From The French Language Today by Adrian Battye and Marie-Anne Hintze an interesting passage re the setting of prescriptive grammar in the 17th cnt French court:
L’Usage for Vaugelas thus remains rather a nebulous concept and in the same preface he goes on to paint the acceptance of the dictates of l’Usage as being very much an act of faith, for just as religious belief in the final analysis defies reason and logical argument, so too, in the linguistic domain, l’Usage must be accepted and one should not expect its decisions to be based on reason. Vaugelas sees himself then as the servant of l’Usage in his Remarques. He is not taking upon himself to make authoritarian statements about what sort of language should be considered correct and socially acceptable, rather he presents himself in his preface as simply recording what are the norms of polite conversation (in the seventeenth century French court).