Alex Byrne in the Boston Review:
How do I know that you have a pain in your leg? Perhaps you tell me or I see you hopping around or grimacing while holding your leg. In short, I know that you have a pain in your leg by observing your behavior (including your verbal behavior). Of course, sometimes this method doesn’t work because you’re faking—but usually it does. How do I know that I have a pain in my leg? In the typical case, not by asking my doctor or seeing myself in the mirror hopping around. It is not immediately clear how best to characterize the method I normally use to find out whether I am in pain, but it is clear that whatever that method is, it is quite unlike the way I have of knowing that you are in pain. And similarly for other mental states. I know that you believe that the pub is open because I see you striding purposefully toward it; I don’t know that I believe that the pub is open by catching a glimpse of myself in a store window, heading for the pub. Usually I do not need to observe myself to find out what I believe. A person, then, has a special way of finding out about her mental states that is quite different from the way she finds out about others’ mental states. Let us mark this fact by saying that we have peculiar access to our mental states. Contrast this with finding out one’s own weight or underwear color: here all the methods of discovery—using scales, undressing, and so on—can also be employed to find out these facts about other people.