What does it mean when Heidegger makes distinctions between categories and existentialia and between being-present and being-in in Being and Time? The distinctions are based on whether or not an entity is Dasein as Dasein's being genuinely differs from other entities. Dasein's being "is essentially concern". This difference does not isolate Dasein from the world, however, so Heidegger's distinction is not a restatement of the traditional subject/object dichotomy.
Heidegger uses "existentialia" and "categories" to classify characteristics of being, but each of the two terms is reserved for a particular type of being. They refer to being which is Dasein or not Dasein, respectively. For example, the characteristics of an entity which make it a rocking chair are subsumed by the categories, whereas the characteristics which make up a philosopher are existentialia. To make this example more meaningful, the distinction which makes the latter entity a Dasein but not the former should be made clear.
The difference between the two sorts of entities is only one of being, but the difference is not arbitrary. Dasein is "...distinguished by the fact that, in its very Being, that Being is an issue for it". That is, Dasein, and only Dasein, concerns itself with being per se. This possible way of being is what Dasein essentially is, and is termed "Being-in" by Heidegger. "Being-in" is potentially misleading, as the expression strongly connotes a spatial (i.e., physical) relationship. One might suppose, for instance, that the term could be applied to a rocking chair; this entity is in the world, if nothing else. That is not what is meant, however.
All entities whose Being "in" one another can thus be
have the same kind of Being - that of Being-present-at-hand...
"Being-in" and "Being-present" are two possibilities of being; one is not merely a special case of the other, though, just as Dasein is not a "...special case...of entities as things that are present-at-hand". At the risk of being redundant: Dasein is unique because it holds an attitude toward the world. Dasein's essence, that thing which makes Dasein what it is and not some other entity, is care. So the term "Being-in" expresses a possible way of being wherein an entity, Dasein, relates to and concerns itself with itself and entities other than itself (i.e., the world). "Being-present", on the other hand, is a way of being which lacks this dimension. A rocking chair simply is; it does not concern itself with other entities in the world and their being, much less with itself. Indeed, Heidegger maintains that such an entity is worldless.
All of this seems to be a rehash of the ideas of subject and object, but Heidegger denies this. For him, these ideas obscure the relationship between Dasein and the world as they imply that "knowing" is some inexplicable process whereby a subject, which exists in its own "sphere", transcends the boundary between itself and the object, encounters the object, and then returns home, so to speak. By placing his emphasis on Dasein's essential being, that of being-in the world, Heidegger hopes to avoid this problem. Dasein does not have to go anywhere. It is not isolated from other entities- it always was and will be amongst them. What sets Dasein "apart" is its attitude. To reiterate, the key word is 'care'. In a collection of entities, it is this concern Dasein has which distinguishes it from the others. This solution to the problem of subject and object and how the former may know the latter is not vacuous. What Heidegger tries to say is that what are often termed subject and object do not differ in kind the way, say, body and spirit do, but rather in orientation. It is the supposition that subject and object differ in kind that gives rise to the epistemological problem. For Heidegger the differentiation of being is between a 'what' and a 'who'.