 Countable Additivity. It ought to be that, for any propositions \(A_1,\) \(A_2,\)…, \(A_n,\)… that are mutually exclusive, if one has credences in those propositions and in their disjunction \(\bigcup_{n=1}^{\infty} A_n\), then one’s credence function \(\Cr\) satisfies the following formula: