128 I. 3
Being and Time

21. Hermeneutical Discussion of the Cartesian Ontology of the 'World'

The critical question now arises: does this ontology of the 'world' seek the phenomenon of the world at all, and if not, does it at least define some entity within-the-world fully enough so that the worldly character of this entity can be made visible in it? To both questions we must answer "No". The entity which Descartes is trying to grasp ontologically and in principle with his "extensio", is rather such as to become discoverable first of all by going through an entity within-the-world which is proximally ready-to-hand—Nature. Though this is the case, and though any ontological characterization of this latter entity within-the-world may lead us into obscurity, even if we consider both the idea of substantiality and the meaning of the "existit" and "ad existendum" which have been brought into the definition of that idea, it still remains possible that through an ontology based upon a radical separation of God, the "I", and the 'world', the ontological problem of the world will in some sense get formulated and further advanced. If, however, this is not possible, we must then demonstrate explicitly not only that Descartes' conception of the world is ontologically defective, but that his Interpretation and the foundations on which it is based have led him to pass over both the phenomenon of the world and the Being of those entities within-the-world which are proximally ready-to-hand.

In our exposition of the problem of worldhood (Section 14), we suggested the importance of obtaining proper access to this phenomenon. So in criticizing the Cartesian point of departure, we must ask which kind of Being that belongs to Dasein we should fix upon as giving us an appropriate way of access to those entities with whose Being as extensio Descartes equates the Being of the 'world'. The only genuine access to them lies in knowing [Erkennen], intellectio, in the sense of the kind of knowledge [Erkenntnis] we get in mathematics and physics. Mathematical knowledge is regarded by Descartes as the one manner of apprehending entities which can always give assurance that their Being has been securely grasped. If anything measures up in its own kind of Being to the Being that is accessible in mathematical knowledge, then it is in the authentic sense. Such entities are those which always are what they are. Accordingly, [96] that which can be shown to have the character of something that constantly remains (as remanens capax mutationum), makes up the real Being of those entities of the world which get experienced. That which enduringly remains, really is. This is the sort of thing which mathematics knows. That which is accessible in an entity through mathematics, makes up its Being. 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,