In Chapter 9 probabilities were known. Utility was the subjective element. Let us now move to decision under ambiguity where probabilities are unknown. Unknown probabilities are more common in real life than known probabilities. Examples are the probability that the earth will warm up by more than 2 degrees in the coming 20 years, the probability of disease when a patient displays given symptoms, the outcome of an upcoming election or the probability that inflation will fall below 2% in the coming year. We may have some idea of the probability of these events, but we are unsure. Moreover, our probability is subjective and will differ from those of other agents.