 Averaging is the distinguishing rationality condition in Jeffrey’s theory. It can actually be seen as a weak version of Independence and the Sure Thing Principle, and it plays a similar role in Jeffrey’s theory. But it is not directly inconsistent with Allais’ preferences, and its plausibility does not depend on the type of probabilistic independence that the STP implies. The postulate requires that no proposition be strictly better or worse than all of its possible realisations, which seems to be a reasonable requirement. When \(p\) and \(q\) are mutually incompatible, \(p\cup q\) implies that either \(p\) or \(q\) is true, but not both. Hence, it seems reasonable that \(p\cup q\) should be neither strictly more nor less desirable than both \(p\) and \(q\). Suppose one of \(p\) or \(q\) is more desirable than the other. Then since \(p\cup q\) is compatible with the truth of either the more or the less desirable of the two, \(p\cup q\)’s desirability should fall strictly between that of \(p\) and that of \(q\). However, if \(p\) and \(q\) are equally desirable, then \(p\cup q\) should be as desirable as each of the two.