Abstract: Bayesianism claims to provide a unified theory of epistemic and practical rationality based on the principle of mathematical expectation. In its epistemic guise it requires believers to obey the laws of probability. In its practical guise it asks agents to maximize their subjective expected utility. Joyce’s primary concern is Bayesian epistemology, and its five pillars: (1) people have beliefs and conditional beliefs that come in varying gradations of strength; (2) a person believes a proposition strongly to the extent that she presupposes its truth in her practical and theoretical reasoning; (3) rational graded beliefs must conform to the laws of probability; (4) evidential relationships should be analyzed subjectively in terms of relations among a person’s graded beliefs and conditional beliefs; (5) empirical learning is best modeled as probabilistic conditioning. Joyce explains each of these claims and evaluates some of the justifications that have been offered for them, including “Dutch book,” “decision-theoretic,” and “non-pragmatic” arguments for (3) and (5). He also addresses some common objections to Bayesianism, in particular the “problem of old evidence” and the complaint that the view degenerates into an untenable subjectivism. The essay closes by painting a picture of Bayesianism as an “internalist” theory of reasons for action and belief that can be fruitfully augmented with “externalist” principles of practical and epistemic rationality.

Keywords: Bayesianism, decision theory, Dutch book, epistemic rationality, epistemology, mathematical expectation, pragmatic, probability, subjective expected utility, subjectivism,