I’m currently writing an essay on the so-called conceivability argument against mind–body identity statements, and whether neutral monism (aka ‘Type-F materialism’ in Chalmers-speak) presents be a suitable response to it. Ever since studying Bertrand Russell’s neutral monism, which accounts for the reality of both mental and physical entities in terms of a single ‘neutral’ type, namely events, I’ve been intrigued by the position, which seems to offer the potential for a reductive analysis of the mental whilst making it intelligible how mental and physical phenomena are so closely interrelated (Descartes’s problem of the union of mind and body).

The basic idea of neutral monism can be summed up relatively simply: the universe is neither wholly mental (a position known as phenomenalism) nor wholly physical (i.e. physicalism), nor comprised of an amalgam of the two (substance or property dualism, dual aspect theory, etc.), but rather of neutral elements that are themselves neither mental nor physical, but out of which everything mental and physical is comprised, or whose properties they supervene upon. The close relation between mental and physical phenomena may then be explained in terms of changes to the neutral elements that underlie the mental (i.e. phenomenal, psychological) and physical (i.e. brain states, nerve firings etc.) entities in question, and which comprise the various aspects of reality with which we are all familiar. It is important to note that this relationship is constitutive, rather than causal, and so what it is for something to be mental or physical is just that it is comprised of the relevant groupings of the underlying neutral elements, whose type and nature is unknown.

In effect, neutral monism solves the mind–body problem by positing a single ‘lowest common denominator’ non-physical ‘substance’ from which everything is comprised, and of which the mental and the physical are simply two different manifestations of. Mental and physical properties are merely different configurations or properties of the underlying neutral elements, and are interrelated due to their being comprised of one and the same stuff. As such, the theory is not unlike Kant’s account of phenomena and noumena in that the nature of noumena is entirely unknown — and quite possibly unknowable — to us, although that is not an essential part of the theory. It is sufficient that reality is, at base, non-physical and non-mental, and that the physical and the mental are merely two different ‘views’ onto the same underlying substrate.

The position is attractive because of its ontological parsimony and because it retains the basic spirit of scientific materialism but without identifying the underlying ‘material’ with the substances that are the subject of current scientific theory. As many scientists would acknowledge, such theories only tell us about the structural or causal properties of the universe, and not of its intrinsic properties, although this is no doubt the aim and aspiration of much modern physics. Nevertheless, most scientists and philosophers assume — justifiably or otherwise — that there must be some bearers of the relevant causal-structural properties, and so some underlying elements out of which the universe is comprised (contrary to the position known as structuralism, which suggests that only structural properties exist). Neutral monism posits the existence of just such elements, but places them beyond the reach of current physics, thus avoiding reducing everything to what the laws of physics tell us about — a position known as physicalism, which becomes philosophically problematic when accounting for the nature of mind and consciousness.

One such problem for the physicalist view concerns the kind of identities between mental and physical entities that it entails. Such terms would, according to Kripke’s modal semantics, have to be necessarily identical or, to put it in common philosophical parlance, refer to the same thing ‘in every possible world’. However, our modal intuitions would seem to suggest otherwise, since — according to Kripke and Chalmers at least — we seem to have little difficulty in imagining pain without C-fibre firings, to take a well-worn example, or exact physical duplicates of ourselves that have no inner experience (aka philosophical ‘zombies’). Whether such intuitions are a reliable guide to reality is itself a much debated methodological issue, but assuming that there is something to this line of argument, it would seem that the apparent contingency of the relevant identity statements calls for an explanation.

Neutral monism seems to offer just such an explanation, and it goes something like this: the mental and the physical are interrelated via their constitutive relations in the underlying neutral (i.e. non-physical and non-mental) substrate. Mental properties pick out the intrinsic properties of elements of this neutral substrate, and are therefore essential properties that pick out the same elements in every possible world. Physical properties, on the other hand, are individuated by their causal role such that being a particular brain event — a neuron firing, for example — is a contingent property of the neutral elements that could, in another possible world, be realised via an intrinsically different instantiation of such elements. Physical events, brain events, etc. are therefore contingent properties of the neutral elements, since they are multiply realisable and need not correspond to the same intrinsic (i.e. mental) properties in every possible world. The apparent contingency of mind–brain identity statements is therefore explained by the hypothesis that physical events (the right-hand side of the identity statement) are picked out via a contingent structural property of the underlying neutral elements, namely that of being a particular brain event, whereas mental events (the left-hand side) pick out an essential intrinsic property of the very same elements, e.g. that of being a particular sensation of redness. Since one is necessary and the other contingent, the two terms can (in theory at least) come apart, thus creating the logical possibility of disembodied minds or philosophical zombies. In practice, this will depend very much upon the relevant properties and configurations of the neutral substrate, so whether such esoteric entities are physically possible remains a moot point (although, to be fair, no-one really thought this anyway — not these day, at least).

The above explanation, apart from being rather tricky to get one’s head around, has a number of things going for it. Firstly, neutral monism constitutes a powerful framework for resolving the infamous mind–body problem. Secondly, it is ontologically very minimal, positing only one fundamental type of entity (the neutral elements), rather than two (the mental and the physical). Of course, this is no different to physicalism itself, which suggests that all other phenomena are constituted of or supervene upon physical properties, but this position is notoriously difficult to maintain, and in any case makes the philosophically dubious assumption that our knowledge of physics is somehow fundamental, despite the fact that such knowledge is only accessible from an irreducibly first-person perspective (although no doubt others will disagree).

Perhaps the main problem with such a view is that it accounts for the nature of phenomenal experience in terms of a kind of object or entity, as opposed to a relation. In many ways, this just seems to be the wrong ontological category to place experience in. It certainly makes sense to think of us as standing in, for example, perceptual relations to objects in the world, and it seems plausible that the phenomenal character of those experiences are a kind of property of this relation — a position known as adverbialism. This idea would be difficult to capture within the framework of neutral monism, since the phenomenal aspects of experience are explained in much the same way as the objects which they are experiences of without reference to their relational nature. The disjunctivist notion of perceptual experience is similarly problematic (although perhaps unnecessary under neutral monism).

Such concerns notwithstanding, the neutral monist position, when stripped of its historical baggage, represents an intriguing and, in my view, underrated response to the conceivability argument, as well as to the issue of the relation between mind and body in general. You might worry that it is a bit of a metaphysical sledgehammer to crack a philosophy of mind nut, but there are many reasons to find the view attractive — not least that we seem to know a lot less about the ultimate constituents of the physical universe than might first appear.