Design just institutions as if you didn't know your place in society.
Rawls's 1971 device. Its critics note the choice depends on assumed risk aversion: behind the veil, a gambler and a cautious person design different institutions, and Rawls assumed the second.
In practice
- A child asked to cut the cake before knowing which slice they get cuts it evenly, and needed no theory of justice to do it.
- A committee designing a policy without knowing which department will bear the cost produces a different answer from one that knows.