Lecture by Jean-Alexandre Perras (resident 2018-2019 of the IEA of Paris) as part of the doctoral and research seminar "Around Jean-Jacques Rousseau", organised by Jacques Berchtold and Christophe Martin under the aegis of the CELLF (UMR 8599 of the CNRS) and ED 3 of the Université Paris-Sorbonne, in collaboration with the doctoral seminar "18th century" led by Jean-Christophe Abramovici, Renaud Bret-Vitoz, Michel Delon and Pierre Frantz.