If materialism were true, we should be able to know about matter by introspection; but we don’t, so it isn’t. For materialism is a theory of the nature of mind—what constitutes mental states—and so we ought to know this nature by knowing about the things that have it; but we don’t. If it is the nature of pain to be C-fiber firing, then knowing what pain is should give us knowledge of that nature; yet we know nothing about C-fiber firing by knowing our own pains. Introspection should reveal pain to be fibrous and staccato, because that is what the neural correlate of pain is; but it is blind to these cerebral facts. We don’t even know that the correlate is extended just by knowing our pains, or indeed that there is such a correlate. Shouldn’t the nature of pain communicate itself to us through our faculty of introspecting pain? Why the epistemic cut-off? There isn’t even a hint of the nature of pain in our introspective knowledge of it, as materialism views this nature; but that is puzzling and unexplained. The most obvious explanation of this lack of physical knowledge is that materialism is not true. Contrast two other theories of mind: behaviorism and functionalism. Our ordinary knowledge of mind incorporates information about the behavioral and functional aspects of mental states: we know that pain leads to withdrawal behavior and that it has a certain functional role vis-à-vis belief and desire. The nature of mental states, according to these theories, is not cut off from our self-knowledge, just as one might expect—we know quite a bit about this nature just by having and knowing about mental states. But in the case of central-state materialism we appear completely in the dark about what really constitutes our mental states, as if behind a brick wall. Thus the theory strikes us as startling, surprising, thrilling even (also rebarbative). It seems like a departure from common sense not a continuation of it.
There doesn’t seem to be any logical necessity about introspective ignorance of the brain: people could know their brain states (conceived as such) by inner sense. Consider a possible world in which people have sensations of pain and also introspective intimations of the cerebral correlates of pain. The pain is felt but so is the corresponding brain activity. In such a world the materialist doctrine would not seem far-fetched or counter-intuitive because ordinary knowledge of pain would include facts about its material nature (according to materialism). People would think, “Oh, that’s why I feel my brain that way when I have a pain, because pain is a state of my brain!” And maybe in that world materialism is in fact true: thus its truth conveys itself to the introspective faculty. The case would be like that of behaviorism and functionalism. There would be nothing puzzling in people’s epistemic situation with respect to the mind. But in our world there is no such knowledge of the real nature of mental states, as materialism conceives that nature; and that is puzzling. The dualist will insist that this is exactly what we should expect, since mental states have no such physical nature. The materialist thus faces a challenge—how to explain our ignorance of the nature of our minds given the materialist doctrine. How can introspection be so blind to the truth? Opponents of materialism will conclude that this is not the nature of mind, which is precisely why we don’t introspect minds in the way we should if materialism were true. The relation between mind and brain naturally strikes us as extrinsic, contingent, correlative, not as a relation of identity or constitution: why, if that is what it is? What the materialist cannot do is point to some aspect of our introspective knowledge that anticipates the truth of materialism—as behaviorism, functionalism, and dualism can with respect to their own theories.
It would be wrong to object that the same is true for other kinds of theoretical identification, as with water and H2O or heat and molecular motion, because here ordinary perceptual knowledge does anticipate the theories in question. That is, our ordinary knowledge of water and heat already represents them as material phenomena of some sort, even if the theoretical details remain to be discovered—they clearly belong with other recognizably material things. But in the case of mental states that is precisely not the case: we don’t already conceive them as material and simply await further information about their material nature. No one thinks that materialism about water and heat is a surprising discovery. Thus there is a crucial distinction between the two cases: our ordinary conception of the mental does not already represent it as material in some way yet to be determined, while for water and heat their material nature is a given. This is because introspection tells us nothing of the mind’s (alleged) material nature, not even of the most general kind. So the question remains: how can the materialist explain our lack of knowledge of the real nature of our own minds? Maybe he can, but the question poses a serious challenge. One would think we have an inkling at least of the real nature of our minds via introspection, but according to materialism we could go through our whole lives and never even think of it.
 One possible explanation might look like this: there is no biological payoff in knowing the material nature of our mental states, so we are not set up to have such knowledge—it’s just a question of whether the knowledge would be useful. I take it the problems with this kind of explanation are obvious (we know lots of useless things, etc.).
 An analogy: an idealist might contend that the real nature of material objects consists in dispositions to cause sensory experiences, but she wouldn’t maintain that this nature is closed to our knowledge, since we are well aware that material objects are associated with dispositions to bring about sense experience. But in the case of materialism the constitutive facts are supposed to lie outside our ordinary awareness of our mental states. That asymmetry cries out for explanation. A dualist will certainly see it as confirmation of his position.