Life does not start when you are born. When you are reborn, your life begins. It is one of the most fundamental and important ideas in existentialist philosophy. The idea that life is just more than what you do. Sartre stated that it is what you do with what is done to you; even when you are born, something is done to you. You did not choose your birth. You must be reborn. You must create yourself. You must make the decision to be a person who struggles to be a person. It's also why Sartre claims that life begins on the other side of despair; for Sartre, existentialist despair is the realisation that you have a responsibility, and that responsibility is referred to as being alive.It is not of your choosing, but it must be of your own creation. That Sartre existentialist theory of life becomes clear once you realise you can live with purpose.