In this paper we examine the notion of impersonality (das Man) presented in Heidegger's Being and Time. Taking as interpretative guideline the analysis of a comparison made by Heidegger between the das Man and the concept of ens realissimum, we maintain that the impersonality has a central ontological function within the program of the fundamental ontology. Like the concept of ens realissimum in the ontoteological tradition, which played the rolle of ground of determination for things in generall, the impersonal manner of being represents the elemental source of all inteligibility. The parallel between the notions of impersonality and ens realissimum enables then a grasping of the social nature of the projection of being by the human understanding of being.