If we buy ten tickets for a new and unfamiliar raffle, Bayes imagined, and five of them win prizes, then it seems relatively easy to estimate the raffle’s chances of a win: 5/10, or 50%. But what if instead we buy a single ticket and it wins a prize? Do we really imagine the probability of winning to be 1/1, or 100%? That seems too optimistic. Is it? And if so, by how much? What should we actually guess?