So far Sartre's account has focused on the solitary consciousness confronting its freedom. But human existence is irreducibly social, and Sartre's treatment of the Other in Being and Nothingness is among the most penetrating, and most disturbing, analyses of intersubjectivity in modern philosophy.
The Other enters Sartre's account through the Look (le regard). When someone looks at you, truly looks at you, as a subject fixing you in their gaze, something ontologically significant happens. You suddenly become an object in the world of another consciousness. Your free subjectivity is caught, pinned, reduced to what the Other sees. You become, momentarily, a thing, an in-itself, for the Other's for-itself.
The phenomenology of shame reveals this structure. When you are caught doing something embarrassing, say, peeking through a keyhole, you feel shame not because you judge yourself, but because you suddenly experience yourself as the Other sees you: vulgar, caught, objectified. Shame is the experience of being made an in-itself by another consciousness. It is the bruising evidence that there is a world in which you are an object, not just a free subject.
This produces what Sartre calls the fundamental structure of all human relationships: an unstable oscillation between two kinds of bad faith. In love, I want the Other to freely choose me as their absolute value, but the moment they do, they are no longer fully free, and so their love feels coerced rather than genuine. In sadism or domination, I try to reduce the Other to a pure object under my control, but a body I have dominated is not really the free consciousness I wanted to master, so domination never fully satisfies. We oscillate between wanting to possess the Other's freedom (love) and wanting to annihilate it (domination), and neither strategy works, because a free consciousness cannot ultimately be either fully captured or fully destroyed.
Simone de Beauvoir's The Second Sex (1949) extends this analysis directly. Women, she argues, have been socially constructed as the Other, as that which defines masculinity by contrast, the sex that is always looked-at, never the looker. Woman has been denied the subject position and assigned the object position structurally, not just personally. Her famous opening line, "One is not born, but rather becomes, a woman", is pure Sartrean existentialism applied to gender: femininity is not a given essence but a situation that is taken up, performed, and can be transformed.
The political implication is significant: bad faith is not only a personal failing, it is a social structure. Systems of oppression work by convincing people that their situation is their nature, that their constraints are their essence. The existentialist analysis of bad faith thus connects to liberation politics: awakening to freedom, while terrifying, is also the precondition for any genuine resistance.