The idea or the original position is to set up a fair procedure so that any principles agreed to will be just.The aim is to use the notion of pure procedural justice as a basis or theory. Somehow we must nullify the effects or specific contingencies which put men at odds and tempt them to exploit social and natural circumstances to their own advantage. Now in order to do this I assume that the parties are situated behind a veil or ignorance. They do not know how the various alternatives will affect their own particular case and they are obliged to evaluate principles solely on the basis or general consideration.
It is assumed, then, that the parties do not know certain kinds or particular facts. First of all, no one knows his place in society, his class position or social status; nor does he know his fortune in the distribution or natural assets and abilities, his intelligence and strength, and the like. Nor, again, does anyone know his conception of the good, the particulars of his rational plan of life, or even the special features or his psychology such as his aversion to risk or liability to optimism or pessimism. More than this, I assume that the parties do not know the particular circumstances of their own society. That is, they do not know its economic or political situation, or the level of civilization and culture it has been able to achieve. The persons in the original position have no information as to which generation they belong. These broader restrictions on knowledge are appropriate in part because questions or social justice arise between generations as well as within them, for example, the question or the appropriate rate of capital saving and of the conservation or natural resources and the environment or nature. There is also, theoretically anyway, the question or a reasonable genetic policy. In these cases too, in order to carry through the idea or the original position, the parties must not know the contingencies that set them in opposition. They must choose principles the consequences or which they are prepared to live with what ever generation they turn out to belong to.
As far as possible, then, the only particular facts which the parties know is that their society is subject to the circumstances of justice and whatever this implies. It is taken for granted, however, that they know the general facts about human society. They understand political affairs and the principles of economic theory: they know the basis of social organization and the laws of human psychology. Indeed, the parties are presumed to know whatever general facts affect the choice of the principles of justice. There are no limitations on general information, that is, on general laws and theories, since conceptions of justice must be adjusted to the characteristics of the systems of social cooperation which they are to regulate, and there is no reason to rule out these facts. It is, for example, a consideration against a conception of justice that, in view of the laws of moral psychology, men would not acquire a desire to act upon it even when the institutions of their society satisfied it. For in this case there would be difficulty in securing the stability of social cooperation. An important feature of a conception of justice is that it should generate its own support. Its principles should be such that when they are embodied in the basic structure of society men tend to acquire the corresponding sense of justice and develop a desire to act in accordance with its principles. In this case a conception of justice is stable. This kind of general information is admissible in the original position.
The notion of the veil of ignorance raises several difficulties. Some may object that the exclusion of nearly all particular information makes it difficult to grasp what is meant by the original position. Thus it may be helpful to observe that one or more persons can at any time enter this position, or perhaps better, simulate the deliberations of this hypothetical situation, simply by reasoning in accordance with the appropriate restrictions. In arguing for a conception of justice we must be sure that it is among the permitted alternatives and satisfies the stipulated formal constraints. No considerations can be advanced in its favor unless they would be rational ones for us to urge were we to lack the kind of knowledge that is excluded. The evaluation of principles must proceed in terms of the general consequences of their public recognition and universal application, it being assumed that they will be complied with by everyone. To say that a certain conception of justice would be chosen in the original position is equivalent to saying that rational deliberation satisfying certain conditions and restrictions would reach a certain conclusion. If necessary, the argument to this result could be set out more formally. I shall, however, speak throughout in terms of the notion of the original position. It is more economical and suggestive, and brings out certain essential features that otherwise one might easily overlook.