Changer la langue (S. Branca) This article shows how the Ideologists at the same time theorists of the sign and grammarians, treated the problems of Usage, of its socio-historical determinations, and of the intervention of scholars and politicians in the field of linguistic change. Is it possible or advisable to improve languages in order to regulate social relations, to provide new sciences with a solid terminological foundation, and finally to ensure by means of a perfect language the transmission of knowledge?