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photosensitive cone in the retina to produce this sense of color—650 nanometers makes people with normal color vision see red, for instance—science is silent on the genesis of the feeling of red, or cerulean, or other qualia . This is the term many philosophers have adopted for the qualitative, raw, personal, subjective feel that we get from an experience or sensation. Every conscious state has acertain feel to it, and possibly a unique one: when you bite into a hamburger, it feels different from the experience of chewing a steak. And any taste sensation feels different from the sound of a Chopin étude, or the sight of a lightning storm, or the smell of bourbon, or the memory of your first kiss. Identifying the locus where red is generated, in the visual cortex, is a far cry from explaining our sense of redness, or why seeing red feels different from tasting fettuccine Alfredo or hearing “Für Elise”—especially since all these experiences reflect neuronal firings in one or another sensory cortex. Not even the most detailed fMRI gives us more than the physical basis of perception or awareness; it doesn’t come close to explaining what it feels like from the inside. It doesn’t explain the first-person feeling of red. How do we know that it is the same for different people? And why would studying brain mechanisms, even down to the molecular level, ever provide an answer to those questions?
    It is, when you think about it, a little peculiar to believe that when you have traced a clear causal chain between molecular events inside our skull and mental events, you have explained them sufficiently, let alone explained the mind in its entirety. If nothing else, there’s a serious danger of falling into a category error here, ascribing to particular clusters of neurons properties that they do not possess—in this case, consciousness. The philosopher John Searle, who has probed the mysteries of mind and brain as deeply as any contemporary scholar, has described the problem this way: “As far as we know, the fundamental features of [the physical] world are as described by physics, chemistry and the other natural sciences. But the existence of phenomena that are not in any obvious way physical or chemical gives rise to puzzlement…. How does a mental reality, a world of consciousness, intentionality and other mental phenomena, fit into a world consisting entirely of physical particles in fields of force?” If the answer is that it doesn’t—that mental phenomena are different in kind from the material world of particles—then what we have here is an explanatory gap , a term first used in this context by the philosopher Joseph Levine in his 1983 paper “Materialism and Qualia: The Explanatory Gap.”
     
    And so, although correlating physical brain activity with mental events is an unquestionable scientific triumph, it has left many students of the brain unsatisfied. For neither neuroscientist nor philosopher has adequately explained how the behavior of neurons can give rise to subjectively felt mental states. Rather, the puzzle of how patterns of neuronal activity become transformed into subjective awareness, the neurobiologist Robert Doty argued in 1998, “remains the cardinal mystery of human existence.” Yet there is no faster way to discomfit a room of neuroscientists than to confront them with this mind-body problem, or mind-matter problem, as it is variously called. To avoid it, cellular neurophysiologists position their blinders so their vision falls on little but the particulars of nerve conduction—ions moving in and out, electrical pulses traveling along an axon, neurotransmitters flowing across a synapse. As the evolutionary biologist Richard Lewontin puts it, “One restricts one’s questions to the domain where materialism is unchallenged.”
    Materialism, of course, is the belief that only the physical is ontologically valid and that, going even further, nothing that is not physical—of which mind