How do you predict something that has never happened before? There's a useful calculation being employed by Wall Street, Silicon Valley and maths professors all over the world, and it predicts that the human species will become extinct in 760 years. Unfortunately, there is disagreement over how to apply the formula, and some argue that we might only have twenty years left. Originally devised by British clergyman Thomas Bayes, the theorem languished in obscurity for two hundred years before being resurrected as the lynchpin of the digital economy. With brief detours into archaeology, philology, and overdue library books, William Poundstone explains how we can use it to predict pretty much anything. What is the chance that there are multiple universes? How long will Hamilton run? Will the US stock market continue to perform as well this century as it has for the last hundred years? And are we really all doomed?