In our series on Big Science Questions, Keith Sutherland looks at conflicting explanations of consciousness and asks how close we are to the 'final frontier'.

Throughout the 1990s - christened the "Decade of the Brain" by former US president George Bush - the scientific understanding of consciousness was commonly called the last remaining challenge, the "final frontier" of knowledge. Suddenly consciousness studies was back on the academic agenda after an embargo lasting nearly a century. According to philosopher John Searle, this embargo was a legacy of behaviourism, which asserted that scientific psychology should restrict itself to the study of observable phenomena, rather than subjective mental states. For a time it was considered such bad taste to raise the topic of consciousness in cognitive science discussions that graduate students would apparently "roll their eyes at the ceiling and assume expressions of mild disgust". So why the big change?

Some argue that consciousness studies regained academic respectability when Nobel laureates Francis Crick and Gerald Edelman started to work in the field in the 1980s. Not so, claims Semir Zeki of University College London, a leading pioneer of cognitive neuroscience. He puts it down to developments in brain-imaging techniques that enable psychologists to study the neural correlates of consciousness in glorious technicolour. But such was the stranglehold of behaviourism and its refusal to acknowledge anything other than the directly observable that even if this technology had been available a few decades earlier, the dreaded c-word would still have been embargoed. Perhaps sociology offers a better explanation for its renaissance. It was, after all, during the 1990s that students educated in the 1960s, many of whom indulged a personal interest in altered states of consciousness, began to take up senior positions in cognitive science departments.

Although it is common to talk about the problem of consciousness, there are, in fact, three separate problems and differing views about how far they overlap. The technical term for the first - described by philosopher David Chalmers as the "hard problem" - is the generation problem: how do material configurations or processes produce conscious experience? In 1866, biologist T. H. Huxley expressed the problem as follows: "How it is that anything so remarkable as a state of consciousness comes about as a result of irritating nervous tissue is just as unaccountable as the appearance of the Djin when Aladdin rubbed his lamp."

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Since then, according to philosopher Jerry Fodor at Rutgers University, we have not made much progress: "Nobody has the slightest idea how anything material could be conscious," he says. "Nobody even knows what it would be like to have the slightest idea about how anything material could be conscious. So much for the philosophy of consciousness."

Few philosophers draw the conclusion that we should waste no more time on the problem, however. Fodor's colleague Colin McGinn agrees that it is insoluble in principle. But he still manages to spend most of his time writing about it. Other "mysterians", such as Oxford mathematician Sir Roger Penrose or philosopher Bill Seager at the University of Toronto, resort to increasingly desperate measures - introducing everything from quantum mechanics to idealism (the theory that only the mental is real) and panpsychism (the doctrine that everything has mental properties) - to try to solve the problem through the "principle of minimum mysteries". They argue that if quantum mechanics is a mystery and consciousness is a mystery, perhaps they are the same mystery.

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The fact that consciousness is a mystery is about the only thing on which Fodor agrees with Massachusetts Institute of Technology psychologist Steven Pinker, author of How the Mind Works (to which Fodor responded with The Mind Doesn't Work that Way ). At Tufts University, Massachusetts, however, Daniel Dennett, another prominent advocate of evolutionary theory, thinks differently. According to Dennett, if we can provide an adequate explanation for cognitive abilities in functional, neurological and evolutionary terms, then there is no "further" problem of consciousness.

Neurophilosophers Paul and Patricia Churchland at the University of California, San Diego, broadly support this view, claiming that, in the eyes of posterity, the "problem of consciousness" will look not unlike the "problem of life". They say that consciousness mysterians are the modern-day equivalent of vitalists, who believed life could not be explained purely by applying principles of chemistry and physics because it depended on a non-physical inner force.

Perhaps the best chance of resolving this dispute lies with a group of scholars that claims that the generation problem of consciousness is a product of the dualistic notion of mind as disembodied and distinct from its physical character, as expounded by 17th-century mathematician and philosopher Rene Descartes. Nicholas Humphrey, a senior research fellow in the Centre for Philosophy of the Natural and Social Sciences at the London School of Economics, sees awareness as an activity of the nervous system. For him, sentience is just an evolved form of such activity, something that enacts rather than perceives. Its primitive origins are therefore observable even in the wriggles of single-celled organisms.

Other writers have drawn parallels with theories drawn from phenomenology, a tradition of continental philosophy that studies consciousness from a first-person perspective. Cognitive neuroscientist Francisco Varela of the CNRS, the French National Research Centre, suggested neuroscience, phenomenology and behavioural science would have to unite to crack the problem of consciousness. Yet another group of writers, including neurobiologist Walter Freeman at the University of California, Berkeley, and philosopher Andy Clark, from Washington University, St Louis, draw on ideas of emergence and chaos theory to dissolve the mind-body problem. These suggest that the properties of consciousness emerge through minute fluctuations in the elements from which it is made and are therefore impossible to predict from those elements.

The second, related, problem of consciousness is the problem of the self. Modern debate on the problem of the self, although owing much to the insights of philosophers John Locke and David Hume, can still be contextualised within two schools of ancient thought. According to Greek philosopher Aristotle, who 'I' am is closely tied to my embodied existence. The psyche and the body form a unity and the psyche involves life functions from locomotion to philosophical contemplation. The opposing view, put by Pythagorean and Platonist thinkers, separated the psyche and the body and led directly to Descartes's theory of a disembodied ego.

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In his "Essay concerning human understanding", Locke addressed the problem of personal identity by proposing memory as the mechanism responsible for continuity of the self over time. When Hume turned his attention to this "self", he found nothing more than a bundle of impressions with no introspective evidence that it existed. He thus dismissed the self as a fiction of the imagination.

Modern philosophy of mind has done little more than refine this observation. Philosopher Sir Anthony Kenny maintains that the self is a "piece of philosophers' nonsense consisting of a misunderstanding of the reflexive pronoun", while Robert Nozick, also a philosopher, agrees that the self is "synthesised in the act of reflexive self-reference" - in other words, it is simply a product of reflection.

Dennett's view of the self as a "centre of narrative gravity" - a theorist's fiction that nonetheless plays a useful explanatory role - has been lent support by work on split-brain patients. Research by cognitive neuroscientist Michael Gazzaniga at Dartmouth College on these patients shows how the concept of the self is constructed by the "interpreter" mechanism located in the left cerebral hemisphere. This function, which can often be dissociated in patients with brain damage, constructs an imaginary narrative consistent enough to maintain a stable concept of self. Social psychologists and anthropologists have also shown that the concept of self varies enormously from culture to culture and that selves are constructed in the domain of social interaction.

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Recent publications, notably James Austin's Zen and the Brain , have tried to integrate philosophical scepticism about the notion of the self with insights from Eastern meditation practice and experimental findings from cognitive neuroscience. Austin's dictum - "consciousness evolves when the self dissolves" - encouraged the American neuroscientist Andy Newberg to try to pinpoint the brain mechanisms associated with the sense of dissolving self traditionally linked to meditation practice and religious mysticism.

The third problem of consciousness, closely related to the issue of the self, is the problem of agency. If the self is a narrative fiction, then who is the author of conscious volitional acts? And, more importantly, how is it possible to reconcile our sense of free agency with the requirement that the agency must be subject to the (deterministic) laws of physics?

There is little agreement as to the degree of overlap between the three problems of consciousness. To Chalmers, sentience may even be a property of all functional systems (including thermostats), whereas selfhood and free action are restricted to sophisticated systems with language. By contrast, Searle argues that consciousness is a natural property of complex biological systems, and claims that the real "explanatory gap" is in the area of volition and free action. Dennett, meanwhile, sees the alleged conflict between determinism and free will as just another of those fake dichotomies that arise when we fail to take the implications of evolutionary design all the way to the wire. Over in the chaos corner, Freeman and his acolytes claim that these are all pseudo-problems caused by a refusal to accept Hume's claim that the notion of "causation" is a construct of human psychology.

Ten years on from the launch of the Decade of the Brain, perhaps we can claim that there has been a little conceptual clarification in the field of consciousness studies, but the final frontier of scientific understanding still looks as distant as ever.

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Keith Sutherland is publisher of The Journal of Consciousness Studies

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