John Rawls died 15 years ago this month, on the 24th of November. According to the moral philosopher TM Scanlon, “John Rawls was widely recognised as the greatest political philosopher of the 20th century. His work revived and reshaped the entire field, and its profound influence on the way justice is understood and argued about will last long into the future.”

His masterpiece is A Theory of Justice, which appeared in 1971. Its opening line says a lot about its content. “Justice is the first virtue of social institutions, as truth is of systems of thought.” It might strike you as a little too lofty, but he brings his subject matter right down to earth in the next sentence. “A theory however elegant and economical must be rejected or revised if it is untrue; likewise laws and institutions no matter how efficient and well-arranged must be reformed or abolished if they are unjust.”

The book was written during the Vietnam War, at a time when America was struggling to come to grips with all sorts of things, not least the idea of civil rights. Political philosophy in previous decades focussed on history and conceptual analysis, not the ragged edges of real world problems, and probably the time was ripe for a big change. Rawls promised a new way to think about justice and fairness, about how our social and political world is organised and whether or not it’s morally right. It sold hundreds of thousands of copies and was translated into more than 25 languages. The ideas in it continue to strike a chord.

Rawls argues for a theory of justice as fairness, and at the heart of his thinking is a remarkable thought experiment. It’s been recast in all sorts of ways, but a version of it goes like this. Imagine people who are asked to choose the principles of a just society. To make certain that their conclusions are fair for everyone, and to ensure that their deliberations are unbiased and impartial—there’s no horse-trading, no bargaining based on differences in wealth or power—Rawls imagines them operating behind “the veil of ignorance.” In short, they don’t know what their place will be in the society they’re setting up. They don’t know their eventual social position, race, gender, natural talents or limitations, inheritance, job, views about religion or what sort of life they might want to pursue. Presented with various principles, and not knowing where they’ll end up or quite what they’ll want, what would they choose?

Rawls argues that those in “the original position” as he called it would focus on securing basic rights and liberties, and they’d want to do something about inequality. They would settle on two principles.

“Each person has an equal right to a fully adequate scheme of equal basic rights and liberties, which scheme is compatible with a similar scheme for all.” “Social and economic inequalities are to satisfy two conditions: first, they must be attached to offices and positions open to all under conditions of fair equality of opportunity; and second, they must be to the greatest benefit of the least advantaged members of society.”

To bring Rawls’s conclusions home, some put his thought experiment slightly differently. Imagine you’re not yet born, but you’re somehow contemplating your prospects, how and where you might end up. If you were to look down at your country, as it is right now, would the thought of landing randomly in a life somewhere in there fill you with comfort or dread?

Is child poverty on the rise? Are homeless people bedding down on the streets while investment properties lie empty? Is there social mobility or are people just stuck? Do those from all walks of life have equal access to the best education? Are women paid less for the same work as men? Is wealth concentrated in the hands of the few? Is healthcare a lottery, depending on postcodes?

You might end up in anywhere down there. Would you cast a longing glance at a fairer society, say Norway, as you hurtled towards your fate? You might, but you might also now have a better idea about justice, and what ought to change in the real world that we’re in.