The privacy of the mind is generally treated as a platitude, but it is seldom (if ever) asked what explains this platitude. Privacy here is best understood perceptually: states of mind are not perceptible by means of the senses. It is not denied that they may be subjects of legitimate inference, or even of interpretative seeing, but they are not objects of perception in the way the body is. I can see your facial features in a way I can’t see your thoughts or feelings or sensations. These are hidden from me, directly accessible only to you. If consciousness is a stream, it is an invisible stream. The senses are defeated by the mind; the two do not work well together. But why is this the case? What accounts for the invisibility of the mind?
First, let us develop a more articulate sense of the problem of privacy. Is the privacy of the mental a necessary truth? It is not an epistemic necessity: it could have turned out that the mind is not private. Picture the child’s inchoate understanding of the mind: it is by no means self-evident to her that the experiences she enjoys are perceptually inaccessible to others. She might be brainwashed into believing that others can see her thoughts (as we are brainwashed into believing that God can see them), and nothing in her understanding would enable her to rule out the truth of this proposition. It is not analytic or a priori that mental states are private. Nothing about how they appear to the child can logically exclude the possibility of perceptibility by others. This is something she must empirically discover. And likewise we adults cannot be certain that our minds are private—the claim is subject to Cartesian skepticism. I believe my mind is hidden from others, but perhaps I am wrong—perhaps I am surrounded by demigods that can literally see into my thoughts. I can’t be wrong that I have thoughts (the Cogito), but I may be wrong about their privacy: “I think, therefore something about me is invisible” is not a logical certainty.
So is the privacy point a contingent truth? It might not be, since epistemic contingency doesn’t entail metaphysical contingency, but for all we have said it might be. It might be that our senses, as they are actually constituted, don’t reveal other minds but that they could be altered in such a way as to overcome that limitation—or the senses of other beings might not be so limited. Or it may be that minds are necessarily inaccessible to our type of senses: nothing like vision, say, could ever reveal the mind of another—though some hard-to-imagine sense might not be so limited. The question is difficult, and it is compounded by an obvious fact: the mind is perceptible to its owner. I am aware of what I think and feel, though you are not. But then there is nothing about the mind itself that logically precludes perception of it; and if so, what obstacle is there to another subject having a similar kind of access? Couldn’t someone else have quasi-introspective access to my mind? It would be wrong to dismiss this point by insisting that introspection isn’t strictly a sense, so that its existence cannot be a reason to allow for a similar sense in others; for that would be equivalent to arguing that we can rule out the inner sense theory of introspection simply by observing that mental states are private. That would be an unconvincing argument, since privacy and the inner sense theory seem perfectly compatible—and indeed that theory has a lot to be said for it. So it is not at all obvious that privacy is a metaphysical necessity: there might well be possible worlds in which minds are perceptually accessible to others. Couldn’t there be reliable causal connections between states of mind and states of observers’ perceptual systems of such a kind that perceptions of the former by the latter were commonplace? Couldn’t there be possible beings that can see into the minds of others? At any rate, I will remain agnostic on the question here, since my question concerns the explanation of the privacy to which we are actually subject. Evidently this is an epistemic contingency, so it should be possible to explain why it holds. What, then, makes it the case that, in our world, minds are invisible?
It is important to appreciate how surprising this privacy really is, despite its familiarity. Not only is it not an a priori certainty; it runs counter to everything we know about the empirical world. It is hugely anomalous, quite bizarre, and very difficult to square with our general scientific picture of reality. Consider the matter biologically. Organisms have evolved a variety of traits useful to their survival. The visible body is the locus of those traits—a collection of functional organs than can be scrutinized with the naked eye (as well as under a microscope). Everything is open to view, public and perceptible. Well, almost everything, because among these evolved traits we have traits of mind—and these are private and imperceptible. This is true for us humans as well as for relatively simple organisms (it is not that the mental traits of reptiles are visible while mammals have evolved private mental traits). Why is this, and how is it possible? What is the point of evolving private mental traits—what does the privacy do for an organism in the way of survival? And how do the genes manufacture such traits? How is invisibility genetically coded? How does the nervous system, itself public and perceptible, generate inner mental states that resist observation? How does the private emerge from the public, both in phylogenesis and in ontogenesis? One would think that all the biological traits of organisms would be alike—all publicly visible—but some are the exact opposite. This cries out for explanation. What is the biological function of privacy, if any? What is it about the mind and the senses that renders the latter unsuitable for perceiving the former? What is the natural ground of privacy?
A first thought would be that it is in the very nature of the mind to be private: that is, we can derive privacy from other constitutive characteristics of mind. Thus we might consider whether intentionality or subjectivity or rationality lies behind privacy—is it because of the former that the latter holds? But this project is quickly dashed: privacy is a logically independent characteristic of mind. Nothing about those other properties of mind entails that mental states should be invisible. So there is no recognized intrinsic aspect of the mind that explains its privacy—nothing in our ordinary conception of it. In particular, nothing about consciousness as such (that we know of) leads inexorably to privacy; the mind could have been public and still had those other characteristics, so far as we can see. Privacy is an add-on, a further trait of mind—as they are add-ons to each other. It begins to seem gratuitous that the mind should insist on invisibility—it lacks a clear rationale. Can we do better?
What about the power of deception? Organisms don’t always want their inner thoughts and intentions to be transparent, so there is survival value in keeping them hidden. Is this the reason the genes ensure that minds remain imperceptible? Hardly: the deception motive would only apply to a limited class of mental states; it wouldn’t apply to many organisms whose mental states are as private as those of professional deceivers; and there are also advantages to possessing public mental states, particularly where ease of communication is concerned. So adaptive deception is not the ground of mental privacy. Nor are the usual physical explanations for invisibility applicable to the mind: optical transparency, camouflage, occlusion, size, speed, and glare. These factors can all result in a physical object being invisible, but they don’t apply to the mind. The mind is not a piece of glass or blends in with the environment or sits behind something or is too tiny or moves too quickly or gives off too much light. And it is not as if it wears a magical cloak of invisibility that it can throw off to reveal its fleshly attributes. Its invisibility is more principled, more inherent. Or perhaps we should say that the limitations of perception with respect to the mind are more inherent to it. But this is puzzling for two reasons: first, mental states are inwardly perceptible, so it is not in their nature to resist all perceptual incursions; second, they have causal powers and exist in the natural world (unlike numbers), so there is no reason why they should preclude the evolution of a sense that can resonate to them. In any case, we have not yet found a convincing explanation for such invisibility as we actually find.
An extravagant thought suggests itself: the reason the mind is invisible is that it is immaterial. We can only perceive material things in space, not immaterial things outside of space. So privacy entails dualism. We can deduce from the invisibility of the mind that the mind is not the body and is not dependent on it. There are a host of problems with this line of thought, which I won’t enumerate; let me make two quick points. The first is that it is not clear that immateriality entails invisibility (even assuming we know what “immateriality” means)—are angels and ghosts logically impossible? Is it our belief in immateriality that explains our commitment to privacy? Second, minds are not imperceptible tout court, since they are objects of introspection; and presumably this is compatible with dualism. Immateriality doesn’t preclude this kind of perception—so why the other kinds? Minds may be imperceptible in their very nature, but it is not because their nature is to be immaterial (whatever that might mean). Nor, to my knowledge, has any dualist attempted to prove that doctrine from the fact of privacy (Descartes was much too astute for that).
A more promising line of thought is that we are formulating the problem wrongly: we are presupposing that visibility is the norm and then fretting over why the mind doesn’t conform to the norm, but in fact, it is the other way about. Actually things are naturally invisible and what needs an explanation is why anything should be detectable by the senses. The question should be why the body is perceptually accessible. There is nothing surprising about the mind not being perceptible, since that is the default condition of the universe; what is surprising is local pockets of perceptibility—and even there the perceptibility may be glancing and superficial. We can imagine a version of Kantian idealism behind this view, or reflections on current physics and “dark matter”. The thought is that reality is inherently removed from our senses and what we say we see is just our own mode of representing it—sense data, mental constructions, subjective phantasms. Then it is simply par for the course that minds are not perceptible—since nothing is! Reality is inherently not set up for the senses, or the senses are not set up for it.
This is a gratifyingly extreme position, but it won’t help us in the present connection, whatever may be said for it generally. For again, the fact is that mental states are perceptually available—to introspection. They are not naturally cut off from our epistemic faculties, like the most hidden of invisible particles or the constituents of dark matter—or even fields and forces. They are very proximate to us from an epistemic point of view. This is what is so puzzling about their third-person privacy: it is not that they are necessarily hidden to others as such that creates the puzzle but that they are selectively hidden. Why is that things that are so transparent from one point of view are so opaque from another? This approaches the status of paradox, unlike generally hidden facets of the universe: so known and yet so unknown. It is as if God gave us one faculty with which to survey mental reality but refused to give us any more—introspection but not vision or hearing or touch or smell or taste. Mental states exist for the apprehension thereof, but the apprehension must only be from a single standpoint. Why not make the mind either universally unperceived or universally perceived? Why make us so acute in one way but so blind in another—and with respect to the very same objects? Those who contend that reality at large eludes perception generally accept that the mind itself is open to immediate inspection—though they are insufficiently puzzled about its selective openness. We make no progress with our question by asserting that reality is generally cut off from our cognitive faculties, because that is signally not the case with the mind. Our explanandum is not that the mind is invisible tout court but that it is invisible to others while visible to oneself; or, as we might say, the fact that something as familiar as the mind is so removed from the world of the senses. One would think that it ought to be perfectly open to perception by others. Everything seems to point in that direction—introspection, biology, the general nature of reality—but for some reason, the mind refuses to yield itself up to the senses.
Is it that the raw materials of mind are themselves imperceptible, so that evolution has no choice but to render the mind imperceptible? A panpsychist might contend that the proto-mental aspects of reality that form the basis of the mind are by nature imperceptible entities, more so than atoms and the like. So there is really no alternative to invisible minds—the properties of the parts transmit themselves to the whole. But this proposal also faces fatal objections. First, it can’t explain introspection: those primitive parts have to be capable of composing inwardly perceptible mental states–but how? Second, they raise the same question: why are they so radically imperceptible? Why does reality consist of both public and private objects? Could there be a sense that brought the proto-mental within its sights? The entities exist in nature, possess causal powers, and are aspects of public objects—and yet they are not perceptible, perhaps necessarily so. It is a puzzle; even a paradox, once we acknowledge introspection. Maybe there is an answer to the puzzle, but we have not yet found it.
What if the respiratory processes of the body were visible while the digestive processes were not? That would seem very odd. But why are the processes we designate as “mental” invisible while the rest are visible? Both are evolved biological traits arising from the same mechanisms and basic materials, so why the dramatic distinction? Suppose we were visited by aliens equipped with minds and senses that enable them to perceive each other’s mental states (perhaps ours too); to them, the idea of mental privacy would seem bizarre. They are surprised to learn that we are mentally blind (as they unkindly put it) and their philosopher-scientists get to work to explain this oddity in our makeup. To them there is nothing natural or inevitable about it, theoretically or practically; they read minds as they read texts. They have perceptual experiences of others being in pain and they think nothing of it. They think we are biological anomalies—they have seen nothing like us despite their extensive inter-galactic travels. Do we have some strange genetic defect? They refer to Earth as “the planet of the blind”, meaning mentally blind. They offer to engineer our deficit away, and are prepared to accept reasonable trading terms for this service (dancing lessons, massages). We accept their offer and wake up after the operation with vivid perceptions of other people’s states of mind—the phenomenology is extraordinary! You don’t feel other people’s pain exactly, but boy do you get a strong sense of it—nothing like that old insipid behavior-based belief that someone else is hurting. Your empathy really shoots up. You find yourself living in a whole new world. I am trying with this story to make strange the predicament in which we actually find ourselves: powerfully conscious of our own mind but only dimly aware of the minds of others. We stare at others and find nothing mental to look at, yet we know that behind the veil lies a mental life as rich as our own. How frustrating it is that we can’t penetrate the veil and observe other minds as they really are! Wouldn’t it be amazing if we could literally see the mental life of a bat? So near and yet so far! And we have no clear idea about why we are thus confined—no explanation of the fact of privacy. Why isn’t the opposite the case—the body is invisible but the mind is visible? Is that beyond God or nature to contrive?
There must be an explanation for privacy, despite its elusiveness, and whatever it is would surely shed considerable light on the nature of consciousness and the mind, as well as on the nature of our knowledge. But we haven’t been able to come up with one, not even an incomplete explanation. It is hard even to get one’s mind around the problem (what would it even be to see into the mind of another?): we have yet another mystery to add to the pile. We don’t know why our minds can’t be seen or heard or touched or smelled or tasted.
 I should say a word about those who believe that the mind is perceptible, as when we see a man’s suffering in his face. I don’t accept that type of description at face (!) value, but even if I did it would not affect the point of this paper, since it would be agreed that other minds are not perceivable except by virtue of expressive behavior. No one supposes that you can see the mind of another when there is no behavior to go on, say when the person is asleep and dreaming. The question then would be what explains this fact.
 The question is difficult but it doesn’t seem that consciousness itself is the root of privacy, its sine qua non, since the same point applies to the unconscious—it too is invisible.
 Would there be any need for speech if the mind were interpersonally transparent? Apparently not, so why isn’t there some selective pressure to evolve transparent minds, thus dispensing with the need for speech? Speech just seems like a rather cumbersome way to get your thoughts across.
 Note too that we cannot see the mind under ultraviolet light or take an X-ray of it (this inaccessibility applies equally to brain scanning machines).
 We could add that the close involvement of mind and brain also makes it surprising that the mind is invisible, since the brain is not. One would think that the mind would be as perceptible as the brain given their intimate connection. (To those who claim that the mind is perceptible because the mind is the brain, I make the obvious reply: even if it were true that there is de re perception of mental states in those states of the brain with which mental states are identical, it would not follow that the brain states are perceived as mental. That is, brain states do not appear to us as mental states.)
 We can easily envisage a form of materialism that accommodates the invisibility of the mental: just identify mental states with states of dark matter!
 Even if physical objects are not perceptually accessible, they are at least represented in perception—it seems to us as if we are seeing them. But that is not true of mental states—it does not seem to us that we see them. So there would still be an asymmetry between imperceptible physical objects and imperceptible mental states.
 Note that I can be experiencing a pain in my foot while being unable to perceive that very pain in myself: I introspect the pain in all its glory but no matter how hard I stare at my foot or my brain I can see nothing of its throbbing existence. I am perceptually cut off from a fact about myself that I know vividly from the inside. So the problem concerns not just my knowledge of other people’s sensations but also my knowledge of my own sensations.
 There is an analogous problem about introspection and the physical world: introspection is limited to the mental world, not extending to physical states of the organism. But why is this—why can’t we introspect our brain states, say? It doesn’t seem logically impossible and yet it never happens. I won’t discuss this problem here, limiting myself to the problem of why perception doesn’t extend to the mind.