This article inquiries into the concept of res in the Cartesian metaphsyics. It argues, on the one hand, that res should not be interpreted in line with the traditional scholastic concept of substratum, and on the other hand, that it is nevertheless a technical notion. In particular, the determination of the ego as res cogitans in Descartes' Second Meditation (AT VII 27) must be interpreted in the light of the ontology of the verœ naturœ of the Fifth Meditation. The subsistence (...) of the ego is indeed grounded on the criterion of identification of the true natures, that is on the inseparability of concepts. The ontology of the verœ naturœ is therefore already at work both in the Second and in the Third Meditation before becoming explicit in the Fifth Meditation. (shrink)