Résumé
The continuum is one of the most difficult mathematical concepts for undergraduate students. We hypothesize that among the difficulties they face in relation with this notion, the graphical evidence provided by the number line fosters the idea ofa dichotomy between discreteness and continuity, hiding the property of density-in-itself, i.e. the intrinsic density with respect to order in a totally ordered set. In this paper, we first provide evidences of the weakness of fresh university students’ knowledgeabout real numbers. Then, we briefly present Dedekind’s construction of real numbers, which relies on the intuitive idea of the continuous line. Finally, we present a didactical situation aimed at fostering the understanding of the relationships between discreteness, density-in-itself and continuity for an ordered set of numbers.