The difference between man and other entities is the former’s sense of temporality. In other words, man’s own self-awareness of himself as existing — or, as Heidegger puts it: Dasein is the being for whom Being is an issue for it. Other entities, be it a dog, raccoon, elephant or a tea-pot, are incapable of projecting their being into the future; or recalling their own past. Lower species live exclusively in the present; for the very moment in which being finds itself. Only man can experience himself in time, and thus never truly static; but rather in a constant state of becoming himself through his finite possibility.
But possibility is understood in contrast to actuality. The traditional positing of the Self-as-substance (i.e., Descartes, Kant, substance ontology generally, etc.) places actuality over possibility; an emphasis of the present-now over the to be/becoming. Substance ontology, in turn, reduces the individual into little more than universalized substance — an entity (albeit, a thinking entity), composed of the same indistinguishable “stuff” (spirit; mind; soul; will to power; etc.). It was this aspect of traditional metaphysics that Soren Kierkegaard, and later Martin Heidegger, went to great lengths to overcome.
Above all, focusing on the present-substance completely misses what it means to be-in-the-world. This analysis simply ignores, or is incapable of taking account of the very fact that we are always in the world relating to it. Heidegger chooses to focus on our “average everydayness” because it is our situational mode of being in the world. In other words, the traditional metaphysical (substance-ontological) view reduces existence to presence; here-now. In the end, it attempts to ground essence without taking into account our existence is to completely disregarded the finiteness of being.
Thus, the temporal self is, in its existence, its own being-towards-possibility: actively involved and always ahead of itself for which its own being matters. We are constantly relating ourselves to the world through ourselves (the self-referential self). Thus, our existence, the relation in which our being is an issue for us. As self-relating entities, we are always encountering the future of possibilities — and if our existence is authentic — then it is our ownmost possibilities.
Properly understood, the Self is neither substance nor statically present — but rather, exists through itself as the positive relation to the world as a projection of constant change always already emerging into possibility [not actuality]. Thus, actuality is becoming possibility — always in a state of constant projection in which each individual Self is always a step in front of itself. It is in this process that each Self comes to understand its own Being: the Being of the Self is the disclosure of the “ability-to-be.” The ability-to-be one’s Self is thus the primordial task of any individual Self insofar as he is said to exists. Likewise, any “what” it means to be able to be must always already be preceded by the “who” that is answering it. To put it more succinctly, there can be no “what” I am without there first and foremost being a “who” I am.
But this is far from the end — for every Self, in its relation to itself through its possibilities, is always already undercut by an “ultimate” possibility — death. Death, understood not as mere cessation of biological life (what Heidegger called mere ‘demise’), but rather as world collapse. My death is my possibility of no more possibilities; my possibility of not being able-to-be. Coming to terms with one’s death as truly “my death” becomes possible when we experience certain existential orientations — fundamental moods that concern the “entirety of a person’s situated existence.” These moods highlight a breakdown of average everyday “being-in-the-world,” and include anxiety/angst, boredom, and despair. Whenever the Self is delivered over to one of these moods, the Self is confronted with the terrifying possibilty that all of its possibilties are subject to a vast and omnipresent vulnerability to total destruction and collapse. This is the revelation of the nothingness of the Self; the abyss of meaninglessness that every Self finds itself situated in while being-in-the-world.