Man is fully responsible for his nature and his choices.

The sentence condenses a central Sartrean claim: there is no fixed human essence that excuses us from what we do. Because we are not born with a predetermined “nature” that dictates our actions, we continually make ourselves through choices, projects, and commitments. This freedom is not celebratory but burdensome: it entails “responsibility” not only for particular acts but for the kind of person one becomes and, in Sartre’s ethical-political extension, for the image of humanity one endorses by one’s choices. The line also rejects appeals to fate, temperament, or social role as ultimate alibis, insisting that even under constraint we remain answerable for how we respond.