Are positive duties to help others in need mere informal duties of virtue or can they also be enforceable duties of justice? In this paper I defend the claim that some positive duties (which I call basic positive duties) can be duties of justice against one of the most important prin- cipled objections to it. This is the libertarian challenge, according to which only negative duties to avoid harming others can be duties of justice, whereas positive duties (basic or nonbasic) (...) must be seen, at best, as informal moral requirements or recommendations. I focus on the contractarian version of the libertarian challenge as recently presented by Jan Narveson. I claim that Narveson’s contractarian construal of libertarianism is not only intuitively weak, but is also subject to decisive internal problems. I argue, in particular, that it does not pro- vide a clear rationale for distinguishing between informal duties of virtue and enforceable duties of justice, that it can neither successfully justify libertarianism’s protection of negative rights nor its denial of positive ones, and that it fails to undermine the claim that basic positive duties are duties of global justice. -/- . (shrink)