In order to discover ethical rules that would promote freedom for everybody, Rawls creates the fiction of an ethical discussion held before we enter the world. Everyone would be required to agree on binding rules of behavior before they had any idea of which place in society they would occupy. It’s his way of defining an ethical system that won’t leave anyone out. Rules could not be tailor-made to serve selfish ends because it’s hard to be self-serving when you don’t know which "self" you’ll be