Bertrand Russell said, "the trouble with the world is that the stupid are cocksure, while the intelligent are full of doubt." Recent research in psychology suggests that our confidence often exceeds our competence. Our brains, researchers claim, take shortcuts and jump to conclusions that flatter us. Can we accurately assess what we know, or are we all subject to the fool's dilemma? Author Laura Penny talks to psychologists and writers about new perspectives on the age-old philosophical problem of human ignorance and self-delusion.