Relational structure is a primitive notion of Heidegger’s Dasein. By analyzing the concept of pros-ti as it emerges from the Heidegger’s 1924 course dedicated to Plato’s Sophist, I outline the Platonic and Aristotelic roots of Heideggerian Mitsein. Arguably the Mitsein makes explicit the instances of the pros ti — in other words, the instances of Aristotle’s concept of relatedness/intentionality that Heidegger ascribes to Plato’s heteron — but giving them an existential value, having Heidegger pursued the shift from realism to phenomenologyexistentialism. (...) The article concludes by emphasizing the relevance of these themes for social ontology. (shrink)