What do you believe to be true? Are you absolutely certain? Can you really know? Does reason exist for its own sake, or is every thought simply an illusion created by your brain? This week, I look at Descartes' revolutionary experiment of doubting everything - everything about the world, how we sense things, even our own reasoning - and attempting to build knowledge back up again. He bases his conclusions on what he finds: the idea of "perfection" leads him to conclude that there must be a god. Is the argument valid? Where do your thoughts come from? How could you feel so sure about something and yet be entirely wrong? Is it possible to use logic to prove anything at all? And at the heart of these questions is one fundamental question that remains unanswered: Are you a person, or are you just using a body?