 Hartley, like Hume, thought that belief was largely a matter of sentiment. “Rational assent” is a matter of the strength of the association between a sentence (or even some of the words in it!) and the word “true” (OM 1, prop. 86) “Practical assent,” a person’s willingness to act, depends on the vividness the proposition—and such can even make “an interesting event, supposed doubtful, or even fictitious, … appear like a real one,” thereby pulling rational assent after it. (Hartley notes, dryly, “the foundation of assent is still the same. I here describe the fact only.”) But Hartley was not content to stop here, an exponent of an urbane skepticism. Belief and assent are matters of feeling, of sentiment, yes—but there are mathematical ways of thinking about the probabilities of belief. And people who think mathematically make better physicians and scientists. (How many trials of an unproven treatment for bladder stones does one have to run, to reach a threshold degree of belief in the results?) In this regard, it is fair to say that Hartley was one of the first Bayesians.