  That is to say: the concretized rule is (morally as well as legally) normative because such normativity is (presumptively and defeasibly) entailed by the (moral) principle that the common good (whose fundamental content is given by the foundational principles of practical reason: 1.1) requires that authoritative institutions take action to specify, apply and enforce some rules on the relevant matters. Social facts make a positive legal rule a reason for action because the desirability of authority as a means of securing common good, and the desirability of the “rule of law and not of men,” are standing and potent reasons for acknowledging such facts as an instance of valid legislation giving presumptively sufficient reason for compliance. Purely positive law that is legally valid is (presumptively and defeasibly) valid and binding  morally—has the moral form or meaning of legal  obligatoriness—when and because it takes its place in a scheme of practical reasoning whose proximate starting point is the moral need for justice and peace, and whose more foundational starting-point is the range of basic ways in which human wellbeing can be promoted and protected, the way picked out in practical reason’s first principles.