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WHAT REMAINS IS NOT ONE PROBLEM ABOUT CONSCIOUSNESS BUT two, which the 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 would probably crack it in this century.
What exactly is the Easy Problem? It's the one that Freud made famous, the difference between conscious and unconscious thoughts. Some kinds of information in the brain--such as the surfaces in front of you, 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 also 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 a mystery.
Although neither problem has been solved, neuroscientists agree on many features of both of them, and the feature they find least controversial is the one that many people outside the field find the most shocking. Francis Crick called it "the astonishing hypothesis"--the idea that our thoughts, sensations, joys and aches consist entirely of physiological activity in the tissues of the brain. Consciousness does not reside in an ethereal soul that uses the brain like a PDA; consciousness is the activity of the brain.
THE BRAIN AS MACHINE
SCIENTISTS HAVE EXORCISED THE GHOST FROM THE MACHINE NOT because they are mechanistic killjoys but because they have amassed evidence that every aspect of consciousness can be tied to the brain. Using functional MRI, cognitive neuroscientists can almost read people's thoughts from the blood flow in their brains. They can tell, for instance, whether a person is thinking about a face or a place or whether a picture the person is looking at is of a bottle or a shoe.