Subjective Probabilities on Subjectively Unambiguous Events

Evidence such as the Ellsberg Paradox shows that decision-makers do not assign probabilities to all events. It is intuitive that they may differ not only in the probabilities assigned to given events but also in the identity of the events to which they assign probabilities. This paper describes a theory of probability that is fully subjective in the sense that both the domain and the values of the probability measure are derived from preference. The key is a formal definition of `subjectively unambiguous event.'