 Philosophically interesting forms of skepticism claim that we do not know propositions which we ordinarily think we do know. We should distinguish such skepticism from the ordinary kind, the claim that we do not know propositions which we would gladly grant not to know. Thus, it is a form of ordinary skepticism to say that we do not know that there are an even number of stars in the Milky Way, but it is a form of philosophical skepticism to say that we do not know that the sun will come out tomorrow. Even though our interest is in philosophical skepticism, we can start our inquiry by thinking about ordinary skepticism.