It is this unconcealment of being that defines Dasein for Heidegger: Dasein is that being for whom being is an issue, that is, for whom being shows up as such. This is why Heidegger begins his inquiry into the meaning of being with an inquiry into the essence of Dasein; The unconcealment of being is an essentially temporal and historical phenomenon (hence the ""time"" in Being and Time); what we call past, present, and future correspond originarily to aspects of this unconcealment and not to three mutually exclusive regions of the homogeneous time that clocks measure (although clock-time is derivative from the originary time of unconcealment, as Heidegger attempts to show in the book's difficult final chapters).