The paper deals with the dialectical dimension of the dialogue De grammatico written by Anselm of Canterbury during his stay in the Abbey of Le Bec. This dialogue between the teacher and the student addresses the question: How grammaticus is both a substance and a quality? In his work De veritate Anselm described the dialogue De grammatico as an introduction to dialectics. The paper tries to show how this Anselm’s dialogue could serve as an explanatory introduction (a textbook) to this liberal art. It seems that the main source of Anselm’s understanding of dialectics was six Boethius’s books called Commentaries to Cicero’s Topica. Therefore, the article presents basic characteristics of dialectics according to the above mentioned Boethius’s treatise (e.g. dialectics as inveniendi et iudicandi argumenti with the help of definitio, partitio and collectio; the description and components of an argument; the importance of the question in a dialectical disputation etc.), while the text of Anselm’s dialogue De grammatico is confronted with those Boethius’s theses.