Every modern person in a pluralistic democracy such as ours must constantly question what he or she assumes to be true about the world. Every cosmopolitan, multi-cultural society in modern history has developed some culture of healthy doubt, and the US is no exception. Doubting our received beliefs is what allows different races, different religions, and those of different political viewpoints to avoid all-out war and, occasionally, even live in something approaching harmony. The opposite of doubt is certainty. And it’s certainty—the belief that no legitimate explanations for the world exist outside one’s own—that marches soldiers onto battlefields and flies planes into buildings.