In this paper, we shall demonstrate that the Cartesian approach to the res extensio and taking it as the most fundamental, essential determination of Vorhandenheit, though Descartes’ ontology is not rich enough to explain the relationship between pure extension and Dasein’s existential spatiality, has its own phenomenological justifications in Heidegger’s view. In order to clarify Heidegger’s stance, we shall first study the distinctive characteristics of existential space as well as its ontological foundations. Meanwhile, existential space proves to be ultimately rest on Dasein’s own existence. Next, we shall turn to the transition from existential space to Cartesian space as a pure extension via the process of Entweltlichung. Cartesian space will thus prove to be a necessary resultant of depriving existential space of its richness and purposiveness of human meanings. It is in the context of this more comprehensive ontology that the relation of Cartesian space with existential one and spatiality of Dasein, on one hand, and the ontological deficiency of Cartesian approach, on the other, would be elucidated. Finally, we shall show that assuming extension as the essential determination of Vorhendenheit, and thus Cartesian space, as a totally meaningless, and deprived of all Dasein’s circumspective concerns which underlie existential space and Dasein’s everyday experience, is not so much refuted by Heidegger’s ontology as regarded to be phenomenologically justified. Moreover, It is so fertile that has paved the way for the subsequent paradigm shift in mathematics and theoretical physics in the 19th century.