Existence precedes essence: humans are not born with a predetermined nature to fulfill

Sartre's foundational claim reverses the traditional assumption that things have a fixed essence — a purpose or nature — that precedes and determines their existence, the way a manufactured tool is designed for a function before it's built. For manufactured objects this makes sense: a knife's purpose precedes its creation. Sartre argues humans are different.

A person simply exists first, without any predetermined nature dictating what they must become, and only through the accumulation of choices and actions does anything like a 'self' or 'character' retroactively take shape. There is no blueprint, human nature, or destiny handed down in advance that a person is obligated to fulfill or discovers within themselves.

This single reversal underlies the whole architecture of the book: if there's no essence preceding existence, then all the traditional appeals to 'human nature' as an excuse or explanation for behavior collapse, and responsibility for what a person becomes falls entirely on the choices they actually make. Nothing about being human comes with instructions — you become who you are only by the choosing itself.