The existentialist Jean-Paul Sartre thought that human beings live in anguish. Not because life is terrible, but rather because we're condemned to be free. We're thrown into existence, become aware of ourselves, and have to make choices. Even deciding not to choose. is a choice.
According to Sartre, every choice reveals what we think a human being should be. A penknife has an essence. There is something that makes it what it is.
It has to have a folding blade, or it wouldn't be a penknife. Its essence pre-exists the actual penknife that someone makes from the design. But according to Sartre, there is no design for a human being. No way we have to be, no God to create a purpose for us, no human nature that fixes how we should live.
In his 1945 public lecture, Existentialism is a Humanism, he declared that for human beings, existence precedes essence. That is the fundamental tenet of existentialism. I create myself through what I do. The choices I make in a world without fixed values.
This makes me, me. I am what I do. What heightens the anguish, though, is that every sincere decision I take presents a picture of what I believe any human being should be like.
In fashioning myself, I fashion humanity. I have to act as if everyone is watching me, and there is no escape.