 The basic upshot of Jeffrey’s theory is that the desirability of a proposition, including one representing acts, depends both on the desirabilities of the different ways in which the proposition can be true, and the relative probability that it is true in these respective ways. To state this more precisely, \(p\), \(q\), etc., will denote propositional variables. Let \(\{p_1, p_2, …, p_n\}\) be one amongst many finite partitions of the proposition \(p\); that is, sets of mutually incompatible but jointly exhaustive ways in which the proposition \(p\) can be realised. For instance, if \(p\) is the proposition that it is raining, then we could partition this proposition very coarsely according to whether we go to the beach or not, but we could also partition \(p\) much more finely, for instance according to the precise millimetres-per-hour amount of rain. The desirability of \(p\) according to Jeffrey, denoted \(Des(p)\), is given by: