Martin Heidegger’s fundamental ontology was supposed to divert philosophy towards the absolutely fundamental issue, which in the history of philosophy, unfortunately, as the author claimed, had been neglected. It was an attempt to designate the real foundation, which western metaphysics – while forgetting about being – had never touched. Thus, Heidegger focused on being. He asked about its sense, the way in which it can be described and explained. The answer was to be found in the analysis of some special being – the human being. Only man, who exists in such a way (Heidegger uses the concept of Dasein) that by his very nature refers to being, is “open” to being. This article shows Heidegger’s explanation based on the analysis of man’s being, connecting the existence of reality with human existence, in its essence making the world thoroughly dependent on man, enclosing the world in Dasein. The world ceases to be the whole reality consisting of particular essences existing independently of man. It becomes “the condition”, the way of being. In the first part, the author presents how the fundamental ontology, while placing being at the centre of the discourse and using the phenomenological method, directs the attention to man. When asking about being, it asks about Dasein, about the way of man’s being. Then, the paper presents the existential analytics, the hermeneutics of Dasein, which is crucial from the point of view of Heidegger’s philosophy. The author explains what Heidegger’s existentials are and what it means that man exists as “being-in-the world”, that he exists in a way of “care”. Finally, it is shown that such an understanding of human being, the way in which he cares about the world and understands the world, i.e. designs it, results in enclosing the world in Dasein. The very sense of reality becomes thus the sense of human existence.