I. Introduction

The conceivability argument (hereafter abbreviated as CA) is an argument against any kind of materialism that takes mental and physical events1 to be numerically identical, but only contingently so. Such arguments have been given by Descartes (1641: §4), Kripke (1980) and Chalmers (1996) and concern the conceivability, and thereby the possibility, of disembodied minds and mindless bodies, the latter of which are often referred to as philosophical ‘zombies’ (ibid. 94) — beings that look and behave exactly like us and yet have no ‘inner life’ or conscious experience. Standard responses to these arguments include a priori materialism (Stoljar 2000), substance or property dualism (Chalmers 1996), and contingent identities between ‘non-rigid designators’ (Jacquette 1987), as described in Section II. Perhaps a less well-known response, however, is based upon the metaphysical doctrine of neutral monism — the view that the universe is neither fundamentally physical nor mental, but composed of ‘neutral’ elements from which all other substances and properties are comprised. This view was famously held by Russell (1921; 1927), Mach (1886) and James (1904), amongst others, although here I shall concentrate less upon the details of neutral monism itself and more upon the structure and plausibility of the response to CA that it facilitates. In particular, I will draw upon recent work by Maxwell (1978) and Stoljar (2001a; 2001b) who, despite describing their accounts as physicalist, could be equally described as holding a form of neutral monism, since the structure (although not their interpretations) of these accounts matches that given by neutral monism.

I begin by describing Kripke’s version of CA in Section II, along with some possible responses to it. In Section III, I present a neutral monist interpretation of Maxwell and Stoljar before highlighting some potential problems for this approach in Section IV. Finally, I conclude that the neutral monist response to CA is successful, but may still be problematic due to the way that it accounts for the metaphysics of perceptual experience.

II. The Conceivability Argument

The type of materialism that CA is opposed to claims that in some, but not all, possible worlds, each mental event can be identified with some corresponding physical event such that they are one and the same thing. This ‘type-B’ materialism (Chalmers 1996: 165) does not mandate that this is the case in all possible worlds — i.e. the entities in question are not necessarily identical — but merely that such identities hold in the actual world, along with a suitable selection of close possible worlds. Moreover, CA applies to both type-type and token-token identity theories, the first of which posits identities between mental and physical event types, e.g. ‘pain = C-fibre firings’, and the latter between particular mental and physical events, or tokens, e.g. ‘my headache at 2pm = these activations of nerve endings within my brain at that time’. For the purposes of this essay I will concentrate mainly upon the latter, but similar considerations apply to both. It is important to note that the relationship that materialism posits is one of identity, and not mere correlation. Consequently, certain pains just are C-fibre firings (to borrow Kripke’s example) and so must possess precisely the same properties — something that is a potential problem for any identity theorist (see Section IV: the grain objection).

Kripke’s version of CA proceeds from what he takes to be a basic intuition about the connection between mental and physical phenomena. Kripke claims that we can imagine a particular pain (e.g. my headache at 2pm) existing without the particular pattern of nerve activations that the materialist claims it is identical with. Had the relevant portions of my brain been replaced with identically functioning pieces of silicon, for example, then it seems that I could have had the same pain in the absence of these particular nerve activations. More generally, we can conceive of minds that exist independently of any body, in which case their ‘pains’ and other mental phenomena would be independent of all physical activity. Of course, the existence and status of such intuitions is highly contentious, and many philosophers (e.g. Stich 2004) question their significance. Even if such do intuitions exist, they may be inaccurate or misleading due to our lacking knowledge of the relevant mental and physical domains (e.g. the laws of physics). Alternatively, they may be the result of exposure — direct or otherwise — to the works of such philosophical luminaries as Descartes and Kripke, or traditional religious beliefs, rather than indicating an innate proclivity to countenance the possibility of immaterial spirits or ‘zombies’. For the sake of argument, however, I will accept that such intuitions are widespread and that we can and do have knowledge of modal facts on the basis of them since, in the absence of this link between conceivability and possibility, it is difficult to see what else could underpin genuine modal knowledge.2

Kripke’s argument also draws upon the modal semantics that he himself created (Kripke 1963). He defines a rigid designator as any linguistic term that identifies the same individual in every ‘possible world’ in which they exist. This category includes most proper names as well as some demonstratives, such as ‘that pain’. Conversely, a non-rigid designator picks out different individuals in different possible worlds, as is generally (although not always) the case with definite descriptions, such as ‘the author of Naming and Necessity’. This picks out different objects in other possible worlds, since it is not an essential property of Saul Kripke that he wrote this particular work.3 Due to the indiscernibility of identicals (i.e. Leibniz’s Law) and the way in which objects in possible worlds are ‘stipulated’ rather than ‘discovered’ (Kripke 1980: 44), all identity statements between rigid designators turn out to be a posteriori necessities. Whilst the detailed arguments for this thesis lie beyond the scope of this essay, the necessity of identity is central to both Kripke’s modal semantics and to CA, as well as being widely accepted by contemporary philosophers and logicians.

Putting all this together with the above intuition concerning the apparent contingency of mind–brain identity statements, and assuming that the relevant terms are in fact rigid designators, Kripke presents the materialist with the following dilemma. Either:

1. (1)The equivalence between mental and physical events is only contingently true, i.e. true in just some possible worlds, in which case such correlations are not identity statements, since this would contravene the necessity of identity, and so type-B materialism is false, or
   

2. (2)Such identities are necessarily true, i.e. true in all possible worlds, in which case the materialist must provide some explanation for their apparent contingency, as established by the above intuitions.
   

Kripke takes it that proponents of materialism must take the latter route if they are to vindicate their theory, although this is by no means the only option. One could, for example, accept the necessity of identity and question the validity of the relevant modal intuitions (although neither Kripke nor Chalmers finds this option plausible). Alternatively, one could reject the semantic framework that gives rise to the problem, claiming that necessary of identity is merely an artefact of the way in which Kripkean possible worlds are specified. In the absence of an alternative modal semantics, however, this option is unattractive as it calls much recent philosophy into doubt; e.g. Chalmers (2004) and Stalnaker’s (2001) two-dimensional semantics. A more promising strategy might be to deny that the relevant mental and/or physical terms are in fact rigid designators. On this view, ‘that pain’ or ‘those C-fibre firings’ might be definite descriptions that only contingently pick out the relevant events, rather than doing so necessarily. In this case, however, Kripke (1980: 331) argues that the relevant non-rigid terms may simply be replaced with suitable rigid designators, and the argument run again to show that these are not identical. A determined materialist might resist this move on the basis that it is question-begging, but this would seem to undermine the materialist’s own claim that pains, nerve activations etc. are natural kind terms, since we do normally think of terms such as ‘water’ and ‘H2O’ as picking out the same thing in all possible worlds. This response does, however, highlight a major assumption in Kripke’s argument, thus creating a point of departure for the kind of ‘non-materialist physicalism’ that is discussed in Section III.

In response to the above dilemma, Kripke considers the attempt to explain the apparent contingency of mind–brain identity statements by analogy with other comparable scientific identities, such as those between heat and molecular motion, or water and H2O. In these cases, Kripke argues that the apparent contingency arises from the presence of a suitable phenomenological counterpart, namely the sensation of heat or the observable properties of water, by which the left-hand term is normally picked out. He then argues that we are liable to mistake the contingency of the causal relationship between this counterpart and the right-hand term for the contingency of the original identity. For example, although it is conceivable that in some possible world the sensation of heat is caused by something other than molecular motion — such as green light, for example — this does not show that heat is not identical to molecular motion, but merely that the sensation by which we detect heat is contingently caused by it. The resulting ‘illusion of contingency’ leads us to falsely conclude that such identities are contingent, when in fact they are necessary, as Kripke’s modal semantics predicts. In the case of pain and other phenomenal states, however, no such phenomenal counterpart is available, since pain (what it’s like to see red, thinking, etc.) is already a phenomenal state. Consequently there is no possible world in which ‘pain’ is distinct from the sensation of pain, since pain simply is the sensation of pain, or else such a world would fail to explain the apparent contingency of ‘pain = C-fibres firing’, since there is no reason why we should identify the non-phenomenal counterpart with the sensation pain. In the absence of any other explanation for the apparent contingency of mind–brain statements, Kripke concludes that such identities must be false and that, by implication, materialism is false.

Some philosophers (e.g. Hill 1997) have responded to this argument by attempting to provide an alternative explanation for the apparent contingency of mind–brain identity statements. Others (e.g Jacquette 1987) have sought to redefine the relevant mental and physical terms in a way that makes the contingency (apparent or otherwise) explicable. Throughout the remainder of this essay, I shall concentrate on one such approach that arises from the theory of neutral monism (Stubenberg 2005), a metaphysical doctrine which posits that the fundamental constituents of the universe are neither physical nor mental, but rather ‘neutral’ elements from which both the mental and the physical are comprised, or whose properties they supervene upon. My account is largely based upon Maxwell (1978) and Stoljar (2001a), but whilst they class their theories as physicalist, I claim that they are characteristically neutral — although since the structure of both theories is substantially equivalent, the difference turns out to be largely terminological.

III. Neutral Monism

The neutral monist response to CA is based upon two key assumptions. The first is that physical science does not tell us everything there is to know about the nature of the universe (Stoljar 2001a: 313; Maxwell 1978: 395). This is not simply due to the incompleteness of current physical theories, but that they can only provide us with a certain type of information about the world, namely its ‘causal structure’ (ibid. 346). Nevertheless, physics does (according to Maxwell) posit the existence of some underlying substance or elements that instantiate the structural properties that we can investigate empirically, such as the observable properties of matter and energy.4 These purely formal properties may be contrasted with the intrinsic properties of the underlying ‘causal network’ (ibid.), which are distinct from, but nevertheless give rise to, all of the causal properties described by physics. It is a logical consequence of this view that it would be possible for a given causal (i.e. physical) structure to be instantiated by a number of different causal networks. This makes the causal properties of any given part of the network (e.g. a C-fibre firing) a contingent, rather than necessary, property of the underlying network, since it does not possess this property in all possible worlds. The second assumption is that both physical and mental events are comprised of, or supervene upon, a single kind of entity which I shall refer to as a neutral element. Whilst the structural properties of these elements give rise to everything we observe in the physical world, as described above, their intrinsic properties correspond to the nature of ‘inner’ mental phenomena, such as sensations, thoughts, consciousness, and so on.5 Neutral monism is thus able to account for the intimate connection between the mental and physical domains by explaining both types of phenomena as being comprised of a single common or ‘neutral’ element.

At this point, neutral monism parts company with Maxwell and Stoljar, who describe the underlying elements as being essentially physical, rather than neutral in character. However, since both acknowledge that physical science only studies the structural (i.e. causal), rather than intrinsic (i.e. mental), properties of these elements, this assumption seems somewhat ad hoc since there is no reason to think of the physical realm as extending beyond the domain of causal phenomena. Stoljar’s (2001a: 313) appeal to the physical properties of objects (‘o-physical’) as opposed to those of physical theory (‘t-physical’) is similarly problematic since such properties are either causal, and therefore relate to structural and not intrinsic properties, or else they are mental, in which case they cannot obviously be classed as physical. Such ‘non-materialist physicalism’ may be contrasted with Russell’s overtly neutral monist account in which the underlying entities are described as being neither mental nor physical in nature, which is the approach I have taken here.6 Terminological differences aside, however, the structure of all of these accounts is substantially equivalent since they each posit that the intrinsic properties of the underlying elements are unknown to physical science, and that these correspond — either individually or in appropriate configurations — to mental phenomena.7 Although positing the existence of an entirely novel type of ‘substance’ about which we know very little might seem to count against the theory, this is offset by the considerable philosophical advantages of the position in explaining the nature of the connection between mental and physical phenomena (although presumably we do know something about its intrinsic nature, since this constitutes the basic elements of experience).8 In support of the position, many thinkers such as Lewis (forthcoming) and Jackson (1988) have found it plausible that the intrinsic nature of the universe cannot be accounted for by the laws of physics alone, thus presenting a prima facie case for the adoption of a neutral monist position.

Expressed within the framework of neutral monism, then, Maxwell’s argument proceeds as follows. Being a particular phenomenal event — e.g. seeing a red patch or feeling a particular pain — is an intrinsic, and therefore essential, property of the relevant neutral element(s). Conversely, being a particular physical event is a structural property of the relevant neutral element(s), and is therefore contingent, since causal structures are not intrinsic to these elements, but multiply realisable through different causal networks which possess the same structural properties. Now the analogy with heat and molecular motion runs precisely in reverse to Kripke’s example, since being a particular brain event is a contingent physical property of the neutral elements which instantiate the relevant brain event in the actual world, whilst being a particular pain is an intrinsic, and therefore necessary, property of the very same neutral elements. This preserves the necessity of identity since such identities hold between two different ways of designating the same neutral elements in this, the actual world, with the physical term (e.g. ‘C-fibres firing’) picking them out by means of a contingent causal property, and the mental term (‘pain’) via a necessary property. The apparent contingency of such identities is explained by the fact that the physical property of being a brain event can come apart from the relevant neutral elements in other possible worlds that have the same causal structure — and therefore the same physical properties — but different intrinsic or phenomenal properties. Or, to paraphrase Maxwell (1978: 380), ‘a given brain event [qua neutral element] might not have been a brain event [qua causal role]’ (original emphasis). Thus there is a possible world in which the neutral element(s) that constitute each pain in the actual world play a different causal role, and so no longer constitute the same — or indeed any — brain event, although the identity between the pain and the actual causal-physical role of these elements remains a necessary one, since they are identical in all worlds where both exist.

Neutral monism therefore explains the apparent contingency of mind–brain identity statements in a manner that exactly parallels the approach that Kripke discounts. However, instead of explaining the ‘illusion of contingency’ in terms of mental events being picked out via one of their ‘accidental’ properties — something that both sides agree is impossible, or at least very implausible (ibid. 375; Kripke 1980: 330) — it is brain events that are picked out by an ‘accidental’ property; namely the property of being a brain event, which is contingent to both the mental and brain events in question (a possibility that Kripke finds ‘self-evidently absurd’ [ibid.]). The metaphysics of neutral monism explains how both of these terms can pick out precisely the same neutral element(s), making it a true mind–brain identity thesis. Physicalism, on the other hand, still turns out to be false, since the underlying entities are (pace Maxwell and Stoljar) not physical but neutral in character. This amounts to a rejection of Kripke’s assumption that physical descriptions are rigid designators since, according to neutral monism, they pick out different entities in different possible worlds by means of their contingent causal role, thereby escaping CA.9

IV. Objections

The plausibility of the above response rests largely upon the plausibility of neutral monism generally. Perhaps the most serious objection to the view, and to identity theses in general, is what Maxwell (1978: 392) calls the ‘grain objection’. If mental events are identical with brain events (or the corresponding neutral elements that comprise or subvene them) then, according to Leibniz’s Law, the properties of any brain event must be identical with the properties of the neutral elements, since they are one and the same thing. But whilst ‘the occurrence of a smooth continuous patch of red in our visual experience’ (ibid.) exhibits both unity and smoothness, it is difficult to imagine how the relevant neutral element(s) or brain event could also possess these properties, since they may be comprised of discrete, logically separable elements. Stoljar (2001a) presents a convincing response to this objection by drawing a distinction between the properties of the contents of an experience, or how that experience represents the world as being, which may be unified and smooth, and those of the experience itself, which may not.

Another potentially more pressing concern for the neutral monist is whether mental events are the sort of thing that can be comprised of intrinsic, non-relational properties at all. Whilst neutral monism does not necessarily entail the existence of so-called ‘free-floating’ qualia — one of the common historical objections to it (Stubenberg 2005) — it seems plausible that many types of experience, and therefore many mental events, are intrinsically relational in a manner that neutral monism fails to capture. Perceptual experience, or the kind of switching scenarios considered by Burge (1979; 1988), show that the contents of thought or experience cannot be captured internalistically, but incorporates elements of the surrounding environment or context. In order to accommodate this insight, the neutral monist would either have to posit that the underlying neutral elements are also externalistically individuated, which would make it difficult to identify them with a particular physical event via its causal role, or else invoke Stoljar’s distinction between representational content, which would be relational, and phenomenal experience, which would not. Both views are problematic and, whilst it lies outside the scope of this essay to explore these problems in greater detail, either may threaten to undermine — or at least complicate — the neutral monist account.

V. Conclusion

I have argued that the metaphysical doctrine of neutral monism provides a successful response to Kripke’s version of CA concerning the apparent contingency of mind–brain identity statements. Furthermore, the structure of this response closely mirrors Kripke’s own analogy with other scientific identity statements, such as ‘heat = molecular motion’ and ‘water = H2O’, but reverses the direction of the analogy such that ‘pain’ is analogous to ‘molecular motion’ and ‘C-fibre firing’ to ‘heat’, rather than (as Kripke assumes) the other way around. This explains both the apparent contingency of the resulting identity statements, and their necessity, as required by Kripke’s modal semantics. The resulting mind–brain identity thesis is, however, non-physicalist, since the bearers of the relevant properties are not physical in any ordinary sense of the term, although the structure of this account closely resembles Maxwell and Stoljar’s ‘non-materialist physicalism’. Its plausibility, however, depends upon that of neutral monist metaphysics as a whole, and whether this adequately reflects the relational nature of experience and representational content. In spite of these concerns, however, neutral monism offers a powerful and flexible framework for explaining the relation between mind and body — a problem that has been a recurrent theme in philosophy ever since Descartes’s Meditations.

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