demonstrates that scepticism about the external world…is self-defeating. What Kant attempts is to prove that any experience whatsoever must conform to the categories and that the experience that is thus produced is of an objective world, not a merely personal subjective creation of each individual. Sceptics about the external world begin from their own experience and argue that they can’t prove that it is really experience of an external world rather than a pure illusion. Kant argues that because they begin from experience, such sceptics undermine their own approach: the existence of an objective external world, always perceived in terms of the categories, is a condition of our having any experience at all. (Warburton, 123)
Warburton, Nigel. Philosophy: The Classics, Third Edition. New York: Routledge, 2006.