Abstract. The paper gives a formal reconstruction of some fundamental patterns of legal reasoning, intended to reconcile symbolic logic and argumentation theory. Legal norms are represented as unidirectional inference rules which can be combined into arguments. The value of each argument (its qualification as justified, defensible, or defeated) is determined by the importance of the rules it contains. Applicability arguments, intended to contest or support the applicability of norms, preference arguments, purporting to establish preference relations among norms, and interpretative arguments are also formalised. All those argument types are connected in a unitary model, which relates legal reasoning to the indeterminacy of legal systems, intended as the possibility to develop incompatible defensible arguments. The model is applied to permissive norms and normative hierarchies, and is implemented in a Prolog program.