 Probability theory was a relative latecomer in intellectual history. To be sure, proto-probabilistic ideas concerning evidence and inference date back to antiquity (see Franklin 2001). However, probability’s mathematical treatment had to wait until the Fermat-Pascal correspondence, and their analysis of games of chance in 17th century France. Its axiomatization had to wait still longer, in Kolmogorov’s classic Foundations of the Theory of Probability (1933). Roughly, probabilities lie between 0 and 1 inclusive, and they are additive. More formally, let \(\Omega\) be a non-empty set (‘the universal set’). A field (or algebra) on \(\Omega\) is a set \(\mathbf{F}\) of subsets of \(\Omega\) that has \(\Omega\) as a member, and that is closed under complementation (with respect to \(\Omega)\) and union. Let \(P\) be a function from \(\mathbf{F}\) to the real numbers obeying: