In 1994 I sat in an auditorium in Tucson, Arizona, as a young man with long brown hair began talking about consciousness. I remember being dimly conscious at first, perhaps because I was hung over, but gradually the sounds he was making woke me up. “There is nothing that we know more intimately than conscious experience,” he said, “but there is nothing that is harder to explain.”

Explaining what he meant by conscious experience, the long-haired man said: “When we see, for example, we experience visual sensations: the felt quality of redness, the experience of dark and light, the quality of depth in a visual field. Other experiences go along with perception in different modalities: the sound of a clarinet, the smell of mothballs. Then there are bodily sensations, from pains to orgasms; mental images that are conjured up internally; the felt quality of emotion, and the experience of a stream of conscious thought.”*

Consciousness is harder than other problems posed by the mind, the long-haired man argued, such as vision and memory. We have inklings how the brain accomplishes these functions, and we can build machines that replicate them, but we have no idea how the brain generates subjective experiences, or how to give them to machines.

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