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So machines can think? Not so fast. Many people would still say no. When they talk about what's inside a human being, they mean way inside--not just the neuronal data flow corresponding to our thoughts and feelings but the thoughts and feelings themselves. You know: the exhilaration of insight or the dull anxiety of doubt. When Kasparov lost Game 1, he was gloomy. Could Deep Blue ever feel deeply blue? Does a face-recognition program have the experience of recognizing a face? Can computers--even computers whose data flow precisely mimics human data flow--actually have subjective experience? This is the question of consciousness or mind. The lights are on, but is anyone home?

For years AI researchers have tossed around the question of whether computers might be sentient. But since they often did so in casual late-night conversations, and sometimes in an altered state of consciousness, their speculations weren't hailed as major contributions to Western thought. However, as computers keep evolving, more philosophers are taking the issue of computer consciousness seriously. And some of them--such as Chalmers, a professor of philosophy at the University of California at Santa Cruz--are using it to argue that consciousness is a deeper puzzle than many philosophers have realized.

Chalmers' forthcoming book is already making a stir. His argument has been labeled "a major misdirector of attention, an illusion generator," by the well-known philosopher Daniel Dennett of Tufts University. Dennett believes consciousness is no longer a mystery. Sure there are details to work out, but the puzzle has been reduced to "a set of manageable problems."

The roots of the debate between Chalmers and Dennett--the debate over how mysterious mind is or isn't--lie in the work of Dennett's mentor at Oxford University, Gilbert Ryle. In 1949 Ryle published a landmark book called The Concept of Mind. It resoundingly dismissed the idea of a human soul--a "ghost in the machine," as Ryle derisively put it--as a hangover from prescientific thought. Ryle's juiciest target was the sort of soul imagined back in the 17th century by Rene Descartes: an immaterial, somewhat autonomous soul that steers the body through life. But the book subdued enthusiasm for even less supernatural versions of a soul: mind, consciousness, subjective experience.

Some adherents of the "materialist" line that Ryle helped spread insisted that these things don't even exist. Others said they exist but consist simply of the brain. And by this they didn't just mean that consciousness is produced by the brain the way steam is produced by a steam engine. They meant that the mind is the brain--the machine itself, period.

Some laypeople (like me, for example) have trouble seeing the difference between these two views--between saying consciousness doesn't exist and saying it is nothing more than the brain. In any event, both versions of strict materialism put a damper on cosmic speculation. As strict materialism became more mainstream, many philosophers talked as if the mind-body problem was no great problem. Consciousness became almost passe.

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