Only about consciousness can we be 100 percent sure. That consciousness exists almost everyone agrees. What consciousness means—that's where arguments and disputations arise. Must consciousness have 'meaning'? Or can consciousness be a random accident, selected by evolution, the 'foam on the waves' of brain activity. But consciousness seems so radically vital.

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Giulio Tononi is a neuroscientist and psychiatrist who holds the David P. White Chair in Sleep Medicine, as well as a Distinguished Chair in Consciousness Science, at the University of Wisconsin.

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