We like to think of ourselves as open-minded, reasonable and realistic people. But evidence from history, psychology and philosophy suggests that we are probably deluding ourselves. Our thinking is heavily influenced by intellectual vices like closed-mindedness, overconfidence, wishful thinking, stupidity and prejudice. Philosopher Quassim Cassam talks about these vices and the role they've played in recent political events. What are they, what's so bad about them, and what can we do to control them?