Skepticism regarding a subject matter is the view that knowledge about the subject matter is not possible. Many subject matters have come under skeptical attack. It has been argued, for example, that it is not possible to obtain knowledge about the external world, about as-yet-unobserved states of affairs, and about minds other than one's own. This entry will focus upon skepticism about knowledge of the external world.

The Cartesian Skeptical Argument

The following skeptical argument is suggested by Descartes's first Meditation. Consider the skeptical hypothesis SK: There are no physical objects; all that exists is my mind and that of an evil genius, who causes me to have sense experience just like that which I actually have (sense experience representing a world of physical objects). This hypothesis, says the skeptic, is logically possible and incompatible with propositions implying the existence of the external world, such as...