In A Theory of Justice (1971), John Rawls proposed the Veil of Ignorance as a method for arriving at principles of justice. Imagine you are designing the rules for a new society, but you do not know what position you will occupy in it. You do not know your race, sex, wealth, intelligence, health, or talents. Behind this veil, what rules would you choose?
Rawls argues that rational people behind the veil would choose two principles: first, equal basic liberties for all; and second, that any social or economic inequalities must benefit the least advantaged members of society.
The Veil of Ignorance matters for freethought because it provides a secular foundation for justice — one that does not depend on divine command or natural law, but on the rational self-interest of people who must live under the rules they create.