Imagine that the U.S.A. has decided to re-found itself, and you have been elected by a large geographical constituency as one of the founding fathers who must negotiate the principles for a new Constitution; further imagine that you are similarly elected to the Constitutional Convention to draft the new constitution under these principles, the Legislature which translates this constitution into law, and the Supreme Court which interprets this law in the light of experience.
You must do your duty by the people who have elected you and the generations to follow, but your electorate has no specific social character and your only mandate is to found a just constitution which will provide stable conditions for social cooperation and a well-ordered society.
How will you conduct yourself in negotiations with your fellow nation-founders? What kind of reasoning can you rely upon? You have your beliefs, but the others hold to different beliefs. And you are going to have to justify your actions to your constituency which is made up of all kinds of people, with all kinds of beliefs and all kinds of interests. You are going to have to explain yourself in a way which will seem reasonable to people who may not share your beliefs and be acceptable to those who do share your beliefs.
This is the thought experiment which John Rawls invites his readers to conduct. Rawls argues that ever since Catholicism and Protestantism fought each other to a standstill in Renaissance Europe, and the separation of Church and State was accepted as unavoidable, "reasonable pluralism" has become a fact of life for modern societies, and a fact which should be welcomed. He argues that if you conduct such a thought experiment, then you would have to come up with a conception of political liberalism something like that which he develops in his own thought experiment written up as Theory of Justice (1971) and more recently, Political Liberalism (1995).
Whether to endorse slavery, free market capitalism, democratic socialism or recreate a landed aristocracy, it is up to the "parties" to decide in due course, on the basis of the founding principles they decide, but given that the constitution must be defensible in terms which will be counted reasonable by the populace at large, Rawls is confident that such a thought experiment would come up with some kind of political liberalism. Rawls regards the relations of production as a secondary question which can be sorted out in due course, once the institutions of representative democracy and the judiciary have been settled and the citizens can legislate the social system.
In Rawls' books this thought experiment is called the "original position" though Rawls describes it in slightly different terms. Rather than supposing one is elected from large geographical electorates, Rawls proposes a hypothetical "veil of ignorance" so that the delegates do not know the social status of those that they represent nor what social position they may occupy in the state to be founded. Otherwise, his thought experiment pretty much matches the current US Constitution, barring political lobbyists, big business control of election campaigns and the naked play of self-interest within the institutions of really existing democracy.
Thus Rawls does much the same as Kant when he re-invented the Revealed Religion of the 18th century Lutheran Church by means of Reason, and Hegel when he set out to discover what was rational in the reality of early 19th century Prussia, but, it has to be said, in a way which is commensurate with a democratic republic of the 20th century, as a "self-standing" conception, limited to that which could be justified from the standpoint of any comprehensive metaphysical, moral or religious doctrine.
Like Kant and Hegel, Rawls does not validate everything that exists in the present-day U.S.A. as rational; he holds that the high cost of US election campaigns which ensures the restriction of nomination of candidates to the very rich, and the lack of an adequate health service and social safety net which ensures that a substantial proportion of the population cannot pursue the good life, are contrary to the requirements of justice. Nevertheless, for Rawls it is the constitution which decides the distribution of wealth and power, not the other way around.
The "original position" which Rawls characterises as a "representation device," is used to argue for "justice as fairness" as a candidate for an "overlapping consensus" "for the right reasons," which can withstand the test of "public reason" by "rational" and "reasonable" citizens who count one another as "free and equal," as a "self-standing" "political" conception, as opposed to a "comprehensive doctrine," and thus create the basis for a society as a "well-ordered system of social cooperation".