This paper, authored by Itzhak Gilboa and David Schmeidler, explores the concept of maxmin expected utility in the context of non-unique priors. The authors address a challenge to Savage's paradigm posed by Ellsberg, who demonstrated that individuals can rank bets without a unique probability measure. They propose a framework where the decision-maker considers a set of possible priors and evaluates bets based on the minimal expected utility over all these priors. The paper introduces the concept of certainty-independence, which is a weakening of the classical independence axiom, and uncertainty aversion, which captures the phenomenon of hedging. The main result of the paper is an axiomatic foundation for the maxmin expected utility decision rule, showing that it can be represented by a utility function and a convex set of finitely additive probability measures. The paper also discusses the uniqueness of the utility function and the convex set under certain conditions and extends the results to a broader set of acts. Finally, it introduces the concept of independence in the case of a non-unique prior and provides a characterization of this independence.This paper, authored by Itzhak Gilboa and David Schmeidler, explores the concept of maxmin expected utility in the context of non-unique priors. The authors address a challenge to Savage's paradigm posed by Ellsberg, who demonstrated that individuals can rank bets without a unique probability measure. They propose a framework where the decision-maker considers a set of possible priors and evaluates bets based on the minimal expected utility over all these priors. The paper introduces the concept of certainty-independence, which is a weakening of the classical independence axiom, and uncertainty aversion, which captures the phenomenon of hedging. The main result of the paper is an axiomatic foundation for the maxmin expected utility decision rule, showing that it can be represented by a utility function and a convex set of finitely additive probability measures. The paper also discusses the uniqueness of the utility function and the convex set under certain conditions and extends the results to a broader set of acts. Finally, it introduces the concept of independence in the case of a non-unique prior and provides a characterization of this independence.