"When René Descartes tried to systematically doubt everything he believed, he succeeded – until it came to his own existence. The very act of questioning whether you exist proves you do, because you must be there for the doubt to be entertained in the first place. But Descartes missed a trick. For even if your own existence is the most certain fact in the universe, this thing called you turns out to be remarkably elusive."Short column in today's Observer about my new book, The Ego Trick.