You must choose the rules for a new society, but you do not know who you will be in it.
You might be wealthy or poor, healthy or ill, part of a majority or a minority. You cannot tailor the rules to your own future place, because that place is hidden from you. What protections would you want everyone to have before you learn where you will land?
Philosopher John Rawls called this imagined starting point the original position, with a veil of ignorance. The people choosing are not blank slates: they know general facts about society, but not their own status, talents, or personal beliefs. The device aims to make the choice fairer by blocking self-serving bargains. It does not mechanically produce one obvious constitution; what principles it supports, and why, remain matters of debate.