As ordinary human beings, we will probably decide not to jump into a deep hole in the side of a mountain if all the information we have is that it looks like a volcano, shakes like a volcano and makes rude noises like a volcano. We may decide otherwise if we know that the smoke and sounds are fake, that there is a safety net and no chance of missing it, and that we will be rewarded generously for jumping. Information, then, is a key factor in making choices.
In the process of developing an impartial theory of justice, it is sometimes better not to know certain things; for this reason, lustitia wears a blindfold. Knowledge of the facts that you have a talent for making the right choices on the stock market and that you deeply enjoy wealth may prejudice you in favour of a free market society with profound differences between rich and poor-unless, of course, you are capable of overcoming prejudice, have no opportunity or incentive to be ruled by prejudice, or are temporarily freed from it. On the other hand, contracting parties must know other things-for one, they must know why they have gathered together at all.
More systematically, the issue looks as follows. In contract theories of justice, seven categories of information can be distinguished. Each of these categories represents a necessary condition for a decision, none of them can be omitted and no other types of information are required. An adequate contract theory of justice must account for the assumptions it makes about each of these seven types of knowledge; an impartial theory will have to do so in terms of impartiality. If we would discard the idea of a contract as a fiction, a vehicle for the imagination, we would still have to explain what kinds and amounts of knowledge are required for an impartial choice of principles of justice. Although there would no longer be contracting parties, we would still want the reader to be able to imagine herself as an impartial being.