 In this regard, what is most interesting in proposition 87 is Hartley’s discussion of probability. Hartley appears to have been one of a small circle of mathematicians who read, and understood, papers making fundamental contributions to the theory of probability by Abraham De Moivre (1667–1754) and Thomas Bayes (1702–61). De Moivre developed a theorem that allows for the determination of the degree of convergence between the frequency of observed events and the underlying probability ratio, for any finite number of events. More important is “the solution to the inverse problem,” which Hartley says “an ingenious friend” communicated to him. This would be Bayes’s theorem—customarily attributed to Thomas Bayes—which deals with determining the probability with which probability ratios can be inferred from observed outcomes, and which provides (a still controversial) foundation for statistical inference. In his discussion of probability theory, and other mathematical topics, Hartley is primarily interested in methods for moving from observation to explanation, of assessing the likeliness—i.e, believability—of hypotheses.