This paper situates the problem of defeaters in a larger debate about the source of normative authority. It argues in favour of a constructivist account of defeasibility, which appeals to the justificatory role of normative principles. The argument builds upon the critique of two recent attempts to deal with defeasibility: first, a particularist account, which disposes of moral principles on the ground that reasons are holistic; and second, a proceduralist view, which addresses the problem of defeaters by distinguishing between provisional (...) and strictly universal principles. The particularist view fails to establish that moral principles have no epistemological import, but it raises important questions about their role in practical reasoning. The proceduralist view fails to distinguish between reasoning by default and reasoning by principles, but it shows that normative principles have a structural justificatory role. The constructivist view recognizes that the moral valence of normative claims vary across contexts, but denies that this is because of holism about reasons. Rather, it defends defeasibility within a constructivist account of reasoning where universality serves as the matrix of judgment. The constructivist view vindicates the justificatory role of universal normative principles, and makes room for some ordinary sources of defeasibility, which are left unaccounted by competing views, and which depend on the agent’s own progress. (shrink)