The article concerns Descartes‘ comment on an anonymous project of universal language. Author argues that Descartes‘ refusal of the project is based on implicit argument against very posssibility of the project. Descartes‘ comment can then be read as a dismissal of quite popular version of entlightenment reform, usually attributed even to Descartes himself, but according to the interpretation proposed here the argument is directed against a recipient of Descartes‘ comment, namely Marin Mersenne.