The year 2019 will mark the 40th anniversary of the Chilean Constitution, approved under the dictatorship of Augusto Pinochet. With multiple and relevant amendments, this Constitution remains in force. However, those who criticize its legitimacy continue to do so on the grounds of its origin. The aim of this paper is to explore, from the standpoint of political and legal theory (John Finnis, Eric Voegelin), why a Constitution’s origin does not necessarily determine its legitimacy. In addition, all of this refers to a deeper and more relevant problem for constitutionalism: its relationship with contractualism. We will argue that reflecting on this is fundamental for public law in modern times.