substantial research program. Its logical form is an existence statement without any verifying instance.
Can we find any analogue of neutral monism elsewhere? Then at least we would know what we are talking about—we would have a model to go by. Here I think we reach the crux: for there is a model, hugely influential historically, that lies behind the neutral monist’s ambitions, and functions as its main inspiration. I mean atomism in the theory of the physical world. According to atomism, seemingly disparate elements of nature can be unified in a common vocabulary, which functions reductively. Thus the four traditional elements of earth, water, fire and air can all be explained by postulating homogeneous atoms that appear in different guises. The atoms are “neutral” in the sense that they appear in each element equally as common factors; the difference arises from their manner of aggregation—specifically, how tightly packed and mobile they are. They are dense and immobile in rocks and other earthy objects, also dense but more mobile in water and other liquids, quite rarified and volatile in fire, and highly dispersed and moveable in air. The unification works by finding a common constituent and then shifting the observed variety to relations between the constituents, specifically relations of proximity and motion. This is a kind of neutral monism of the four elements—and it works. It is actually true that the fourfold reality reduces to a single reality! The natural world turns out to be a lot more homogeneous than we supposed; the ancient atomists’ dream turns out to be sober fact. This provides a boost to the flagging spirits of the aspiring mental-physical unifier—maybe such an atomistic monism can supply the unification we seek. So we declare that mind and body must be composed of atoms of some sort that are shared between them; the variety or divergence we observe is but a superficial reflection of different relations between these underlying atoms. As the same physical atoms can occur in fire and water, so the same neutral atoms can occur in pain and salt. The atoms just combine differently, producing pain in one case and salt in another. The neutral monist has thus provided a model for how his conjectured theory might be true. He isn’t stuck just flapping his hands with a faraway look in his eye.
The trouble is, of course, that this kind of atomism is completely implausible as a theory of the mental and the physical. In the case of traditional atomism we are dealing with four types of physical phenomenon, but that is precisely what is not true of the mental and the physical. The atoms that work to unify physical phenomena don’t work to unify the mental with the physical. We would need a completely new type of “neutral” atom—a hitherto undiscovered particle—in order to vindicate the type of atomism suggested by the neutral monist. But we have no evidence of any such particle, nor even a clear conception of what we are talking about. So the model limps—in fact, it never even gets moving. It operates rather as a mirage, like illusory water on the desert horizon. It makes us think that we have a real theory-sketch in hand, which we just need to fill out; but in reality it distracts us from the nature of the problem. It gives us false hope. We still don’t know what neutral monism would look like if it were true. Citing the atomist precedent is yet another instance of trying to understand the mental-physical divide by reference to something quite different, i.e. divisions within the physical domain.
Does this mean that neutral monism must be false? No: it means that we don’t know how it can be true. We have no clear conception of what its truth might be like. It can’t be like idealism or materialism because they are not neutral; it can’t be stated by recourse to topic-neutral vocabulary because that vocabulary lacks the requisite expressive power; and it can’t be modeled on the example of classical atomism because it is a problem of a completely different order. Anything we can cite as a possible format for the theory fails to do what is required of it, and nothing else suggest itself. All we can say is: if neutral monism is true, then it must take a form that transcends what we can currently understand. Nor is it like anything we can currently understand. Perhaps it will entail abandoning wholesale our current conceptions of the mental and the physical (a kind of “error theory”)—we are systematically deluded about the real nature of these categories. Maybe reality is fundamentally different from the way we naturally conceive it, and possesses a unity we cannot even dream of. Or perhaps the whole idea of unity is itself a mistake. In either case we have nothing substantial on which to base our hopes for the theory called “neutral monism”. It is a theory without precedent or precise formulation. That doesn’t make it false, but it does make it close to unintelligible.
 We might label it “Janus-faced monism”: it has to provide a unitary vision from two directions of gaze.
 Compare all those well-known analogies to empirically discovered identities in the physical sciences such as “Heat is molecular motion”.
 By “unintelligible” I mean unintelligible to humans, not contradictory or otherwise necessarily false. It might be a true theory we can never grasp, even in outline. At present it amounts to not much more than the proclamation, “There must be something unitary out there otherwise the world would make no sense”.