What rules would you pick for society if you didn't know who you'd be in it?

Rawls argued that you would choose fairness, because the veil of ignorance would take away your ability to rig the rules for yourself. The original position is a hypothetical position from which people choose principles for the basic structure of their society without knowing which position they would occupy in it. Behind the veil, no one knows their ethnicity, social status, gender or, crucially for Rawls, anyone's ideas of how to lead a good life.

If you do not know whether you will be rich or poor, strong or frail, you will not write rules that favour any one group. In A Theory of Justice (1971), Rawls expected two principles to emerge. First, each citizen is guaranteed a fully adequate scheme of basic liberties compatible with the same liberties for all. Second, the difference principle permits inequalities only if they benefit the worst-off members of society. His reason is that morally arbitrary factors, such as the family one is born into, should not determine one's life chances.

The choice behind the veil is not obvious, though. Rawls expected the parties to follow the maximin rule, looking at the outcome for the least advantaged position. But John Harsanyi and William Vickrey, who described the same thought experiment earlier, derived proofs that a rational observer would instead adopt a utilitarian framework, because people who weigh outcomes by expected utility would maximize average or total well-being.

So the same device is used on both sides. Behind the veil you might be cautious like Rawls or averaging like Harsanyi, and much of the debate is over which attitude to risk is rational.