The article reconstructs the notion of juridical system in H.L.A. Hart from the analysis of the terms language and use of the language, which are used by the author from the particular tradition of analytical thought I suggest that both the reconstruction of core themes in Hart’s book and the analysis of the conditions embedded in discursive productions are fundamental to understand Hart’s perspective. The hypothesis states that the relationships between the law and moral are a first and necessary step to build up a model of society adapted to a previously formulated theory. If the hypothesis is true, the rational legitimization of discourse may be considered from the point of view of a theory of argumentation.