For Heidegger, Dasein is an entity with a special mode of being. A human being is this entity and it is human beings in general who enjoy this singular kind of being, which can be referred to as Existenz, or existence. It is a general motif for Heidegger that human existence is existential in contrast to existentiell. The latter connotes a third person perspective of objectivity (ontic) whereas the former is in terms of a first person perspective of lived experience (ontological).
Furthermore, Existenz is constituted by several unique phenomenological features. Perhaps the most crucial of these features is being-in, which designates the way in which a human entity is always in-a-world. A world for Heidegger is a totality of referential significance wherein the entities encountered have a special salience in terms of our bodily comportment and self-understanding. Rocks do not “have” a world although they can be said to crudely “exist” in the world ontically. Moreover, in every case, Dasein is “mine” in the sense that humans – tautologically – are always individuals and thus cannot exist as two individuals simultaneously. If a human is defined as an entity, then an individual Dasein cannot be two separate entities (two individuals) at once. I think this structure of “mineness” is based on common sense notions of individuality and does not constitute any special connotation of egotistical existence. It simply states that Dasein is an individual human entity capable of being distinguished from other human individuals.
Moreover, a rock’s existence is qualitatively different than the existence or Existenz of humanity. A rock exists in the world merely in terms of a spatial or ontic arrangement. It is “next to” or alongside everything else in the world but it does not dwell in the world in the same way humans do. The rock does not have a “home” in the world nor is it familiar with the world it exists in. By contrast, in virtue of the referential structure of significance, humans can be said to live or dwell within a wordly world that they are for the most part familiar with. To use a classic example, humans do not experience a hammer as if it were an objective conglomeration of materiality but rather, we see it as something-for-hammering. It is familiar to us in terms of a referential web of usability.
However, our relation to these phenomenal worlds are not just in terms of a tacit understanding of how to use equipment; there is a component of explicit understanding. Heidegger refers to this crucial feature of Dasein as its understanding of being. We understand entities in terms of their “what” and their “that”. That is, we understand the world in terms of entities as entities e.g. we explicitly understand that, conceptually, hammers exist and we tacitly understand what it is: something-for-hammering called a “hammer”. This understanding is done tacitly on the level of pragmatic usability but it is also done explicitly in terms of our capacity for conceptual language. A useful model for this tacit-explicit dynamic is the postulated dorsal and ventral neural streams in visual processing. Roughly speaking, the dorsal stream is for tacit motor reactivity and the ventral stream is for more explicit object-recognition The understanding of being is also evident in our basic grammar for we utilize the structure of “is” in order to make sense of the world. “Today is Saturday.” “The cat is on the mat.” Thus, a central feature of Dasein’s uniqueness is our special mode of grammatical language wherein the world becomes perceptually structured in terms of thatness and whatness i.e. in terms of an understanding of being, understanding entities as entities.
Furthermore, Dasein can be distinguished as an entity with a special mode of being in terms of the existential structure of its being. The structure is generally referred to by Heidegger as “care”. Care-structure can be defined in terms of a human being-ahead-of-itself and being-already-in-a-world. Being-ahead simply means that humans routinely think and act in terms of future-oriented projects. We live for future possibilities and as Heidegger sometimes says, we are our possibilities. This capacity to live “in the future” constitutes one of the most basic existential structures of human existence and is exemplified by phenomena such as marriage and parenting, wherein our everyday behavior is structured by future possibilities. Moreover, we also live through our past as when Heidegger says “the other ‘end’ is the ‘beginning’, ‘birth. Only the being ‘between’ birth and death presents the whole we are looking for.”
Being-already-in-a-world simply means that humans always find themselves in a world of signification. Worldhood is not subjectively bestowed upon the world by our consciousness, but rather, is an a priori determination of phenomenal reality by virtue of the referential web of significance given through the milieu of public language and practice.
Lastly, we can distinguish Heidegger’s conception of humanity from the classical tradition of substantive metaphysics. This tradition has always sought to define humanity in terms of an ever present ground e.g. the One, God, self-knowledge, transcendental subjectivity, etc. Heidegger’s approach is in stark contrast to these metaphysical approaches in the sense that he does not isolate a continually present substance that grounds the human self but rather, locates the constitutive features of humanity in terms of their thrown involvement in a public world with a dynamic body. There are no founding “onto-theological” structures to Dasein’s existence because human being is always shifting dynamically in accordance with the complex, changing world we occupy. Because Dasein is a thrown possibility, and not a given presence, the “essence” of Dasein lies in its existence i.e. being-in-the-world, Existenz.