This article aims at reminding the place of comparative grammar in the Saussure work and the central role it played in the development of his ideas about linguistics. Hence, the postulate of the differential identity of linguistic entities is rooted in the sui generis method which Saussure has spontaneously adopted, from an early age, in order to restore the morpho-phonological system of the Indo-European. By degrees, in the name of the differentiality principle, the author of the Écrits de linguistique générale has been brought to call into question the assumed universality of the categories and to contest the very possibility of a “science” of etymology or of a “historical grammar”. His critical vision, which did not concede to its Zeitgeist, wholly preserves its relevance up to now.