Problems of Other Minds
We speak of the problem of other minds, in the singular, but it is instructive to disentangle different strands in what is so described. Does the difficulty of knowing other minds stem from the nature of mind or from the contingent limits of our faculties of knowing other minds? We humans know other minds (insofar as we do) by the observation of external behavior by means of our senses, particularly vision. We train our eyes on other people from some distance and see their facial expressions and bodily movements, as we hear sounds emanating from their oral cavity. If we are far away our eyes and ears register very little of the behavior that (we think) gives rise to knowledge of other minds, and even at close quarters our senses fail to register minute bodily changes. The human senses can only take in so much of what happens to the body. And they are quite blind to other facts that might provide evidence of the other’s state of mind: chemical events in the interior of the body, invisible brain alterations. All this could be observed by an observer with suitable senses, but we humans gain our knowledge of other minds through a limited subset of the available evidence—we just look and listen from a certain distance, in a certain light, etc. Our knowledge has a slender perceptual basis, determined by the character of our senses. Many animals drink more deeply of the workings of the body and therefore have access to evidence for mind that is denied to us. Smell is the obvious route to knowledge of other minds for a dog or cat; and perhaps such animals feel less removed from other minds than we do. The dog can “smell fear” and detect mood by responding to subtle (for us) biochemical markers. A possible being could taste our hormones and arrive at conclusions about our mental state. The human way of knowing other minds is just one way of knowing them, based on a particular set of perceptual capacities.
Let us call this the perceptual problem of other minds: it arises from the contingent circumstances of our actual perceptual faculties used for knowing other minds. And the skeptical thought will be that these faculties are inadequate for affording genuine knowledge of other minds. Seeing a person’s face from six feet away, say, is just not an adequate basis for knowledge of what is in that person’s mind at the time. Superior perceptual access (think dogs) would be adequate, but the actual perceptual access we have to other people’s bodies is insufficient to ground a claim to knowledge. It is as if we perceive just a few traces of what is going on inside, so that our normal dealings with people don’t afford what is needed for knowledge—it’s all just guessing. Maybe the problem doesn’t apply to dogs (or Martians), but with humans the normal perceptual basis is just too exiguous. But this problem is not the only problem faced by our claim to know other minds: there is also what might be called the logicalproblem of other minds. This is the problem that no matter how much perceptual access we might have to a person’s body—his internal organs, biochemistry, neural activity—such facts could never, as a matter of principle, afford an adequate basis for knowledge of other minds. For there is a logical gap between the physical manifestations or accompaniments of mental states and mental states themselves. The former are public and objective while the latter of private and subjective. We are thus faced by a deep inferential chasm that cannot in principle be crossed. So even if the perceptual problem could be solved the logical problem would remain.
Now some may pooh-pooh the logical problem, finding it overly fastidious epistemologically; they find this an uninteresting form of skepticism. But even if that were so there would still be the perceptual problem, and that problem looks much closer to common sense. It is really not clear (it will be said) that our ordinary modes of perceptual access to people can warrant the kind of confidence we typically repose in our claims about other minds (though we often readily admit deep ignorance about such matters). At any rate, there are two problems here, requiring different kinds of answer. I think most philosophers have had in mind the logical problem when considering the problem of other minds, but the perceptual problem needs to be treated in its own right; there is something specially worrying about our claims to knowledge of other minds that doesn’t apply to the problem of the external world (say). There is no problem about our knowledge of the external world that might be resolved by extra perceptual faculties or an enhancement in acuity. It is not that dogs know the external world better than humans! The problem of other minds is not a skeptical problem like any other—a reflection of the logical gap between evidence and conclusion—but has a distinctly human aspect: it results from our actual modes of perceptual access. We know just from our daily dealings with people that we are vastly ignorant of what is going on inside them—their thoughts, motivations, and emotions. That is why it is so easy to evoke skepticism about other minds in people: even children can see that it is abominably difficult to know whether others see the world in the same colors they do, for example. This is not a matter of some abstract epistemological principle about the logical gap between evidence and conclusion; it results from awareness that we perceive little of decisive relevance to psychological attributions. We are accordingly natural skeptics about other minds. We wouldn’t be if we had greater perceptual access to relevant facts, such as what is happening in the gut and brain; then we would be at most philosophical skeptics about other minds.
And there is another problem I shall call the phenomenological problem: we just don’t have any experiences of the presence of other minds—it doesn’t seem to us perceptually that there are other minds. It seems to us that there are external objects, including human bodies, but we don’t likewise have experiences in which it seems to us that we are seeing a mind (pace Wittgenstein). The phenomenology of our knowledge of other minds does not include perceptual appearances of those minds. By contrast, we do have our own minds before our minds when we introspect, so it really does seem to us that we have a mind: but it doesn’t likewise seem to us that other people have minds. And if there is no such phenomenology, then nothing forces on us a belief that we tend to hold, namely that there are other minds. So we can’t cite this phenomenology as a defense of our ordinary beliefs. We have these beliefs based on shaky inference from scant perceptual clues. The skeptic will say that without an appearance of other minds we are manufacturing a belief to which we are not entitled. The belief is not grounded in our primitive awareness of the world but arises from something like custom or habit: it is more like religious belief than belief in natural facts. And indeed who can say that their beliefs about other minds are grounded in incontestable facts of experience? We are really just making the best of a bad epistemological job in forming our beliefs about other minds as we do, making the most of whatever clues come our way. It is not that vision (or hearing) is somehow well suited to discovering truths about other minds; it is ill suited to that task but we have nothing else to rely on. We can see a person’s face and we can search there for indications of inner realities, but it is not that other minds chose this method as the most revealing way of making themselves known. It is really a miracle that we can know anything about other minds this way at all. If people had no faces, we would be much more skeptical about other minds than we already are. We rely on scraps and hints, and we know it. 
 The problem of the external world has neither a perceptual nor a phenomenological component: it is not that we only have access to a worryingly limited subset of perceptual data in forming our ordinary beliefs about things, and it is not that there is no phenomenological datum of external objects. The problem is purely an abstract logical problem about evidence and belief, namely that perceptual data don’t logically entail material-object beliefs (the same holds for the problem of induction). The problem of other minds is in a class by itself.