 Expected utility theory consists of three components. The first is a utility function that assigns real numbers to consequences. The second is a probability function that assigns real numbers between 0 and 1 to every possible event. The final component is an “aggregation norm” that holds that the value of an act is its expected utility value relative to these two functions, and that rational preferences track expected utility value (see the entry on  expected utility theory).   Each challenge to EU theory can be thought of as a rejection of one or more of the three components as normative, and alternatives generally replace or extend the relevant component.