Cartesian Doubt

In his Meditations on First Philosophy, Descartes says that you have to begin by asking yourself the question what can I know with certainty? And in the first and second of those six meditations he tackles this question. He uses skeptical doubt as a tool for arriving at the point of certainty that he thought was required as the bedrock for his inquiry. But it turns out for a day cut that there is one thing that you cannot be fooled about.