  Heidegger turns next to the question of “who it is that Dasein is in its everydayness” (Being and Time, Introduction to IV: 149). He rejects the idea of Dasein as a Cartesian ‘I-thing’ (the Cartesian thinking thing conceived as a substance), since once again this would be to think of Dasein as present-at-hand. In searching for an alternative answer, Heidegger observes that equipment is often revealed to us as being for the sake of (the lives and projects of) other Dasein.