The article aims at elucidating the argumentation in Anselm's Proslogion by relating some aspects of it to the early medieval theory of argument. The focus of the analysis is on the "single argument" (unum argumentum), the discovery of which Anselm announces in the Preface to the Proslogion. Part 1 of the article offers a preliminary description of the single argument by describing the reductio ad absurdum technique based on the notion "that than which a greater cannot be thought". Part 2 discusses the ideas about arguments and argumentation that Boethius presents in Book One of his In Ciceronis Topica. Part 3 draws attention to some early medieval sources (Abelard, Lanfranc, Anselm) that are witness to the importance of the Boethian ideas in Anselm's time. Finally, Part 4 argues that Anselm looked at his single argument in the Boethian framework and that the term "that than which a greater cannot be thought" should be identified as his single argument.