Is any form of dualism of the mental and the physical defensible?

I. Introduction

The debate over the nature of mind and body has occupied philosophers ever since Descartes first proposed that these were two distinct substances that interacted through some mysterious union (Descartes 1640). This essay examines whether this or any other form of dualism can provide a coherent explanation of mental and physical phenomena, and the relationship between the two. I will begin by presenting a brief account of substance dualism and some of the problems associated with it before turning to property dualism, and in particular the theory of neutral monism as advocated by Spinoza and Russell. The way that this theory accounts for mental and physical properties will be examined along with one of the main objections to it: how to account for the subjective nature of experience. Finally, I conclude that neutral monism is a defensible form of property dualism that offers a highly plausible explanation for the nature of the complex and integrated world in which we live.

II. Substance Dualism

Substance dualists, such as Descartes, hold that there are two types of thing in this universe: minds—variously referred to as spirits, souls, God, and so on—and matter (ibid.). Mind is regarded as a spontaneous, animate, thinking substance without any physical properties such as location, shape or extension, whereas matter is the familiar stuff and energy of the material world that is described by the physical sciences. Each of these substances provides a substrate that supports the various observable properties of the mental or physical world, respectively. Although this view has considerable intuitive appeal as it accounts for the marked difference between mental and physical phenomena as we experience them, it runs into problems when we try to account for the way that mind and matter interact, such as when our mind causes our body to move, or when the mind perceives a physical object as a result of sensory input to the body.

Philosophers have attempted to overcome this difficulty in various ways. The first of these, interactionism, argues that mind and body are causally connected and so influence one another, either through some part of the brain (Descartes 1650) or simply by virtue of their regular and constant conjunction.1 However, it is unclear how two different substances of entirely different natures could possibly interact without them both occupying the same spatiotemporal realm, which according to substance dualism, mind and matter do not. An alternative theory, epiphenomenalism, argues that mental phenomena, although substantial, are simply accidental by-products of physical brain processes, and have no causal power in their own right. Although intelligible, this view assigns only secondary importance to mental phenomena such as thought and consciousness. This is at odds with our everyday experience, which dictates that mental phenomena enter into causal relationships with both physical matter and other mental events. Finally, various forms of parallelism state that there are no causal connections between mental and physical substances, and that the two only proceed in apparent agreement due to the intervention of God, as in occasionalism, or as the result of a pre-established harmony between all the various substances in the universe set up by God. Neither of these arguments is taken very seriously today, and indeed none of these accounts seems capable of explaining both the existence of mental and physical phenomena, and the way that the two interact. Many philosophers have taken this to indicate that substance dualism is inherently flawed, and instead favour some form of monism, in which everything is comprised of a single substance, be it physical, mental or neutral in nature. One such theory is neutral monism, which I shall spend the remainder of this essay evaluating.

III. Neutral Monism

The theory of neutral monism was first articulated by Spinoza in contrast to the dominant substance dualist theories of his time (Garrett 1999: 871). As the name suggests, instead of attributing mental and physical properties to two different types of substance, neutral monism posits a single substance, neither mental nor physical in nature, but which exhibits both mental and physical properties.2 This theory is sometimes known as dual aspect theory, although this name is somewhat misleading as Spinoza actually thought there to be an infinite number of different properties, of which the mental and physical were the only ones comprehensible to us. By accounting for both types of phenomena as different aspects or properties of the same substance, neutral monism, as with physicalism and all other types of monism, eliminates the problem of mind-body interaction. Any causal links between mental and physical events can be explained in terms of changes to the single underlying substance, which manifest themselves as observable changes in its mental and/or physical properties. If such properties are simply different aspects of the same basic material, they are bound to exhibit a strong degree of correlation and apparently causal behaviour, as both arise from exactly the same source. Despite being a single-substance theory, neutral monism is still a form of dualism because, unlike physicalism or identity theory, it preserves the notion of distinct mental and physical properties, rather than trying to explain one purely in terms of the other. Neutral monism is thus a form of property dualism in which both types of property are considered to be different aspects of a single underlying reality.

More recently, neutral monism has been defended by Russell (1921; 1958: 140–60), who developed his own distinctive version of the theory that takes into account recent advances in both theoretical physics and neuroscience. According to Russell, the world consists not of substances but of events. What we perceive as physical substances are in fact the result of a long causal chain of events involving billions of microscopic interactions between fundamental physical particles—quarks, photons, electrons, and so on—reaching from the object of observation to our eyes, nerves and, ultimately, our brain. These interactions give rise to further events corresponding to perceptions, thoughts and ideas, but in every case the intrinsic nature of the phenomena, be it mental or physical, is some form of event or interaction rather than any type of substance as we would normally conceive it.

The physics supporting Russell’s view is well established, as scientists have long known that what we perceive to be solid matter is largely empty space containing a swarming mass of energy packets, or quanta, which are continuously flickering in and out of existence (Gribbin 1984). Our perception of physical properties such as solidity, extension and even gravity are the result of precisely such microscopic interactions between the matter/energy in our own bodies and the matter/energy in the things that we perceive, mediated by the exchange of fundamental particles, which are themselves theoretical entities describing a particular class of interactions or events. However, Russell takes this idea one step further. He suggests that the events that give rise to physical properties are also the source of mental properties, such as thought, memory and consciousness. In the same way that certain patterns or groupings of events correspond to, say, the hardness of a tabletop, or an individual electron, different patterns or groupings of the same or similar events may correspond to the experience of seeing the colour red, for example. To put it another way, if a brain surgeon was to examine her subject’s brain under a powerful microscope, the brain cells, neurotransmitters, and so on that she would see only tell part of the story. Only those groupings of events that give rise to the physical appearance of the grey matter in the subject’s brain would be visible, whilst other groupings of such events remain completely hidden from view. It is these hidden groupings of events that correspond to the patient’s conscious (or otherwise) thoughts, sensations and memories, which are completely inaccessible in the physical realm, and are instead manifest as mental phenomena. Viewed in this way, mental and physical properties are simply two different descriptions or aspects of the same underlying reality, which consists only of events or interactions that may be described as purely mathematical entities.

It should be stressed that Russell’s account should not be taken as a reduction of the mental to some aspect of the physical (physicalism), or of the physical to the mental (idealism), although he has been accused of both (Stubenberg 2005). Rather, he points out that both mind and matter are equally incomplete descriptions of reality, and must be understood as partial accounts of a third, as yet unknown realm, in whose ‘complete science, the word “mind” and the word “matter” would both disappear, and would be replaced by causal laws concerning “events.”’ (Russell 1927: 226). Instead of trying to eliminate mental or physical properties by equating one to the other, Russell’s neutral monism places both types of attribute into a single comprehensive framework, which may, in theory at least, be described by a single set of consistent and universal natural laws. Such laws would concern only events and their causal relationships, making neutral monism a highly parsimonious, and therefore ontologically very attractive theory.

IV. The Problem of Subjectivity

However, there is one remaining issue that must also be addressed by any complete theory of mind. As noted by Nagel (1974), conscious experience has a peculiar subjective character that is not adequately described by a purely objective account of the phenomena.3 The difference between a conscious entity, such as a human being, and an unconscious one, such as a chair, is that it is ‘like something’ to be human; i.e. there is a subjective quality to our experience that cannot be captured by any reductive form of description, no matter how detailed or complete the description may be. Although this is no more a problem for neutral monism than it is for many other theories, the neutral monist is able to explain such subjective qualities, or qualia, as additional properties of the underlying neutral entities, or in Russell’s case, events. However, Russell makes a more sophisticated claim that is borne out by recent analyses of brain structure and the nature of consciousness (Edelman & Tononi 2000).

In essence, we experience qualia when our mind relates present events, internal or external, to those that have already been processed and incorporated into memory. These past events are brought to bear upon present stimuli—say the thought or colour of a blade of grass, for example—by brain and/or mind processes in order to create what we recognise as conscious experience, or what Edelman and Tononi refer to as ‘the remembered present’ (ibid. 102). Although it may be impossible to explain consciousness in terms of its physical structure, as it has none, it is possible that it may instead be described as a set of events that, when they occur in sequence at the correct speed, create the experience of conscious qualia. In other words, consciousness is a process rather than a thing. This hypothesis is supported by the fact that it is almost impossible to imagine a totally static or unchanging conscious state, and so intuitively at least, consciousness is a time-dependent phenomenon. This emerging view of how the brain generates consciousness fits well with the choice of events as the fundamental building blocks of nature, as events by definition exist in time, and is another point in favour of Russell’s theory of neutral monism.

Edelman and Tononi go on to identify two different levels of consciousness, which they term primary consciousness, i.e. basic awareness of objects and the environment, and higher-order consciousness, corresponding to conscious or symbolic thought and self-awareness. These two categories reflect the division of conscious phenomena into what we might call first-order knowledge, corresponding to direct experience, sensations and perception of the world, and second-order knowledge, which represents abstract thoughts, concepts and knowledge about experience, such as my awareness of the fact that I had cornflakes for breakfast this morning.4 Viewed in these terms, it may be impossible to define conscious experience (first-order knowledge) in purely objective terms (second-order knowledge), as these can never fully capture the intrinsic nature of the original experience. Because consciousness is essentially experiential, its subjective qualities are lost in the translation to the purely descriptive language of second-order knowledge, which is always one step removed from the phenomena it describes. If this is the case, then the problem of subjectivity is a pseudo-problem arising from the nature of language, and the best that can be hoped for is a suitably accurate description of mental and physical phenomena that matches the available facts, which is exactly what Russell’s theory of neutral monism provides. This was what led Russell ‘to regard the traditional problem of the relation of mind and matter as definitely solved.’ (Russell 1959: 103–4).

V. Conclusion

Substance dualism is unable to explain the interaction between mind and body because it falsely divides the world into two distinct and irreconcilable elements: the mental and the physical. Property dualism, however, in the form of neutral monism, offers a convincing alternative that not only solves the mind-body problem, but accommodates recent advances in physical science, such as explaining how apparently substantial objects can be formed out of nothing but empty space and packets of energy. If both physical and mental properties arise directly from the same source, rather than one depending upon the other for its existence, they can be fitted into a single framework of natural laws where any apparently causal interaction is easily explained in terms of changes to the single underlying substance. By positing events as the neutral entities upon which all other properties depend, Russell’s theory of neutral monism has the additional advantage of explaining why time is so crucial to our experience of consciousness, a fact which is borne out by recent developments in applied neuroscience. This goes some way towards explaining why traditional reductive arguments have been unsuccessful at capturing the subjective nature of conscious experience, although in the final analysis, this may prove to be impossible due to the limitations of ‘second-order’ descriptions in capturing the nature of ‘first-order’ experience. As such, Russell’s theory is not only defensible, but is a highly plausible explanation for the nature of our world that finally unites the ghost of mental phenomena with the machine of the physical universe.

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