Grammar, Logic, Semantic. Two Conflicting Doctrines in the Thirteenth Century: Roger Bacon and the Modistae The complex interplay between grammar and logic in the 12th and 13th centuries is an important element in understanding the development of these two disciplines. Two 13th century doctrines (the Modistae's and Roger Bacon's) show a strong coherence in their logico-semantic and grammatical theories, but rely on diametrically opposed assumptions about language: the Modistae seek to base themselves on the universal and stable (logical and grammatical) properties of a timeless language, Bacon on the variability of language and the freedom of the speaker to change the meaning of words as of constructions, thus enabling mm to take into account the evolution of language. The differences between the two conceptions explain the contradictory answers of the Modistae and Roger Bacon to the important linguistic questions of the latter half of the 13th century.