  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, for each belief, there is some contrary hypothesis that cannot be ruled out. It need not be the same hypothesis for every belief, but skeptics usually buy wholesale instead of retail, so they seek a single hypothesis that is contrary to all (or many common) beliefs and which cannot be ruled out in any way.