isolation is powerful but misleading. For a start, while humans have accumulated a vast store of collective knowledge, each of us alone knows surprisingly little, certainly less than we imagine. In 2002, the psychologists Frank Keil and Leonid Rozenblit asked people to rate their own understanding of how zips work. The respondents answered confidently — after all, they used zips all the time. But when asked to explain how a zip works, they failed dismally. Similar results were found when people were asked to describe climate change and the economy. We know a lot less than we think we do about the world around us. Cognitive scientists call this ‘the illusion of explanatory depth’, or just ‘the knowledge illusion’.