Mind: A Brief Introduction (John R. Searle) / quote 20 of 138
Three grades of solipsism
The view that I am the only person who has mental states is called “solipsism.” Solipsism comes in at least three different grades. One, the most extreme form: I am the only person in the world who has mental states; and indeed in some forms, nothing exists in the world except my mental states. Two, epistemic solipsism: maybe other people have mental states, but I can never know for sure. It is quite possible that they do but I have no way of finding out, because all I can observe is their external behavior. And three: Other people do have mental states, but I can never be sure that they are like mine. For all I know, what I call “seeing red,” if you could have that very experience you might call it “seeing green,” and if I could have your experience that you call “seeing red” I would call it “seeing green.” We both pass the same color blindness tests because we both make the same discriminations in our behavior. If asked to pick out the green pencil from a box of red pencils, we both pick the same pencil. But how do I know that the inner experiences you have that enable you to discriminate are similar to the ones I have that enable me to discriminate?