In epistemology , the problem of explaining how it is possible for one person to know anything about the quality of another person's inner experience, or even that other people have inner experiences at all.
According to a standard example, because each person's pain sensation is private, one cannot really know that what another person describes as pain is really qualitatively the same as what one describes as pain oneself. Though the physical manifestations the other person exhibits can be perceived, it seems that only the other person can know the contents of his mind. The traditional justification for belief in other minds, the argument from analogy, was given its classic formulation by John Stuart Mill : because my body and outward behaviour are observably similar to the bodies and behaviour of others, I am justified by analogy in believing that others have feelings like my own and are not simply automatons. In the mid-20th century the argument from analogy was severely criticized by followers of the later Ludwig Wittgenstein . An approach to the problem of other minds from the perspective of existentialism is contained in Being and Nothingness (1943), by Jean-Paul Sartre .