 On the one hand, propensities are said to constrain the degrees of belief, or credences, of a rational agent. Recall the ‘applicability to rational beliefs’ criterion: an interpretation should clarify the role that probabilities play in constraining the credences of rational agents. One such putative role for propensities is codified by Lewis’s ‘Principal Principle’. (See section 3.3.) The Principal Principle underpins an argument (Lewis 1980) that whatever they are, propensities must obey the usual probability calculus (with finite additivity). After all, it is argued, rational credences, which are guided by them, do.