In dreams we often remember things that never happened; at best therefore we can be sure of our present momentary experience, not of anything that happened even half a minute ago. And before we can so fix our momentary experience as to make it the basis of a philosophy, it will be past, and therefore uncertain. When Descartes said 'i think', he may had certainty; but by the time he said 'therefore i am', he was relying on memory, and may have been deceived.