Raffoul (1998:67) – Ser como substância

[…] Descartes   presupposes, then, a certain meaning of Being, whose elaboration is not guided by a phenomenal content, but by a traditional idea of philosophy: the idea of Being as substantiality, that is, as Vorhandenheit.

Thus the Being of the "world" is, as it were, dictated to it in terms of a definite idea of Being which lies veiled in the concept of substantiality…. The kind of Being which belongs to entities within-the-world is something which they themselves might have been permitted to present; but Descartes   does not let them do so. Instead he prescribes for the world its "real" Being, as it were, on the basis of an idea of Being whose source has not been unveiled and which has not been demonstrated in its own right.-an idea in which Being is equated with constant presence-at-hand [standige Vorhandenheit]. (GA2  , 96/128-129)

Presence-at-hand thus becomes the mode of Being which improperly determines the ego cogito. Due to the lack of differentiation between the entity that we are and intra-worldly entities, be they zuhanden or vorhanden, and because of the illegitimate reduction of all meaning of Being to that of substantiality (i.e., constant presence) alone, Descartes   misses the proper Being of Dasein, existence, the opening to Being, Being-in-the-world. This is why emphasizing Descartes  ’s uncritical use of the ancient and medieval categories makes the task of a preliminary understanding of substantiality all the more urgent: that understanding alone could make accessible to what a "profound extent the medieval ontology has influenced the way in which posterity has determined or failed to determine the ontological character of the res cogitans" (GA2  , 25/46). Now, if one tries to grasp the ontological underpinnings of the cogito on the basis of the Greek interpretation of Being as "substance" (ousia, parousia, hypokeimenon), it appears immediately that substance covers up the temporal-ontological meaning of presence (Anwesenheit). Ancient ontology, according to Heidegger, is an ontology that orients itself toward nature in the broad sense, and determines it as parousia, ousia. Now, for Heidegger, the highly polysemic term ousia, grasped in its fundamental signification, signifies but one thing: constant presence. "Substance" is the ontological title of an entity which is always present-at-hand.

[RAFFOUL  , François. Heidegger and the subject. Atlantic Highlands, N.J: Humanities Press, 1998]