Jean-Paul Sartre's Being and Nothingness (1943) takes Heidegger's ontological analysis and pushes it toward an ethics of radical freedom that was both exhilarating and terrifying to its postwar readers. For Sartre, human consciousness, the for-itself (pour-soi), is defined by its difference from things. A stone is what it is: in-itself (en-soi), fully determined, coinciding with itself. Consciousness is defined by negation: it is always what it is not, always a step ahead of itself, never simply identical with any fixed nature.
This ontological structure means freedom is not a property humans happen to have, it is the very structure of consciousness. You are condemned to be free. You cannot not choose. Even choosing not to choose is a choice. Even fully conforming to social expectations is a choice, it is just a choice to pretend you have no choice.
Bad faith (mauvaise foi) is the attempt to flee this freedom by pretending to have a fixed, determined nature, treating yourself as an in-itself, a thing with an essence that precedes and determines your choices.
The waiter who is too waiter-like, whose movements are too precise, too quick and too regular, whose face expresses perhaps a trifle too solicitously, a trifle too eagerly, is playing at being a waiter. [...] He is playing, he is amusing himself. But what is he playing? We need not watch long before we can explain it: he is playing at being a waiter in a café.
— Jean-Paul Sartre, Being and Nothingness (1943), trans. Hazel Barnes
Sartre's waiter is a perfect illustration. The waiter who embodies the role so completely that he seems to have no inner life beyond it, who moves with mechanical precision, adopts the proper expression, never breaks character, is performing bad faith. He is trying to be-waiter the way a stone is a stone: totally, completely, without remainder. But a stone does not choose to be a stone. The waiter chooses, every morning, to show up and play the role. That choice, and the freedom it expresses, is what bad faith hides from.
Bad faith can also work in the opposite direction: the spirit of seriousness pretends that values are built into the world independently of human choice, that something is really, objectively important, so you don't have to take responsibility for having chosen to care about it. Both forms share the same structure: evading the anguish of having to own your choices.
Sartre's ethics follows directly: authenticity requires acknowledging your freedom and taking full responsibility for your choices, without appeal to nature, God, social role, or psychological determinism as excuses. "I couldn't help it" is almost always bad faith. The only genuine response to being condemned to be free is to embrace it, to choose fully, acknowledge that you are choosing, and take responsibility for what you become.
Simone de Beauvoir, Sartre's partner and a major existentialist thinker in her own right, challenged the individualism of this account. In The Ethics of Ambiguity (1947) she argued that freedom cannot be exercised in isolation: your freedom is conditioned by the freedom of others. You cannot be genuinely free in a social world that denies freedom to others. Authentic existence is therefore necessarily political, it requires working toward the conditions that make freedom possible for everyone.
Albert Camus, meanwhile, rejected the existentialist label altogether. His The Myth of Sisyphus (1942) addresses the same raw material, groundlessness, contingency, the absence of inherent meaning, but refuses what he calls the existentialists' "philosophical suicide": the leap to God (Kierkegaard), Being (Heidegger), or radical commitment (Sartre) as a way of resolving the tension. For Camus, the tension must not be resolved. The absurd, the confrontation between the human demand for meaning and the universe's "unreasonable silence", must be lived with clear eyes, in full revolt, without false hope.
The struggle itself towards the heights is enough to fill a man's heart. One must imagine Sisyphus happy.
— Albert Camus, The Myth of Sisyphus (1942), trans. Justin O'Brien