Here's a simple idea that explains a lot about how the world works: "I know that you know that I know." It might sound complicated, but it's the reason we stop at red lights or believe money has value says Harvard psychologist Steven Pinker. But when people start to have their own truths, when facts are not shared, we can get in to trouble. Dr Pinker unravels this idea that makes in the world go round in his new book, When Everyone Knows That Everyone Knows: Common Knowledge and the Mysteries of Money, Power, and Everyday Life. Dr. Pinker is coming to New Zealand in February for an evening of smart, stimulating conversation. Tickets are available here.