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and consciousness are the paramount examples—can even exist in the sense of being a measurable, real entity. (This approach runs into problems long before minds and consciousness enter the picture: time and space are only two of the seemingly real quantities that are difficult to subsume under the materialist umbrella.) For a sense of the inadequacy of equating what neurons do with what minds experience, consider this thought experiment, based on one first advanced by the Australian philosopher Frank Jackson. Imagine a color-blind neuroscientist who has chosen to study color vision. (Jackson called her Mary.) She maps, with great precision, exactly what happens when light of a wavelength of 650 nanometers falls on the eyes of a volunteer. Shelaboriously traces the pathway that analyzes color through the lateral geniculate body of the thalamus, along the sweeping fibers of the optic radiation, into the primary visual cortex. Then she carefully notes the activation of the relevant areas of the visual association cortex in the temporal lobe. The volunteer reports the outcome: he sees red! So far, so good. The neuroscientist has precisely described the stimulus—light of a precise wavelength. She has meticulously traced the brain circuits that are activated by this stimulus. And she has been told, by her volunteer, that the whole sequence adds up to a perception of red.
    Can we now say that our neuroscientist knows, truly and deeply knows , the feeling of seeing red? She certainly knows the input, and she knows its neural correlates. But if she got out of bed the next morning to find that her color blindness had miraculously remitted, and her gaze fell on a field of crimson poppies, the experience of “red” at that instant would be dramatically and qualitatively different from the knowledge she had gained in the lab about how the brain registers the color red. Mary would now have the conscious, subjective, felt experience of color.
    We needn’t belabor the point that there is a very real difference between understanding the physiological mechanisms of perception and having a conscious perceptual experience. For now, let’s say the latter has something to do with awareness of, and attention to, what is delivered for inspection by the perceptual machinery of the central nervous system. This conscious experience, this mental state called a sense of red, is not coherently described, much less entirely explained, by mapping corresponding neural activity. Neuroscientists have successfully identified the neural correlates of pain, of depression, of anxiety. None of those achievements, either, amounts to a full explanation of the mental experience that neural activity underlies. The explanatory gap has never been bridged. And the inescapable reason is this: a neural state is not a mental state. The mind is not the brain, though it depends on the material brain for its existence (as far as we know). As the philosopher ColinMcGinn says, “The problem with materialism is that it tries to construct the mind out of properties that refuse to add up to mentality.”
    This is not exactly the view you find expressed at the weekly tea of a university neuroscience department. For the most part, the inevitable corollary of materialism known as identity theory—which equates brain with mind and regards the sort of neuron-to-neuron firing pattern leading to the perception of color as a full explanation of our sense of red—has the field by the short hairs. The materialist position has become virtually synonymous with science, and anything nonmaterialist is imbued with a spooky sort of mysticism (cue the Twilight Zone theme). Yet it is a misreading of science and its history to conclude that our insights into nature have reduced everything to the material.
    The advent of materialism is widely credited to Isaac Newton, who is considered the intellectual father of the view that the world is an elaborate windup clock that follows immutable laws.