Heidegger’s most important and distinguished feature from the other philosophers, to us, is his linguistic analysis when he explain his ideas and his putting forward conceptual innovations. The word Dasein that Heidegger used it as a synonym for "human entity" and settled it in the basis of his ontology is one of those terms, possibly the most important one. Dasein is ground, meaning and end of Being. Besides, it is an incomplete Being. It only can reach its totality and completeness with death. Thus, Heidegger sometimes called it as sum moribundus (I am in dying). In this study, we try to examine the nature of its possibility to get its identity as a being-toward death. And in this context, we try to take up its relation with Cartesian subject (cogito sum).