H. P. Grice's Wildest Dreams

dreaming, argument: One of the principal arguments against the certainty of perception in Descartes. Dreaming is taken by Descartes to be a kind of experience we have when we sleep, with a content consisting mainly of imagery. In a dream, while I am actually lying undressed in bed, I can think that I am dressed and seated near the fireplace. Now I think that I am awake, and am reading this book with a pen in my hand. But since I have been deceived by similar illusions whilst asleep, I cannot know with certainty that these apparent perceptions do not belong to a dream. There seems to be no logical criterion to distinguish waking experience from dream experience. I have reason to doubt any of my perceptions on the grounds that it could be an illusion from my dreams. We cannot distinguish between deception and non-deceptive perception. Thus my perceptions on any given occasion might not be veridical, and could be merely illusory. Any perceptual beliefs I form on their basis could be false. This argument raises the problem of the external world and has played a great role in modern Western philosophy. “In the dream argument Descartes recognises that conflicts may occur among perceptions of any sort – even among those that bear the sensory marks of perception under the most ideal conditions.” Frankfurt, Demons, Dreamers and Madmen