Our knowledge of mind is conspicuously confined: we know mind in our own case quite clearly and distinctly, but we can at best speculate about mind as it occurs elsewhere. Nothing else in nature is quite like this: we don’t know shape, say, in only one instance while being ignorant of its many other manifestations—as if I could know perfectly well that this table is oblong but not what shape anything else is. But in the case of things like experience of color I know with certainty what mental states I have, but I am at a loss to know what color experiences others may be having. In all the extension of the concept experience of red my knowledge is confined to just one instance.  This is the problem of other minds: I have confined knowledge of mind, though mind itself is not correspondingly confined.
In fact, it is true to say that there is no reason whatever in the body and brain of other organisms for attributing anything mental to them. If an alien intelligence examined organisms on earth it would not suppose, on that basis alone, that those organisms have minds. The only reason we attribute minds to others is that they are observed to be outwardly similar to us and we know that we have minds. If we didn’t know by introspection that we have a mind, we would not suspect the presence of mind in others. There is nothing in the outward form and substance of an organism to suggest anything beyond an elaborate physical machine; mind comes into the picture only because we know by introspection that we have a mind and suspect that others do based on their analogy to us. And we only believe that we have a mind because we have a special kind of access to mind in our case; otherwise we would take ourselves to be automata. Our knowledge of mind rests essentially on introspective access to mind, which applies only in a single case; hence our knowledge of mind is notably confined compared to the full reality of mind. Mind is everywhere in the biological world, but it is evident in only one place, i.e. oneself.
What would it take to really know the minds of others? The obvious answer is: introspection of their minds. I would need to know others as I know myself. But that is problematic to say the least, for then wouldn’t they have to be me? Maybe my introspective faculty could be modified and somehow hooked up to the minds of others, so that I had direct access to other minds—then I would not have confined knowledge of mind. But it is not easy to see what this could amount to, so this route to knowledge of other minds is apparently blocked. What else would do the trick? Presumably this: that we could read off the mind of another from the facts of his body and brain. Suppose that there exists an a priori deductive link L between the body and the mind such that by knowing L one could know the mind of another. We can tell that another body has L and we can infer deductively that anyone who has Lmust have a mind—indeed, we can tell what kind of mind she has given the specific way L is configured. Then we would have solved the problem of other minds: we would have a cast-iron demonstration of the existence and nature of other minds—by hypothesis. We don’t know what L is as things stand, but this is what would liberate our psychological knowledge from its current confinement. We don’t know other minds because we lack knowledge of what confers mind on a physical organism; we have to make do with mere (putative) correlates and symptoms of mind, not its actual basis in the organism.
What this amounts to is that we can only solve the problem of other minds by solving the mind-body problem.  When we solve the mind-body problem we thereby solve the problem of other minds. Accordingly, the measure of the difficulty of the mind-body problem is the other minds problem. Solving the mind-body problem would be (or would be a crucial part of) solving the problem of other minds, because it would require grasping the intelligible link between body and mind, which is the only thing that could solve the problem of other minds. In both problems we need to identify what makes an organism have a mind—what grounds, explains, and guarantees mentality. So the two problems are indissolubly connected—are indeed aspects of the same problem. That problem is discovering what leads from body to mind—what bridges the gap that confronts us. So to solve the other minds problem we need to solve the mind-body problem, and to solve the mind-body problem we must find something that can be used to solve the other minds problem.
The interesting question here is whether we can’t solve the mind-body problem because we can’t solve the other minds problem. Are our epistemic limitations with respect to other minds the underlying reason that we find the mind-body problem so difficult? To put it more sharply, if the idea of solving the other minds problem in the way I suggested makes no sense, does it follow that solving the mind-body problem is impossible? If we can’t solve the other minds problem that way—it simply wouldn’t work to make others minds accessible to us—should we conclude that solving the mind-body problem is also impossible? For a solution to the mind-body problem woulddeliver such a result: but no such result is feasible; therefore that problem cannot be solved. Is it that we are cognitively closed to the mind-body link precisely because we are cognitively closed to other minds? Does epistemic confinement with respect to other minds lead to epistemic closure about mind and body? Is consciousness mysterious because other minds are mysterious? Is our ignorance about the mind-body connection a reflection of our ignorance about the minds of others? Is the difficulty of the other minds problem a marker of the difficulty of the mind-body problem? If we can never overcome the inaccessibility of other minds, as a matter of deep principle, does that mean that we can never solve the mind-body problem—since the latter solution wouldsolve the other minds problem? If we are necessarily ignorant of the minds of others, are we as a consequence necessarily ignorant of how our own mind links to our body?
Consider functionalism. This theory does two jobs: it gives an account of how other minds can be known, and it offers to explain the emergence of mind from body. The two jobs are performed by the same thing: a specification of the “causal role” of an internal state. It is in virtue of causal role that a body gives rise to a mental state, and it is by knowing that a causal role is instantiated that we know the mental state of another. Thus we know that someone has a certain mental state because we know that she has a property (a functional property) that constitutes that mental state. Functionalism has the right form to solve both the mind-body problem and the other minds problem, by connecting the two problems. The only trouble with it is that it is implausible: causal roles don’t determine mental states, constitutively or evidentially. There is a conceptual gap between instantiating a causal role and instantiating a particular type of mental state (“inverted qualia” and “absent qualia”—inverted spectrum and zombies, respectively). What is significant for present purposes is that functionalism treats the other minds problem as easily solvable (it’s a variant of behaviorism), so that the solution to the mind-body problem does not need to undertake anything too demanding. But if we think that we really can’t know the minds of others—that they are radically inner and private—then the demands on the theory are a lot more difficult to meet, since we need to provide an account of the mind-body connection that can be used to deliver knowledge of other minds. And if the latter is impossible, so is the former. Nothing we can come up with would give us the kind of insight into other minds that we have into our own mind—nothing could answer the skeptic about other minds—so nothing we can devise will suffice to explain the emergence of mind on body. Since other minds are necessarily hidden, the solution to the mind-body problem must be inaccessible, because it would supply a way of opening up the minds of others. If T is the correct theory of emergence, then T will render other minds transparent; but other minds can never be rendered transparent; therefore T will never be known. T would give us knowledge we can never possess, so we can’t discover T. If a theory in physics had the consequence that we can determine the position and velocity of a particle simultaneously, then we would know that that theory cannot be discovered by us, since we demonstrably cannot measure position and velocity together. Our knowledge of mind is necessarily confined to our own case, but it would not be if we could deduce mind from body according to T; so we can’t perform any such deduction, i.e. the mind-body problem is insoluble by us.
It is different for God: God can see right into every mind that exists, not just his own mind, with the same clarity that those minds have about themselves; and so there is no epistemic limit that attends his grasp of the mind-body connection. When God grasps the nature of emergence he can use that theory to gain access to the minds of others; but we do not have his omniscience when it comes to minds in general. Nothing we can imagine could lead us to see the minds of others, so as to put a stop to skepticism about other minds; so we could not be in a position to gain such knowledge—which is what a solution to the problem of emergence would supply. To put it as strongly as possible, it is logically impossible to solve the problem of other minds; but knowing T would make it possible; therefore we cannot know T. The intuitive reason is that T contains an a priori demonstration that a body configured thus and so must give rise to mental states of specific types. T must straddle the explanatory gap, but that implies that it can solve the problem of other minds—and that problem is simply not soluble by creatures such as us (in contrast to God). Our knowledge of mind is necessarily confined to our own case, and this implies that we cannot have knowledge of the relation between mind and body that would render that limitation moot. Solving the mind-body problem would involve solving the problem of other minds; but that is never going to happen, so forget solving the mind-body problem.
This entire line of argument rests on a basic assumption—the extreme intractability of the problem of other minds. I am not here suggesting that we have no reason at all to attribute minds to others (though I don’t think that position should be ruled out); my point is just that our epistemic access to other minds is nothing like our access to our own mind. In the latter case skepticism is ruled out, but in the former case it notoriously is not. I know with certainty what is going on within my (conscious) mind, but that is not the case with my beliefs about other people’s minds. Here it seems that we can be certain that we cannot have the kind of access to their minds that we have to our own mind. What would it even be to examine another person’s body and discover therein exactly what is on his or her mind—as I can direct my introspective faculty to what lies within and come up with infallible knowledge of my mind? But if we could solve the problem of emergence by discovering a theory that contained entailments between body and mind, then we would be able to penetrate with certainty into the minds of others—as in the case of functionalism. Functionalism is false because it makes the problem of other minds too easy—we just need to make an inventory of the causal roles of internal brain states. But once we accept that this problem is deep and ineradicable we must question whether we can solve other problems that impinge on it. I could not know my own mind by surveying my body and brain, I must use my introspective faculty; nor could I know the minds of others by a similar survey, and introspection is ruled out in their case. So our access to minds is inherently confined; it is necessarily restricted to a single instance. But a solution to the emergence problem would not be so confined—it would enable us to read the minds of every other creature with a mind. It would open up the world of mind to our epistemic faculties, rendering it completely transparent, removing all barriers to knowledge—as if we could suddenly see before us what had for so long remained hidden. But this is a fantasy; so we are not going to make any discoveries that render it a reality.
 My point here is not that in no sense of the word “know” can I know that another person is experiencing red; it is just that there is a pronounced epistemic asymmetry between first-person knowledge and third-person knowledge. One sort of psychological knowledge is not open to skeptical doubt while the other sort is. Thus I know for certain that I am experiencing red; in the case of others I am inferring or surmising or speculating, and I might be wrong.