Credit: Getty/Gaetan Charbonne

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The young woman had survived the car crash — after a fashion. Five months after parts of her brain had been crushed, she could open her eyes, but didn't respond to sights, sounds, or jabs. In the jargon of neurology, she was in a "persistent vegetative state". In crueller everyday language, she was "a vegetable".

So imagine the astonishment of British and Belgian scientists when they scanned her brain using a kind of MRI (magnetic resonance imaging) that detects blood flow to active parts of the brain. When they recited sentences, the parts involved in language lit up. When they asked her to imagine visiting the rooms of her house, the parts involved in navigating space and recognising places ramped up. And when they asked her to imagine playing tennis, the regions that trigger movement joined in. Indeed, her scans were barely different from those of healthy volunteers. The woman, it appears, had glimmerings of consciousness.

Try to imagine what it is like to be that woman. Do you appreciate the words and caresses of your distraught family, while racked with frustration at your inability to reassure them they are getting through? Or do you drift in a haze, springing to life with a concrete thought when a voice prods you, only to slip back into blankness? If we could experience this existence, would we prefer it to death? And if these questions have answers, would they change our policies toward unresponsive patients — making the heated political battle over the brain-damaged Terri Schiavo in 2005 look like child's play?

A report of this discovery in November 2006 was just the latest shock from a bracing new field: the science of consciousness. Questions once confined to theological speculations and grandiose late-night student debates are now at the forefront of cognitive neuroscience. With some problems, a modicum of consensus has taken shape. With others, the puzzlement is so deep as perhaps never to be resolved. And the discoveries have shaken some of our deepest convictions of what it means to be human.

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IT SHOULDN'T BE SURPRISING that research on consciousness is alternately exhilarating and disturbing. No other topic is like it. As French philosopher René Descartes noted, our own consciousness is the most indubitable thing there is. The major religions locate it in a soul that survives the body's death to receive its just deserts, or to meld into a global mind. For each of us, consciousness is life itself, the reason why Woody Allen said, "I don't want to achieve immortality through my work; I want to achieve it by not dying". And the conviction that other people can suffer and flourish as each of us does is the essence of empathy and the foundation of morality.

To make scientific headway in a topic as tangled as consciousness, it helps to clear away some red herrings. Consciousness surely does not depend on language. Babies, many animals, and patients robbed of speech by brain damage are not insensate robots; they have reactions like ours, which indicate that someone's home. Nor can consciousness be equated with self-awareness. At times we have all lost ourselves in music, exercise, or sensual pleasures, but that is altogether different from being knocked out cold.

What remains is not one problem about consciousness but two, which the prominent Australian philosopher David Chalmers has dubbed the Easy Problem and the Hard Problem. Calling the first one 'easy' is an in-joke: it is 'easy' in the sense that curing cancer or sending someone to Mars is easy. That is, scientists more or less know what to look for, and with enough brainpower and funding, they will probably crack it in this century.

What exactly is the Easy Problem? It's the one that Sigmund Freud made famous: the difference between conscious and unconscious thoughts. Some kinds of information in the brain, like your daydreams, your plans for the day, your pleasures and peeves, are conscious. You can ponder them, discuss them, and let them guide your behavior. Other kinds, like the control of your heart rate, the rules that order the words as you speak, and the sequence of muscle contractions that allow you to hold a pencil, are unconscious. They must be in the brain somewhere, because you couldn't walk and talk and see without them, but they are sealed off from your planning and reasoning circuits, and you can't say a thing about them. The Easy Problem, then, is to distinguish conscious from unconscious mental computation, identify its correlates in the brain, and explain why it evolved.

The Hard Problem, on the other hand, is why it feels like something to have a conscious process going on in one's head — why there is first-person, subjective experience. Not only does a green thing look different from a red thing, remind us of other green things, and inspire us to say "that's green" (the Easy Problem), but it actually looks green — it produces an experience of sheer greenness that isn't reducible to anything else. As Louis Armstrong said in response to a request to define jazz, "When you got to ask what it is, you never get to know". The Hard Problem is explaining how subjective experience arises from neural computation. The problem is 'hard' because no one
knows what a solution might look like or even whether it is a genuine scientific problem in the first place.

And not surprisingly, everyone agrees that the hard problem (if it is a problem) remains unsolved.

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