 Bayes’s theorem is a very central element of the probability calculus (see Joyce 2019). For historical reasons, Bayesian has become a standard label to allude to a range of approaches and positions sharing the common idea that probability (in its modern, mathematical sense) plays a crucial role in rational belief, inference, and behavior. According to Bayesian epistemologists and philosophers of science, (i) rational agents have credences differing in strength, which moreover (ii) satisfy the probability axioms, and can thus be represented in probabilistic form. (In non-Bayesian models (ii) is rejected, but (i) may well be retained: see Huber and Schmidt-Petri 2009, Levi 2008, and Spohn 2012.) Well-known arguments exist in favor of this position (see, e.g., Easwaran 2011a; Pettigrew 2016; Skyrms 1987; Vineberg 2016), although there is no lack of difficulties and criticism (see, e.g., Easwaran 2011b; Hájek 2008; Kelly and Glymour 2004; Norton 2011b).