According to Chalmers, a theory of consciousness should, at the very least, "...give the conditions under which physical processes give rise to consciousness, it should specify just what sort of experience is associated. And we would like the theory to explain how it arises, so that the emergence of consciousness seems intelligible rather than magical. In the end, we would like the theory to enable us to see consciousness as an integral part of the natural world. Currently it may be hard to see what such a theory would be like, but without such a theory we could not be said to fully understand consciousness." (p. 5) To what extent does Chalmers make progress, judged by his own criteria? He touches on a few of the scientific findings which might reveal physical processes which give rise to consciousness, but that is not what his book is about. His real concern is to suggest what an appropriate theory of consciousness would be like. This is a book by a philosopher addressed primarily to philosophers. Chalmers writes well and many of the chapters will be accessible to the general reader. However, non-philosophers will find the technical nature of some of the chapters heavy going (and might be better served by his more succinct presentation of his views in Facing up to the problem of consciousness, in the Journal of Consciousness Studies, 2, 200-219, 1995). Given the fluid state of consciousness studies, progress towards the shape of an appropriate theory, fully argued, will nevertheless be of interest to many.
In the 20th Century, philosophy of mind has been predominantly reductionist, arguing that consciousness will eventually be shown to be nothing more than a physical state or function of the brain. By contrast, Chalmers joins a small, but growing band of theorists who maintain that consciousness can only be understood within a non-reductionist science of the mind. His central argument against reductionism hinges on the claim that consciousness is naturally supervenient but not logically supervenient on physical states. That is, it actually supervenes on brain activity, but it is conceivable that it might not. This, he argues, makes consciousness different from all other properties, including emergent biological properties such as life. If we know all the microproperties of a physical system and their relationships, we necessarily know the macroproperties - for example, according to Chalmers, given that the physical facts are as they are, it is inconceivable that life could be other than it is. In this sense, a higher order property such as life is nothing more than its combined, physical constituents. By contrast, he can conceive of a zombie which is physically indistinguishable from himself, but which lacks consciousness. Consequently, consciousness cannot be a physical property. Nor can it be reduced to physical properties. There are, he claims, no other non-physical properties in this sense.
This tension between what is natural (or actual) and what is logical (or logically possible) runs throughout the book, sometimes uncomfortably. Many scientists, for example, will be dubious about the actual knowledge of macroproperties that is provided by knowledge of microproperties. Morphogenesis (the creation of life forms) for example, does not seem to be very well accounted for by gene theory (at present), let alone quantum mechanical events. So even physical wholes might, in some sense, exhibit properties not reducible to the sum of the parts. Chalmers' response to this would be that in such cases we have insufficient knowledge of the parts; if we had complete knowledge of genes, subatomic particles, and so on, it is logically inconceivable that macroproperties such as morphogenesis would not be understood. This insistence on complete knowledge provides Chalmers with an escape from any possible counter-example, as he can always claim these to be cases where our knowledge of microproperties is incomplete. However, in actuality, our ability to account for all complex forms and functions in terms of the activities of physical microproperties seems to be more a matter of faith, than logical necessity. Functions, for example, often need to be understood not just in terms of their constituent processes but also in terms of the environments which embed them. And the reducibility of psychological beliefs, desires and so on to physical microproperties seems counterintuitive; how, for example, could even a complete knowledge of quantum mechanical events provide an understanding of why one needs to catch a 94 bus?
A simple, definitive argument against the reducibility of phenomenal properties to physical ones would nevertheless be very welcome, and this is Chalmers' more central concern. Unfortunately, the force of Chalmers' argument rests entirely on what he finds to be conceivable - and its consequent weakness, that different theorists might claim to be able to conceive of entirely different things. Zombies are not just functionally indistinguishable but also physically indistinguishable from humans. A reductionist might therefore claim that he cannot conceive of such a creature lacking consciousness. If consciousness just is a state of the brain, then it inconceivable that a creature with a brain in that state could lack it. Faced with this standoff, the only recourse would seem to be to take a vote (about what is conceivable) - hardly a satisfactory way to resolve the issue. Chalmers nevertheless goes on to assemble a range of standard arguments against reductionism from the literature, claborates these where necessary, and gives a critique of the counter-arguments. His mastery of philosophical terrain is impressive, and taken together, his arguments pose a serious challenge for materialist theories of mind.
Not all of Chalmers' ontology, however, is so straightforward. Sometimes for example Chalmers describes his position as a doubleaspect theory - which poses the question double-aspects of what? According to Chalmers, phenomenal properties and their physical correlates in the brain will be structurally coherent, in the sense that they will encode the same information. On these grounds Chalmers justifiably describes his position as a doubleaspect theory of information. This much is plausible (I presented an identical dual-aspect theory of information in "Is human information processing conscious?" in The Behavioral and Brain Sciences, in 1991). However, at other times, to avoid positing some transcendental ground for physical and phenomenal properties Chalmers describes his position as naturalistic dualism - in which consciousness becomes basic in the same sense that energy is basic in physics. This raises the question, If phenomenal and physical properties are equally basic, distinct, and not grounded in something more fundamental, then what is it that relates them to each other so precisely? Alternatively, if phenomenal properties supervene on physical ones (as he argues throughout the book), then why regard the phenomenal properties as basic? It is up to Chalmers to decide which position he wishes to defend (the rest us will have to wait and see).
Further problems arise from Chalmers' psychofunctionalism perhaps the most innovative (and controversial) aspect of his theory. Although Chalmers refers in much of his work to the need for psychophysical bridging laws, it eventually becomes clear that these are psychofunctional rather than psychophysical. That is, he believes there to be an invariant relationship between physical and phenomenal properties, but it is the way a system functions, and not its physical embodiment that determines what that system experiences. According to Chalmers a machine that functions in a way that is indistinguishable from humans has experiences that are indistinguishable from humans (a version of strong Al). This would be true whether the system is made out of silicon chips, beer cans, or the population of China - provided only that in their detailed activity, these systems instantiate the same causal relationships, i.e., function in the same way. Chalmers' combination of a non-reductive phenomenology with standard functionalism is, to my knowledge, novel within philosophy of mind (standard functionalism claims that consciousness is nothing more than brain functioning) - and, given that the phenomenology of consciousness has proved to be the stumbling block of functionalism, it is not surprising that Chalmers' position has attracted considerable interest.
Chalmers develops this position from two thought experiments, which he describes as fading qualia and dancing qualia. In these he considers the familiar scenario in which the neurones of the brain are gradually replaced by silicon chips which exactly replace the functioning of the neurones they replace. As the replacements progress, do the qualia gradually fade? Or, if one were able to switch between one's normal brain and a replacement silicon brain (with exactly the same functions) would the qualia dance? According to Chalmers if one replaced the functions exactly one could not notice the difference either externally in terms of behaviour, or internally in terms of what one experiences.
Whatever one may think about the fading/dancing qualia arguments, the view that bodies don't matter for what we experience is highly counterintuitive. On Chalmers' account, not just machines made of silicon chips might experience in the way that humans do, but so would virtual minds (instantiated in the symbol manipulations of programmes) and even systems consisting of symbols written on bits of paper by the population of China, provided only that the causal relationships governing the creation of those symbols, simulate those of the human mind's symbol manipulations accurately. Processes within the human brain normally thought of as unconscious would also have to be conscious in Chalmers' system (by virtue of their functioning) - in which case the conscious/non-conscious distinction loses its meaning. The theoretical cost of this position to consciousness studies is considerable. If the conscious/non-conscious distinction cannot be made, how could one investigate the conditions for consciousness in the human brain which rely on contrasts between neural conditions adequate or not adequate for conscious experience? How could one make sense of the extensive experimental literature on the differences between preconscious, conscious and unconscious processing?
And what of psychodynamic theory - is all talk of a personal or transpersonal unconscious just muddled thinking?
Note that Chalmers is forced into this uncompromising position by his fading/dancing qualia argument (whatever functions is conscious by virtue of its functioning). Given this, all brain functions must be conscious. Consequently, he maintains that those functions which do not seem to enter into our consciousness must be autonomously conscious (they are conscious to themselves). This leads to the extravagant claim that there are as many distinct consciousnesses cohabiting in the human brain as there are distinct functions.
Nor does Chalmers see any reason to draw the line at brains or systems which simulate the functioning of the brain. If consciousness of given sorts is invariably associated with functioning of given sorts then all forms of functioning are associated with experiences, irrespective of their embodiment. This panpsychofunctionalism (my term for this) is quite different from panpsychism (the view that all material forms are accompanied by forms of experience). If true, then not only do thermostats experience in ways that relate to their function (sensing hot and cold), but so does rain falling in that it functions to make the earth wet - and even rainbows experience something relating to their production of beautiful sensations in the human mind. The central difficulty for Chalmers is that functioning is observer-relative. Chalmers' defense is that the structure of physical systems does, to some extent, constrain their potential functioning. But this really misses the point. Does the Chinese statue on my desk really experience something different when it functions as an incense holder rather than as a decorative object? And if it does both simultaneously, does it really simultaneously have both experiences? As John Searle has commented in his recent New York Times review of Chalmers' book, "It is rather as if someone got the result 2+2=7 and said, 'Well maybe 2 plus 2 does equal 7'."
In many ways this book provides an impressive display of expertise and argument and it has been well received by many (although not all) philosophers of mind. But it gives rather confused directions, in my view, to some future theory of consciousness. I find it hard to take issue with its antireductionist stance, or its view that consciousness is somehow basic, or that information may have a central role to play in any theory of how phenomenology relates to the brain (I have argued for the same positions myself). But double-aspect theory isn't interchangeable with naturalistic dualism, conscious processes do need to be distinguished from non-conscious ones in the brain, thought experiments with unfalsifiable conclusions need to be distinguished from actual experiments, and I very much doubt that the stiff whisky I have just swallowed is aware of its function in sustaining my ability to write this review. Philosophers, quite rightly, feel free to contemplate possible worlds as well as actual ones. But once one has explored where the logic of an argument leads, one still has to consider whether to temper logic with common sense.