The scepticism of the Enlightenment was neglected in the studies that Popkin devoted to early modern scepticism. Generally, Popkin only attributes a preparatory, or methodological role to the scepticism of the Enlightenment. Indeed, Popkin himself has come back several times to the picture he had drawn of the Enlightenment, notably in light of the works by Baker, Olaso or Tonelli. However, it was only to accept that scepticism was more developed than he had first thought, and never to change his (...) mind about the influence that it had on this time-period. Through the analysis of forms of scepticism that were totally neglected by Popkin (i.e., clandestine scepticism and egoism), this article proposes a complete re-evaluation of scepticism in the Enlightenment. Rousseau is particularly interesting here because he has known both the clandestine scepticism, and the egoism movement. This article takes him as a case study in order to test the validity of its thesis concerning the influence of this underground scepticism on the Enlightenment as a whole. (shrink)