The final kind of argument derives from René Descartes (1641). I do not seem justified in believing that what I see is a lake if I cannot rule out the possibility that it is a bay or a bayou. Generalizing, if there is any contrary hypothesis that I cannot rule out, then I am not justified in believing that what I see is a lake. This is supposed to be a common standard for justified belief. When this principle is applied thoroughly, it leads to skepticism. All a skeptic needs to show is that, f