Individual humans know embarrassingly little about the world, and as history progressed, they came to know less and less. A hunter-gatherer in the Stone Ageknew how to make her own clothes, how to start a fire, how to hunt rabbits and how to escape lions. We think we know far more today, but as individuals, we actually know far less. We rely on the expertise of others for almost all our needs. In one humbling experiment, people were asked to evaluate how well they understood the workings of an ordinary zip. Most people confidently replied that they understoodthem very well - after all, they use zips all the time. They were then asked to describe in as much detail as possible all the steps involved in the zip's operation. Most had no idea. This is what Steven Sloman and Philip Fernbach have termed 'the knowledgeillusion'. We think we know a lot, even though individually we know very little, because we treat knowledge in the minds of others as if it were our own. 21 Lessons for the 21st Century