Ruth Barcan Marcus is famous for her influential work in modal logic. Her perhaps most distinctive contribution to the subject, the so-called Barcan formula, states that if it is possible that something has a certain property, then there is something that possibly has that property. The Barcan formula gave rise to an extensive debate concerning the interpretation of systems of modal logic. In recent years, Marcus has been writing on issues concerning belief and rationality rejecting received accounts of propositional attitudes and attempts to systematize epistemic logic. This talk discusses this lesser known part of Marcus’s contribution to philosophy and logic.