Scott Sturgeon presents an original account of mental states and their dynamics. He develops a detailed story of coarse- and fine-grained mental states, a novel perspective on how they fit together, an engaging theory of the rational transitions between them, and a fresh view of how formal methods can advance our understanding in this area. In doing so, he addresses a deep four-way divide in literature on epistemic rationality. Formal epistemology is done in specialized languages—often seeming a lot more like mathematics than Plato—and so can alienate philosophers who are drawn to more traditional work on thought experiments in epistemic rationality. Conversely, informal epistemology appears to be a lot more like Plato than mathematics and, as such, it tends to deter philosophers drawn to formal models of the phenomena. Similarly, the epistemology of coarse-grained states boils down everything to a discussion of rational belief—making the area appear a lot more like foundations of knowledge than anything useful for the theory rational decision, such as decision-making under uncertainty. The Rational Mind unifies work in all of these areas for the first time.