Sartre believes that there is no human nature in the sense of a defining set of characteristics that are necessary for something to qualify as human (essence).
Humans define themselves through their actions. Humans are what they make of themselves. What distinguishes a human from a mere object, is that the human can plan, consciously, who or what he/she wants to be and then work towards that. This makes each human responsible for who she/he is.
Yet each individual human is also responsible for all humans. This is because each choice an individual makes for him/herself, creates a vision of who he/she ought to be. This makes the claim that what one is choosing is more valuable than other consciously rejected alternatives. This creates a priority of value for all, not just the chooser. Sartre believes this because he believes that anything that is better for the individual is perceived as better for all. He says, “In fashioning myself I fashion man.” (P.489.)
In The Second Sex, de Beauvoir uses an existentialist foundation for her argument for woman’s liberation. Current views of the nature of women are inauthentic because they are based on socio-cultural beliefs and inequities of power, instead of freedom. Acceptance of the current circumstances leads to Bad Faith in those individuals that deny their freedom and believe the cultural stereotypes. Even those who act to change current conditions in order to promote freedom suffer from anguish because they still feel guilty about not living up to other people’s ideals. Change requires all humans to recognize the freedom and equality of all men and women as progress and not incompatible with good sexual relations.