 He became well-versed in the historical and comparative method and adopted its mentalist tradition and systemic view of language.In his first major publication, "Le problème de l’article et sa solution dans la langue française" (The problem of the article and its solution in the French Language) (1919), Guillaume set out to apply the comparative method to the uses of the articles in Modern French, in order to describe their mental system located in the preconscious mind of the speaker rather than in pre-historical time.He was to pursue his research into the system of articles for the next 20 years.