The article examines some aspects of Brentano's ontology, starting with his 1862 dissertation "On the ambiguity of Being according to Aristotle", as well as its influence on the philosophy of M. Heidegger. The author shows that the ontology of the early Brentano is not limited to ousiology, since it includes a discussion of the field of mental being (ens rationis, ὂν ὡς ἀληθές) and it is in this aspect that he influences the young Heidegger. Following Aristotle, Brentano assigns a central (...) role to the ontology of essence, which in the late period leads him to the position of reism, but in the lectures of the middle period Brentano discusses the problems of intentional inexistence, thanks to which projects of "new types of ontology" by Husserl and Mainong appear. The author believes that Heidegger was also influenced by these ideas. Of course, there are fundamental differences between the positions of Brentano and Heidegger, but the similarities are quite large. In particular, if Brentano, highlighting the real and true areas, gives preference in favor of the former, then Heidegger's ontology is built on the second member of this opposition. In particular, everything that can be given is called being by Heidegger, because we are talking about intentional being, about ens rationis, or ὂν ὡς ἀληθές. (shrink)