Most people take themselves to know various things about the external world, the past, and other minds. We assume, for example, that we know that there are cars and trees, that we were alive yesterday, that others are happy or sad, and that they believe various things. Moreover, we believe that almost everyone knows many of the same kinds of things. We don't assume, for example, that such knowledge is the purview of only a handful of philosophers. Still, many of us are skeptical about some claims to knowledge. Suppose someone claims to know who will win the World Series in ten years or that there is life on some other planet. We are skeptical about such claims to knowledge. We deny that those who make such claims really do know what they claim to know. We might also deny that such claims are justified or reasonable. We might be skeptical about the claim that such beliefs are justified.

While almost all of us are skeptical about some knowledge claims, skeptical arguments in philosophy typically purport to show that we do not know or are not justified in believing many of the things we ordinarily assume we know or reasonably believe. These arguments often conclude that we do not know or are not justified in believing anything about the external world, the past, or other minds. In some cases, these surprising conclusions follow from premises that seem quite plausible.