This book argues that moral philosophy should be based on seven scientific principles of theory selection. It then argues that a new moral theory—Rightness as Fairness—satisfies those principles more successfully than existing theories. Chapter 1 explicates the seven principles of theory-selection, arguing that moral philosophy must conform to them to be truth-apt. Chapter 2 argues those principles jointly support founding moral philosophy in known facts of empirical moral psychology: specifically, our capacities for mental time-travel and modal imagination. Chapter 2 then shows that these capacities present human decisionmakers with a problem of diachronic rationality that includes but generalizes beyond, L.A. Paul’s problem of transformat...