Now, since Da-sein is the being that must be interrogated in order to ask after the meaning of being, an analysis of Da-sein must be completed. It is the analysis of Da-sein that is meant to yield a horizon to investigate the meaning of being. This means going over several concepts. Da-sein and its possibilities, the world / referential totality, the average everyday “who” of Da-sein, and an introduction to care.
So Da-sein is not objectively present; it does not have a way of being in the way a “thing” does. Analyzing Da-sein’s existential structure is thus not a question of psychology, anthropology, biological, neurology, etc. These disciplines all taken humans as an objectively present object to be studied. Heidegger isn’t dismissing their activities; he merely wants to do what Kant might have called an “ontological analytic of the subjectivity of the subject.” I mention Kant because Heidegger’s project has a similar a-priori interest.
If you want to ask about what Da-sein is, the answer is that Da-sein is its possibilities. That is, Da-sein’s average understanding of itself is in terms of what it is not - the world. It takes up projects and roles and understands itself in terms of them. Heidegger characterizes the fundamental constitution of Da-sein as being-in-the-world.
That’s a loaded sentence. Being-in-the-world sounds as if Heidegger is saying Da-sein is inside a world - like we are inside this room. This obvious interpretation needs to be dispelled immediately. That kind of spatial idea - of things next to one another - is a category. Relationships in three dimensional space is a matter for objectively present entities.
Rather, being-in-the-world (BitW) is an existential. It’s not about the location of your body; it’s not something we have and could do with out. BitW is described as something like “dwelling” or a “familiarity” with the world. The world here is an ontological concept; just like “being-in” is not about being in this room, and just like Da-sein is not human, Heidegger’s “world” is not the Earth.
Rather, BitW is about being absorbed in the world; Heidegger will eventually refer to this as fallenness. The relationship between Da-sein and the world is not a subject/object relationship; Da-sein is not a subject that somehow needs to get out to the world. This is Heidegger’s major rebuke to the Cartesian view of things; a dualism between mind and extension has no place in B&T. We are absorbed in the projects and possibilities that we find in the world. We encounter useful things that exist as part of the referential totality of the world. The upshot of this is that our closest association with the world is not a perceptual cognition, but a taking care.