Imagine having to design the rules of a society without knowing what place we would occupy in it. We do not know whether we will be born rich or poor, whether we will be healthy or disabled, intelligent, vulnerable or dependent on others. Nor do we know whether we will have opportunities or need help to live with dignity. Under these conditions, we would probably accept a society with guaranteed basic rights, genuine opportunities and mechanisms to protect those in disadvantaged situations. Not because we know that one day we will need that protection, but because we cannot rule out that possibility.

However, the rules we defend often depend on the position we occupy. We demand freedom when we have choices, security when we are afraid and solidarity when we need help. We often consider whatever benefits us to be fair and question whatever benefits others. Imagining that we could occupy any social position forces us to examine our principles without the comfort of knowing in advance whom they will favour. Perhaps a just society is not one in which everyone receives the same, but one whose rules we would accept even if we ended up in the least privileged position.