But what therefore am I? A thinking thing. What is that? I mean a thing that doubts, that understands, that affirms, that denies, that wishes to do this and does not wish to do that, and also that imagines and perceives by the senses.
I therefore suppose that all I see is false; I believe that none of those things represented by my deceitful memory has ever existed; in fact I have no senses at all; body, shape, extension in space, motion, and place itself are all illusions. What truth then is left? Perhaps this alone, that nothing is certain.