From the fifteenth to the nineteenth century two mass-processes (related to politico-economic phenomena) transformed the conditions of communicative interaction and the study of languages in Europe: 1) gradual loss of universalism (decline of Latin) and disappearance of Latin-vernacular bilingualism; 2) standardization of the vernaculars by grammars and dictionaries (‘grammatisation’). ‘Exotic’ languages were described; Sanskrit — a unique phenomenon — became the basis of genealogical/typological classifications. From an epistemological perspective, language study, divided among the ‘arts’ of the trivium (grammar/logic/rhetoric), changed after the Renaissance, becoming part of the philosophy of mind (Port-Royal grammarians, Locke). To show the erosion of the borders of the three ‘arts’, the author analyzes two texts (by Lamy [seventeenth century] and Condillac [eighteenth century]) and concludes that both exemplify belief — common to ‘rationalism’ and ‘empiricism’ — in the interdependence between mental operations and grammatical forms. This resulted in a unified science of language. Comparative linguistics (nineteenth century.) led to a reflection on language history and scientific methods for gathering/verifying data: the eighteenth-century epistemological model imploded, cognitive linguistics became marginalized. However, some late nineteenth-century linguists aimed at elaborating a general linguistic theory, filling the gap between a cognitive foundation for linguistics and methods for comparing/classifying languages synchronically and diachronically: Steinthal, Paul, Bréal.