Thinking about consciousness is no longer the preserve of philosophers; it is part of science too. Below, Kam Patel outlines the main issues

When I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never catch myself at any time without a perception, and never can observe anything but the perception.

David Hume

A Treatise of Human Nature (1740)

Most of us may feel we can identify with the 18th-century philosopher David Hume's inner world of thoughts and feelings. It is a private world which we would recognise as existing in some way within ourselves. But explaining the nature of what Hume thought of as his "bundle of internal perceptions" - and what 20th-century thinkers prefer to call "consciousness" - is a problem philosophers have wrestled with for hundreds of years.

In the past ten years the question has moved beyond philosophy; igniting a surge of interest among scientists working in disciplines as diverse as neuroscience, cognitive science, psychology and artificial intelligence.

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Although a bewildering number of approaches are being deployed, it is possible to trace a number of key assumptions underpinning much of this interdisciplinary work.

Most scientists and philosophers assume that consciousness emerges from the operation of nerve cells in the brain (neurons). A notable opponent of this (seemingly obvious) view is the mathematician Roger Penrose, who controversially argues that consciousness is generated by quantum activity that occurs in tiny parts of the brain's nerve cells called microtubules. Penrose also maintains that computers can never emulate human understanding and therefore never attain consciousness, a claim that has attracted fierce criticism from the artificial intelligence community.

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More generally, arguments over approaches adopted by most researchers and thinkers in the field become focused when two fundamental issues are considered. The first of these revolves around the relationship between conscious experience and the person, organism or artificial system owning that experience. The second concerns the ability of a person, organism or system to describe and understand the conscious experience of another.

Some thinkers, such as John Searle, base their theories on the essential privacy of the conscious experience and take what can be called a "first-person perspective" on the problem. Searle argues that consciousness should be seen as a "high level feature" of the workings of the neuronal system. But he stresses the privacy of that feature: "There is a sense in which each person's consciousness is private to that person, a sense in which he or she is related to his pains, tickles, itches, thoughts and feelings in a way that is quite unlike the ways others are related to them". This means, he says, that the essence of consciousness - a subjective, qualitative phenomena - cannot be described purely by examining the subject's behaviour or through efforts to build computational models of consciousness. Attempts to study the conscious experience as if it were a third-person phenomenon are, according to Searle, doomed to failure. Meanwhile some others committed to the first-person point of view argue for "qualia", the absolute, inexplicable, unassailability of first-person sensations such the redness of red or the thrill of seeing a beautiful sunset.

Putting forward a counter-argument to those in the first-person camp are thinkers who argue that while consciousness may appear to be a purely subjective experience, this should not prevent the development of a description of the conscious experience from a third-person perspective. The philosopher Daniel Dennett believes that the power of an objective, third-person driven analysis of consciousness is "woefully underestimated" by the first-person camp. He believes that it is quite possible objectively to capture everything about a person's conscious experience by scrupulous, patient and subtle experimental analysis. "You let the subjects tell you what it is like to be them. You consider everything they say and what they say is part of your data. But you don't necessarily give it credence because the subject may not be right about their own subjective world," he explains. Dennett says that the third-person approach is leading to the discovery and prediction of new kinds of experience in people. "The fact that we can manipulate the very subjective experiences of people shows that we can understand it from a third-person point of view. So the claim that there is this ineffable residue that cannot be got at is shrinking fast."

For Dennett, scientific work on consciousness "is where the action really is". He says there has been a huge expansion in theoretical studies of consciousness because of advances in technology. This has made it possible to frame hypotheses that investigators simply did not have a language for a few years ago. For Dennett, conceptual advances in consciousness research go hand in hand with the availability of new technology.

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At the leading edge of scientific research on consciousness is Francis Crick, father of molecular biology. Crick, based at the Salk Institute in San Diego, and his collaborator Christof Koch, have focused their efforts on the visual system. By the end of the century they hope they will be in a position to offer a neural explanation or "correlate" for visual awareness. This, they hope, will allow them to develop a coherent theory about consciousness which tackles the complexities of the subjective experience.

Crick says that the first-person experience should never be taken at face value in investigations of its nature. Visual psychology has shown that people deceive themselves enormously as to what they think is going on in their brains. For instance, people think they can see equally clearly in all directions whereas it is easy to show that they see most clearly in the centre of their gaze. The reason for thinking that we have all-round clear vision is because the eyes are busy moving about all the time, enabling the brain to fill in missing information in our field of view.

Another example of cerebral disinformation is human motivation: people say one thing when they are obviously motivated by something else. Crick says: "You cannot just take people's verbal reports about what they say on their own terms. You have to test them in a lot of circumstances. The subjective experience is what you might call real but it is not reliable in the sense that it is a true account of what is going on in your brain."

Crick and Koch believe that the sense of privacy in the conscious experience arises from the way the brain works. Crick explains that different areas of the brain can be thought of as being arranged in a pseudo-hierarchy with each "level" coding input-output information in a particular way. One level will perhaps code for movement, another for colour and so on. At each level the information is recoded so that what neurons are responding to is very different at each stage. And when it comes to the brain initiating speech or other kinds of motor output, the information has to be recoded again. Crick says: "This means that you cannot actually find out from the output what is going on inside the brain because the information is being recoded at all these stages. That is the explanation for why it is private. You can give an account of it but you cannot actually say what it is like. You cannot actually say what it looks like, or feels like to you in any way that explains it to other people. And that is what you would expect from our particular way of looking at the brain."

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Considering the state of research on consciousness in general, Crick says it is not clear how rapid progress is going to be. He points out that even the most common assumption - that neuronal activity gives rise to conscious events - has not yet been proven, though he believes that it will be. He likens the lack of this evidence to the position people were in when they worried whether a vital force was needed to explain living things: "We now think that is unlikely because we understand the system so well. But in earlier times it was a reasonable assumption that you didn't have to have a vital force but you could not establish it. With consciousness we are in a state of ignorance. That is the main point. Most of the things we would like to know we do not know yet". While there has been an explosion in the number of people working on the problem, Crick says that this does not mean that the field is not in a "very confused state", with scientists disagreeing among themselves as much as the philosophers do.

He would like to see much more experimental work, particularly in neuroscience, being done and says that what is really lacking in the field is ideas. And it is experimental facts that he hopes will provide investigators with new ways of looking at the problem: "A lot of people would be loath to agree with that - they believe they can do it all in their heads," he says laughing.

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