Everyone knows they are going to die. Almost nobody lives as though it has any bearing on Tuesday.

Heidegger's claim in Being and Time (1927) is that the bearing is total, and that the not-noticing is the thing requiring explanation rather than the natural state of an adult who has other things on.

The move that makes it an argument

The everyday idea of death is of an event, somewhere ahead: I am alive now, at some point I will not be, and the two are different times.

Heidegger's first move is that this misses what death is for the one who dies. It is a third-person fact, the kind you could hold about anyone — mortality rates, actuarial tables, other people's funerals. As a first-person matter it is not an event at all, because there is no experience of being dead and the experience of dying is still living.

What there is instead is a structure. You exist ahead of yourself — always underway toward something, always taking up possibilities — and what you are ultimately underway toward is the end of having any. That is not a fact about your future. It is the shape of your present, and it is operating whether or not you attend to it.

The four features, and what each rules out

The argument rests on four properties, and they do more work than the usual summaries suggest.

Non-relational. No one can die your death for you. Almost everything else a person does is substitutable in principle — someone else could do the job, make the argument, raise the child — and this is not. It is the one possibility that isolates you completely, which is why it is the route to what is yours.

Uncircumventable. Every possibility you take up is bounded by the possibility of having none. You cannot plan around it or outflank it; it is not a risk to be managed.

Certain. Not probable. The only genuinely certain thing about a human life, and the certainty is of a strange kind, because it is not established by evidence and no new evidence could unsettle it.

Indefinite. Certain and unscheduled at once. This is the feature that does the real work: a known date would make death a deadline, and a deadline can be prepared for and then forgotten about until it approaches. Not knowing when means it bears on now, and on every now.

Why the avoidance is organized

Heidegger's most useful observation is that the flight from this is not individual cowardice but a social arrangement.

Death gets handled publicly as something that happens. People pass away. It is an occurrence, regrettable, statistically normal, and above all other people's. The whole apparatus is a way of acknowledging death constantly while keeping it at the distance of a fact about the world rather than a structure of mine.

He calls the anonymous subject of this arrangement das Man — the They, or the One, as in what one does. Nobody is das Man and everybody participates in it, which is precisely why it is so hard to get out from under. There is no individual to disagree with.

What changes if you take it up

Not the circumstances. This is the part most often got wrong, and Heidegger is explicit: authentic existence does not require different possibilities from the ones already in front of you. It is taking up the same ones as yours.

What changes is that they become finite, ordered and non-substitutable. A life of unlimited time has no structure, because nothing has to be given up for anything else and no choice excludes another. The recognition that there is an end is what makes a choice a choice, and it is why the analysis belongs to authenticity rather than to morbidity.

Sartre disagreed on exactly this. In Being and Nothingness (1943) he treats death as an external event — something that happens to me from outside, an absurd interruption rather than my ownmost possibility. On his account it cannot ground anything, because it is precisely the point at which my freedom stops rather than a structure my freedom has. That disagreement is the sharpest one inside existentialism, and it is a disagreement about whether finitude is mine or merely happens to me.