Heidegger begins the book with a discussion of exactly why this question of being has been covered over throughout the tradition. Part of the answer is that we always already understand being, as I’ve said. There are, however, prejudices that lie in the way of asking this question; being is seen as either being a universal concept, an indefinable concept, or the most self-evidence concept. Heidegger’s retort is that none of these prejudices actually take being as a problematic; they cover it over with the covert judgements of common sense.
Because we are concerned here with questioning, we must start with the structure of the question itself. You don’t ask questions about subjects of which you are 100% ignorant, and you don’t ask random sources. You look for a likely place to enquire. So there are 3 elements here. You need an idea of what you are asking after - being. An idea of what you are actually asking - the meaning of being. And who you are asking. The very structure of questioning answers the “who” – only one being actually asks anything.
The who is Da-sein. Literally in the German, there-being. Being there. Da-sein is the human way of being; thus it is not the same as human. Da-sein is not homo-sapien. Keep in mind the ontological difference here; ontically, homo sapien; ontologically, Da-sein. It’s very easy to slip into equating Da-sein and homo sapien, but to do so is to miss the whole point of the ontological difference; calling Da-sein human or vice versa is to equate an entity with its being.
Why is Da-sein the privileged entity? Because it already has something like an understanding of being. Da-sein has a pre-ontological understanding of being — this is strictly separate from knowledge. It’s not cognitive, either. Understanding is a way of being; this isn’t a question of epistemology. We understand the “is” without being able to conceptually define it. This is what makes Da-sein ontically different from other beings - we are concerned in our being about being. Da-sein related to existence understandingly; the structures by which we do this are existential structures. You might hear echos of Kant’s a-priori transcendental structures here.
Da-sein understands itself in terms of existence, as well. It understands itself through possibilities that come to it through various means; these are existentiell possibilities. These are the possibilities we understand ourselves through; it is an existentiell possibility to be a student.
Rephrased, existentiells are our projects. The stuff we do. They are ontic characteristics specific to Da-sein. Existentials, on the other hand, are what you might think of as our ontological side; the way(s) in which we exist understandingly. One example is mood, attunement.
So here we have Da-sein as the privileged entity that we can question. The problem is that Da-sein is entangled in a particular tradition - a philosophical tradition that covers over the ontological difference and the question of being. What Heidegger wants is a “productive appropriate” of the tradition - a destructuring. It’s worth emphasizing that Heidegger’s intended destructuring is a positive move; it does not “bury the past in nullity.” Heidegger is the hero of the tradition; he’s saving everyone from themselves. This discussion of destructuring and the tradition’s concern over presence appears on pages 20 to 22, for the Derrida fans here.
I guess we all have a passing familiarity with the whole metaphysics of presence thing. Well this is where it appears. The tradition - beginning with Parmenides - takes the objective presence of beings as the guide for interpreting being. This is a rephrasing of what has already been said about onto-theology; the tradition takes a being, a being that is present - in both a spacial and temporal way - and uses this to discuss being as such. This is a “making present.” This table, here, now, is our guide to being. Really.
This presence is to Heidegger only one way of being. This German phrase Vorhandenheit is variously translated as present to hand or objective presence. Another way of being is Zuhandenheit - translated as ready to hand, or handiness. This refers to a particular usefulness of an entity, within a certain kind of context, which Heidegger calls the referential totality of the world. I’ll elaborate on both of this later, but for now I’ll just say that both objective presence and handiness are ways of being for entities that are unlike Da-sein. Da-sein cannot be made present and cannot become handy. This will all be elaborated later on; I just want the terms introduced here.
All right, I’ll pause here to summarize the terms and claims that will continue to be important.
The ontological difference is that difference between being as such and entities. Being is the background upon which something appears as the thing that it is. Entities are basically things - anything you care to name is an entity. All entities have being, but the being of an entity is not itself an entity. This difference has been ignored by the tradition since Plato; entities (usually one entity in particular) are always privileged over being. That entity is judged in terms of its objective presence; all that is real is seen as fully present. Heidegger wants to ask after the meaning of being; the entity that must be asked is the only entity that is concerned about being in its being - Da-sein. Da-sein relates to the world understandingly through ontological existential structures; it relates to itself through ontic existentiell projects.