Existentialism has a reputation for gloom that its best writers would have found funny. It is not a philosophy of despair. It is a philosophy of what follows once you stop expecting the universe to hand you a purpose.
It was never a school that founded itself. Most of the people filed under the label either rejected it or never heard it. Its ancestors are a Danish theologian writing in the 1840s and a Russian novelist writing about men who reason themselves into ruin. It became a movement in Paris in the 1940s, in a city under occupation and then reckoning with what people had done during it, which is part of why freedom and complicity sit at its centre.
Existence before essence
The core claim is short. A paper knife has an essence before it exists: someone intended a blade for cutting, then made one. A person does not. You arrive first and the definition comes later, assembled out of what you actually do. There is no human nature waiting to be discovered and lived up to. There is a life being made while you live it.
Life must be lived forwards, but it can only be understood backwards.
Freedom as a weight
If nothing was decided in advance then the choices are yours, and so is the responsibility. The existentialists were unusually honest about how unwelcome that is. They named the dizziness it produces, and they named what we do about it: bad faith, the small daily manoeuvre of pretending there was no choice. The waiter playing at being a waiter, the person who says they are simply made this way. The point is not that they are lying to anyone else. They are managing themselves.
The absurd
Camus reached the same terrain by another road. The absurd, for him, is not that the world is meaningless; it is the collision between a person who wants meaning and a universe that does not answer. His conclusion was neither suicide nor resignation but revolt: living fully inside the gap, without appeal. That is a warmer philosophy than its reputation suggests, and it was written by someone who had watched an occupation from close range.
Never only French, and never only men
Simone de Beauvoir gave the movement its most rigorous ethics and turned its method on a question the men had not asked: one is not born a woman but becomes one. Freedom is always situated, and some situations are built to constrain it. Frantz Fanon extended the analysis to colonised existence, where another's gaze decides what you are before you speak. Hannah Arendt asked what politics means for beings who are plural and mortal. Merleau-Ponty insisted that the freedom in question belongs to a body, not a mind hovering above one. Heidegger, whose vocabulary much of this borrows, joined the Nazi party in 1933 and never adequately accounted for it, a fact the tradition carries rather than resolves.
The atlas follows this line from Kierkegaard's Copenhagen to the Paris of Sartre and Beauvoir, and outward to Fanon and the thinkers who took the argument somewhere it had not been. The map earns its keep here: existentialism is the rare philosophy where where someone stood changes what they were able to see.