A Theory of Justice derives its central argument from a thought experiment in which a rational person is called upon to determine the best type of society for themselves. But for the purposes of Rawls' argument, they cannot know anything about what their economic status will be in this society. They may be fabulously wealthy or dirt poor. Rawls describes this situation as a "veil of ignorance," and it is essential to his point. He argues that in such a situation, a rationa person would choose a society and an economic system that best provides for those people who are worst off, because there is at least as good a chance that they might wind up in that group as in any other.
Clearly, the conditions for the thought experiment Rawls proposed are not plausible, in the sense that no person can be completely ignorant of their interests (though, as Marxists have suggested for more than a century, they may be persuaded to develop a false consciousness of their interests.) But Rawls is attempting to do is, in his words, to
...present a conception of justice which generalizes and carries to a higher level of abstraction the familiar theory of the social contract as found, say, in Locke, Rousseau, and Kant.
His object is not to present a plausible situation that an individual might actually face in their lives, but to establish principles that
free and rational persons concerned to further their own interests would accept in an initial position of equality as defining the fundamental terms of their association.
These principles "would govern all further agreements," i.e., they would be the foundation for the institutions of society. What Rawls intends to do is to establish a rational basis for an economically and socially just society. The whole point is to put men in an implausible situation, stripped of the privileges and inequalities that our society, clearly not founded on the principles of fairness, has accrued to some people. "The choice which rational men would make" in such a hypothetical situation, Rawls claims, "determines the principles of justice." His theory should be understood as a theory, but one which he hopes can inform our discussions about liberty, social justice, and the complex relationship between people and society.