Ius quod necessitas constituit, Senatusconsultum est. Jacques Cujas and the foundation of the normative power of the Roman Senate . Jacqes Cujas identifies in his Paratitla, in his Observationes and extensively in the texts of his posthumously collected lectures, necessity to be the source and foundation of the normative power of the Roman Senate. In doing so, he revisited, rectified and refined the achievements of the medieval jurists, he connected literary and legal sources with philological precision and historical awareness, and achieved results, which are also particularly useful to the contemporary historiographical and historical legal reflection. This study attempts to render all these aspects, on the basis of the numerous testimonies in the Opera omnia, showing the context and the implications.