The assertion that nothing can be known may mean just what it says; nothing whatever can be known about any subject. In this form, skepticism is clearly self-refuting, for if nothing can be known, skepticism can be known. Further, another person may, if he feels like it, set up the opposite assertion, something can be known; and the utter skeptic has no way of disproving it, for any arguments on his part would be a confession that something was known on which an argument could be based. The thorough-going denial of all knowledge is a dogma that destroys itself. As a wild mood of despair, it is comprehensible; but as a reasonable view, it cannot even be formulated.
(Brightman, Introduction to Philosophy, 70)