Paul Katsafanas explores how we might justify normative claims as diverse as 'murder is wrong' and 'agents have reason to take the means to their ends.' He defends an original account of constitutivism--the view that we can justify certain normative claims by showing that agents become committed to them simply in virtue of acting, and argues that taking this view promises to resolve longstanding philosophical puzzles about the metaphysics, epistemology, andpractical grip of normative claims. Katsafanas' account employs a nuanced theory of action, which draws on recent empirical work on human motivation as well as a model of agency indebted to the work of Nietzsche, in order to answer the greatest challenge for any constitutivist: he argues for a conceptionof action that is minimal enough to be independently plausible, but substantial enough to yield robust normative results.