Diderot’s le Fils Naturel is generally seen as containing the first developed theories with regard to a new genre of dramatic literature, that will later become known as ‘the dramatic genre’. However, the composite structure of Diderot’s book, which is made up of a framework-story, a dramatic model and a theoretical discussion, also represents the crystallization of a slow turn in the uninterrupted line of the countless possible relations between performance and public. Diderot starts a change: the birth of an intimate and private bond, that is set up in such a way as to involve the spectator in a continuous and destablising game between reality and fiction, of which s/he is both protagonist and witness.