 Textbooks in philosophy often refer to the problem of other minds. At a superficial glance it can look as if there is agreement about what the problem is and how we might address it. But on closer inspection one finds there is little agreement either about the problem or the solution to it. Indeed, there is little agreement about whether there is a problem here at all. What seems clear is that there was a period in philosophy, roughly around the mid-twentieth century, when there was much discussion about other minds. The problem here has most commonly been thought to arise within epistemology: how do I know (or how can I justify the belief) that other beings exist who have thoughts, feelings and other mental attributes? One standard line of reply to this question has been to appeal to analogy, another to best explanation. A less standard approach has been to appeal to criteria. Connected with this approach, and fueled by the later work of Ludwig Wittgenstein, some philosophers argue that the real problem of other minds is conceptual: why do I so much as think that there are other thinking, feeling beings? Both the conceptual and the epistemological problems may be thought to be connected with a more general metaphysical problem of understanding what minds and mental states are.