1. What is the Problem of Other Minds?
That other human beings are mostly very like ourselves is something about which almost all of us, almost all of the time, are certain. There are exceptions, among them philosophical sceptics, and perhaps those suffering from some abnormal mental condition. We do not, of course, believe that we always or even mostly know about others' inner lives in detail, but we do not doubt that they have an inner life, that they experience the physical world much as we do, rejoice, suffer, have thoughts, beliefs, feelings, emotions, and so on. But what, if anything, justifies our certainty? Philosophers cannot agree on what underpins this most basic of human beliefs.
Unsurprisingly, given that human beings are social, if not all necessarily sociable beings, this lack of agreement is more than a case of philosophers engaging in some abstractly theoretical controversy and contestation. The different positions taken affect our view of what it is like to be the kind of creature we are, and possibly are affected by our view of who we are and our human situation.
There is general agreement among philosophers that the problem of other minds is concerned with the fundamental issue of what entitles us to our basic belief that other human beings do have inner lives rather than whether we are able in specific cases to be sure what is happening in those inner lives.
However, there are (at least) two problems of other minds. There is the epistemological problem, concerned with how our beliefs about mental states other than our own might be justified. There is also a conceptual problem: how is it possible for us to form a concept of mental states other than our own. It is generally thought that the materials used to fashion the epistemological problem are the very same materials that produce the conceptual problem. The conceptual problem is generally raised in the context of solving the epistemological problem. One view here is that there can only be an epistemological problem if the conceptual problem is solved, but solving the conceptual problem solves the epistemological problem (Malcolm 1962a). That would be just as well since otherwise the epistemological problem would still be with us. More straightforwardly, some have thought that the conceptual problem is the difficult one without, tantalizingly, showing how easy it is to solve the epistemological problem (Nagel 1986, 19–20).
Despite the above proposals, and allowing for philosophy's notorious lack of common agreement, it remains worth noting that philosophy provides no generally agreed solution to the problem of other minds.
1.1 The Epistemological Problem
The epistemological problem is produced by the radical difference that holds between our access to our own experience and our access to the experience of all other human beings. We often know directly that we are in a certain mental state. Typical cases would be where we are in serious pain, are itching, are smelling a rose, seeing a sunflower, are depressed, believe that today is Tuesday, and so forth. We do not always know directly that we are in the mental state we are in but what is striking is that we never have direct knowledge that other human beings are in whatever mental state they are in. It is this stark asymmetry that generates the epistemological problem of other minds.
The asymmetry is a matter of what is known directly and not known directly, and the specific kind of knowledge. It is not a matter of what can be observed, perceived, felt, as opposed to what cannot be observed, perceived, felt. Were I able to observe the mental states of another human being that would not mean that I did not have a problem of other minds. I would still lack what I needed. What I need is the capacity to observe those mental states as mental states belonging to that other human being. They would have to be experienced by me as someone else's mental state. My experience of the other would have to come accompanied by that guarantee, attached as it were to an epistemological label. The situation would only then be as it is in my own case. I would only then be in possession of the direct knowledge that I and all of us forever lack.