 Degrees of belief in the [0,1] interval are introduced to quantify the strength of belief attitudes. For our purposes, we will take propositions to be the objects of partial belief. It is also possible to assign degrees of belief to sentences in a formal language, but for the most part nothing hinges on the approach we choose (see Weisberg, 2011). Accordingly, let \(\mathbf{A}\) be a field of propositions over a set \(W\) of possibilities. A function Pr: \(\mathbf{A} \rightarrow \Re\) from \(\mathbf{A}\) into the set of real numbers, \(\Re\), is a (finitely additive and non-conditional) probability measure on \(\mathbf{A}\) if and only if for all propositions \(A, B\) in \(\mathbf{A}\):