The present work is based on Joseph Raz work and consists in a theoretical investigation about the concepts of legal norms and systems in the boundary of analytic positivism. The analysis of the models developed by Austin, Kelsen and Hart prove the necessity of reformulation of the criteria of identity of Law. A criterion that takes into account the activity of the primary law-applying organs is superior because recognizes the fact that the identified norms, in a certain way, are connected with reasons for action. Because this practical dimension of the norms can not be disregarded, the criterion proposed by Raz proportions a better and more faithful description of a legal system.