 This illustrates a general issue with update rules like Jeffrey Conditionalization: to apply them, we first need to determine the new probabilities to assign to the evidence. From there we can determine the new probabilities of other propositions. But this essential bit of input is something for which we don’t have a rule; it’s a sort of loose end in the formal system, something that’s left up to us as users of the model. For some discussion of the epistemological significance of this point, see Field (1978) and Christensen (1992).