This article studies the origin of the control of the constitutionality of laws in France, particularly the project Sieyes presented to the National Convention on the 2 and 18 thermidor an III (20 July and 5 August 1795) which culminated in the idea of a constituent power and separation of powers. The authors tries to demonstrate that in the French tradition characterized by the refusal to use instruments to guarantee the integrity of the constitution and to limit the powers of the legislator, the project of Sieyes is an attempt to create a neutral power by the constitutional jury, a politico-judicial organ, responsible for controlling the constitutionality of laws, for proposing the change in the constitution, and for the exercise of an natural equality.