This article examines Montesquieu's concept of natural law and treatment of legal customs in conjunction with his theory of moral psychology. It explores his effort to entwine the rational procedural quality of laws with the substantive principles that sustain them. Montesquieu grounds natural law in the desires of the human being as ‘a feeling creature’, thus establishing the normative force of desire and making right action attractive by engaging the passions rather than subordinating them to reason. As a result, natural (...) law generates both political norms and the motivations that drive political actions. It provides a standard for assessing the ostensibly jumbled multiplicity of legal customs in human societies. And it reminds us that there is more to political theory than the rational justifi-cation of norms, that central to any account of liberal constitutionalism must be an effort to show the attractions – and not only the justice or the rationality – of right action. Key Words: law • Montesquieu • moral psychology • motivation • natural • law • norm justification • passions. (shrink)