The article is devoted to the question of being in Heidegger’s fundamental ontology, namely, his claim to understand the ancient idea of being. In his work “Contributions to Philosophy (From Enowning)”, Heidegger distinguishes between the understanding of being as the being of everything that exists and his own understanding of being as such, out of connection with things. He calls the first point the “leading question” and attributes its authorship to Aristotle, the second one – “the main question”. The article discusses the relationship of these issues, the possibility of their formal coincidence and discrepancy between them. The first part of the article introduces the context of Heidegger’s thought. It deals with the “ontological difference” that Heidegger introduces in the framework of “Being and Time”, as well as with what transformation this difference undergoes in his later works, after the “turn” (“Kehre”). In the second part of the article, attention is drawn to the convergence and divergence of the “leading question” from the “main one” by clarifying the ontology of Aristotle. Particular attention is paid to various ways of understanding existence, in particular – an incidental and accidental existence. At the end of the article, the attempt will be made to reconstruct the direction of Heidegger's thought, based on the results of consideration achieved earlier.