Norman Malcolm, "Defending Common Sense"

Moore's reply to the skeptic, to simply claim that he knows that this is a hand, or that he knows he is a human being, is mistaken because it assumes that saying "I know this is a hand" or "I know I am a human being" makes more sense than "I don't know this is a hand" or "I don't know I'm a human being". The proper response to the skeptic is to say that neither the assertion nor the denial that one knows that this is a hand makes sense (in a philosophical context).