Consciousness is said to be "one of the last great mysteries for science". It is at once the most familiar thing in the world and the most difficult to explain. Oddly enough the great successes of modern neuroscience only seem to make consciousness harder to understand.
The central mystery has fascinated philosophers for thousands of years; that the universe seems to contain two completely different kinds of thing. On the one hand are bodies and brains - physical objects that we can touch and measure; on the other are conscious experiences - private and subjective feelings that we cannot get at directly. We can ask people what they are experiencing, record their words, and measure what happens in their brains, but somehow this doesn't seem to capture the "what it's like" of subjective experience.
Right now, for me, the sky is a very faint pale blue streaked with early morning wisps of delicate pink. But how can science measure this? We can't even tell whether you and I are having the same experience when we both say we are seeing blue. My pale blue might be your bright orange. This "what it's like for me", is what philosophers call "qualia"; the intrinsic properties of the experiences themselves. So the mystery is this - how can a few pounds of living neurons inside a skull create qualia? No one knows.
No one knows, but at least now they are arguing about it. When I started my research, more than 30 years ago, no serious scientist would even admit to an interest in consciousness, and I was very much on my own. I had had many strange experiences and was obsessed with trying to understand them, but the science simply wasn't there to do the job. Then, gradually, I found I was in the midst of a hot topic. Brain scanning and other advances in neuroscience meant we could at last peek inside a living brain, but how could we find consciousness there?
The problem got me hooked, and so I gave up my university job to read all I could on the subject and write a textbook; a project that left me well informed but even more baffled. Everyone seemed to disagree. So I decided that I needed to ask the experts what they meant - face to face. I travelled the world talking to some of the world's finest thinkers and put together a book, Conversations on Consciousness.
Perhaps the key thinker in this debate is the young Australian philosopher David Chalmers. I caught up with him in Tucson Arizona where, for many years, he has organised the famous "Toward a Science of Consciousness" conferences. He said that scientists researching vision, memory, thinking or emotions were just tackling "easy problems". Even if they solved all those there would still be something else left to explain - consciousness itself - and this he called the "hard problem".
The phrase stuck, and now Chalmers's hard problem has become something of a Holy Grail for consciousness studies. Scientists and philosophers are falling over themselves to become the one who solves the hard problem. The trouble is, no one knows how to set about solving it.
At one extreme are those who think a revolution in physics is the answer. The Tucson anaesthetist Stuart Hameroff is one such theorist. "Every day," he told me, "I put patients to sleep and wake them up and it's still incredible. You wonder - where do they go?" He has teamed up with the British mathematician, Sir Roger Penrose, to argue that the brain is a quantum computer and the conscious self depends on quantum effects in the microtubules - tiny tubular structures inside every cell of the body. They are convinced that this is the way forward, but no one else I talked to shared their enthusiasm.
Far more common are the neuroscientists who think that if we just get on with the "easy problems" we will eventually solve the hard one. Pre-eminent among these is the late Francis Crick, who won the 1962 Nobel prize for discovering the structure of DNA. Aged 60 and after nearly half a century of work in biology, he changed tack totally - turning his attention from the mystery of heredity to that of consciousness.
Shortly before his death from cancer in 2004, an 88-year-old Crick invited me to his home in Southern California. Within minutes, he was demanding that I come up with a crucial experiment. When I tried my best, he dismissed it out of hand. "All that's nonsense," he said, "because it's based on pure psychology and you're not talking about neurons."
Crick had no time for the speculations of psychologists or philosophers - all they do is argue, he said, and never make discoveries. He was convinced that what we need to do is put the hard problem aside and get on with studying the neural correlates of consciousness; that is, measure what is going on inside the brain when a person has a conscious experience. In this light, he looked for the consciousness neurons - the parts in the brain that are active when someone has a conscious experience.
He likens the hard problem to an ancient conundrum - the nature of life itself. Back in the 19th century, biologists were convinced that they would find a special "life force" that breathed life into plants and animals and departed at their death. Of course, no such force was ever found, Crick himself contributing to its demise. The answer turned out to be that when you understand how living things work, you realise they don't need any special force at all. Could the same be true for consciousness?
Pat and Paul Churchland certainly think so, and the pair, who are both professors of philosophy at the University of California, San Diego, take a strong line on Chalmers and his hard problem. "I don't see how you can tell, by looking at a problem, how difficult it is," says Pat. "There are many examples where people thought a problem was unsolvable, and turned out to be wrong." For the Churchlands, there is no "mystery of consciousness". For them, when we grasp how the brain's visual system processes colour information, the problem of qualia will be solved.
The most extreme view, however, is posited by Tufts University professor, Daniel Dennett.In his book Consciousness Explained, he denies the existence of qualia and says that there is no such thing as "consciousness itself". Dennett believes that if we start from our intuitions about consciousness then we are doomed to failure. For example, he argues, some people may feel as though they have a little conscious self somewhere inside their head, which is the subject of the stream of experiences. He believes that the brain possesses no central controller; no inner screen where the images could appear; and no one inside to experience them. There is no magic process that somehow turns ordinary nerve activity into conscious experiences. We must, he told me, throw out all of these perfectly natural, but misguided ways of thinking about consciousness.
But how? Turning your intuitions inside out is terribly hard, but if Dennett is right then most of the others I spoke to are completely wrong. Quantum physics will not help one jot, and no one will ever find Crick's "consciousness neurons".
I would love to pop into the Tardis, jump forward a few years, and see who turns out to be right. For now at least, consciousness looks set to remain one of our greatest mysteries.
Susan Blackmore will be speaking about 'Conversations on Consciousness' on Saturday 10 June at the Cheltenham Science Festival ( www.cheltenhamfestivals.com; 01242 227 979)