Have you ever wondered whether other people see colors the way you do? Perhaps when you look at newly sprouted grass, it looks to you the way ripe tomatoes look to me. Yet if it does, how could I ever know that it does? We could both take tests that are used to identify color blindness and that would require us to do such things as identify numerals written in red on a background of green. However, this would not resolve the issue since it could be that when you see the red numeral two, you see the same color I see when I look at grass, the only difference being that you have learned to call that color “red.” If only I could jump into your skin and find out what your experience of the numeral was like, then I would know whether you see the color red the way I do. But since that is impossible and since there seems to be no test that could reveal an answer, it seems that we are left with an irresolvable mystery: I can never know what it is like to have your experiences.