“Imagine you’ve been asked to create the principles and rules of a new society. Now imagine, when making the rules, that you don’t know your position, sex, intellectual capacity or disability in that society.”
Our primary social tendency is to ensure our survival. Any tool or power we come to possess, we use for the same purpose. Law making is one of the highest powers available to anyone. Left alone, the law makers will naturally be biased towards oneself. Veil of ignorance uses the same bias to counter it.