 A last comment: MacFarlane is not explicit as to whether bridge principles are to be understood as synchronic norms—norms that instruct us which patterns of doxastic attitudes are, in a specified sense, obligatory, permissible or reasonable at a given point in time; or whether they are to provide diachronic norms—norms that instruct us how an agent’s doxastic state should or may evolve over time. To illustrate the distinction, let us consider Co+ (aka  IMP)  once again. Understood synchronically, the principle should be spelled out as follows.