After having been neglected for most of the twentieth century, the subject of consciousness has become fashionable. Amazon lists 3,865 books under "consciousness," a number of them new releases of the last year or two. What exactly is the problem of consciousness, and why exactly is it so difficult, if not impossible, for us to agree on a solution to it?

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There are lots of explanations in science and philosophy that are not in the form of equations. In fact, equations are rather rare in biology. Think of the germ theory of disease or the theory of evolution. What we are interested in, in these cases, are causal mechanisms, not equations. What causes disease symptoms? What is the causal account of the evolution of human and animal species from simpler forms of life? And now, what causes consciousness?