Someone stays in a career because it is what their training pointed at. They did not choose it so much as arrive at it, and the reasons they would give are all true: the qualification, the years, the salary, what people expect.

The existentialist claim is that the training explains the position and does not excuse it, because staying is a choice being made again every morning. The fact that it does not feel like a choice is the thing to be explained, not the defense.

That gap — between a life you are living and a life you could give an account of as yours — is what the tradition means by authenticity.

Why the default is the other thing

The claim that makes the concept serious is that inauthenticity is ordinary, not a failure.

Most people, most of the time, do what one does. The possibilities available are the ones the surrounding culture has already arranged; the opinions are the ones that are held; the shape of a life is the shape lives have. None of this requires bad character or even inattention. It is the path of least resistance and it is comfortable, and the discomfort it avoids is real.

So authenticity cannot be a state you reach and then possess. It is a way of taking up an existence that has to be renewed, because the pull back toward the default is constant and does not weaken with practice. Anybody who describes themselves as having become authentic has said something the tradition does not permit.

What it is not

The word has been so thoroughly absorbed into ordinary talk that the most useful thing a definition can do is mark it off from the three things it is routinely confused with.

Not sincerity. Sincerity is saying what you actually feel. You can be entirely sincere about a self you have never examined, and a sincere person can be as thoroughly inauthentic as a dishonest one — more so, because sincerity feels like it settles the question.

Not self-expression. The popular version treats authenticity as removing obstacles to an inner self that is already there and waiting. The existentialists hold the opposite: there is no pre-existing self to express. Which is why acting on impulse is not automatically authentic — an impulse can be as unexamined as a script, and usually is.

Not individualism. The serious versions all insist that a life is lived among others and that authentic relations with other people are part of what is being described, not an obstacle to it. Beauvoir made this the center of her ethics, and it is what separates the tradition from the self-help literature that borrowed its vocabulary.

Two routes in

Kierkegaard gets there first, through the individual who must choose rather than be carried, and the choice is a leap because nothing outside it can justify it in advance.

Heidegger makes it systematic in Being and Time (1927), and his route runs through mortality: what makes a life yours rather than one anybody could be living is the recognition that it ends, and that nobody can do the ending for you. That argument is being-toward-death.

Sartre approaches from the failure rather than the achievement. He is far more precise about the ways people flee their freedom than about what living in it would look like, and the name for the flight is bad faith.

The objection worth taking

MacIntyre and the virtue-ethics tradition argue that authenticity is parasitic. Told to be true to yourself, you still need some account of what is worth being true to — and the content, when anybody supplies it, turns out to be borrowed from the older traditions existentialism claimed to have got past.

The defense is that the tradition is not trying to supply content. It is naming a relation to whatever content you have, and the relation is a real thing that a list of virtues does not by itself give you: you can hold entirely correct values in a way that is not yours.

Whether that is a sufficient answer is open. Charles Taylor's The Ethics of Authenticity (1991) is the best attempt at it, and the useful takeaway is his: the ideal is degraded into narcissism easily and constantly, and the degraded version does not refute the real one.