Martin Heidegger's Being and Time (1927) does not begin with ethics or psychology but with ontology, the question of the meaning of Being. Heidegger argues that Western philosophy has forgotten the question of Being since the pre-Socratics; it has busied itself with beings (things that exist) while ignoring the more fundamental question of what it means for anything to be at all. Heidegger's method is to investigate Being through the one being for whom Being is an issue: Dasein, the being whose mode of being is existence, the human being.
Heidegger describes everyday Dasein as thrown (Geworfenheit): you find yourself already in a world you did not choose, a historical period, a language, a social situation, a body. You are always already underway. But Dasein is not merely passive; it is also projection (Entwurf): Dasein is always oriented toward possibilities, understanding itself in terms of what it is moving toward. The structure of human existence is therefore temporal: rooted in a past you did not choose, moving through a present that is always structured by future possibilities.
In everyday life, Heidegger argues, Dasein falls into what he calls das Man, the anonymous "they." You do what "one does," think what "one thinks," value what "they" value. This is not a failure of character; it is the default mode of existence, necessary, unavoidable, and often comfortable. But it is also a way of fleeing from the specific weight of your own existence, from the fact that you are this particular Dasein with this thrown situation and these specific possibilities to take up or abandon.
Authenticity (Eigentlichkeit, literally "ownedness") is Heidegger's name for the mode of existence in which Dasein takes ownership of its thrown possibilities rather than hiding in das Man. This is not about being unusual or nonconformist, you can dress like everyone else and still be authentic. It is about the relationship between you and your existence: whether you are genuinely inhabiting your choices or merely going along with what "one does."
The pathway to authenticity runs through Being-toward-death (Sein-zum-Tode). Death, for Heidegger, is the one possibility that is always mine, non-relational (no one can die my death for me), certain, and indefinitely imminent. Most of the time, Dasein flees from this awareness, death is acknowledged in the abstract ("we all die someday") while being evaded in the concrete ("but not me, not now"). Authentic existence requires holding death before oneself, not morbidly, but as the horizon that individualizes: when you genuinely confront your finitude, the anonymous claims of das Man lose their grip, and your own most possibilities come into focus.
Heidegger's concept of anxiety (Angst) is the mood that discloses this structure. Unlike fear, which always has a specific object, anxiety has no object: it is the vague but pervasive sense that existence itself is groundless, that the world of everyday significance has no ultimate foundation. In anxiety, das Man's reassurances fall silent. What remains is the naked fact of existence, uncanny, groundless, and one's own.