Veil of ignorance
The veil of ignorance, along with the original position, is a concept that has been in use by other names for centuries by philosophers such as John Stuart Mill, John Rawls, and Immanuel Kant whose work discussed the concept of the social contract. John Harsanyi helped to formalize the concept in economics. The modern usage was developed by John Rawls in A Theory of Justice. It is a method of determining the morality of a certain issue (e.g., slavery) based upon the following thought experiment: parties to the original position know nothing about their particular abilities, tastes, and position within the social order of society.
The veil of ignorance blocks off this knowledge, such that one does not know what burdens and benefits of social cooperation might fall to him / her once the veil is lifted. With this knowledge blocked, parties to the original position must decide on principles for the distribution of rights, positions, and resources in their society. As Rawls put it:
...no one knows his place in society, his class position or social status; nor does he know his fortune in the distribution of natural assets and abilities, his intelligence and strength, and the like.—John Rawls, A Theory of Justice 
The idea then, is to render obsolete those personal considerations that are morally irrelevant to the justice or injustice of principles meant to allocate the benefits of social cooperation.
For example, in the imaginary society, one might or might not be intelligent, rich, or born into a preferred class. Since one may occupy any position in the society once the veil is lifted, the device forces the parties to consider society from the perspective of all members, including the worst-off and best-off members.
David and Bathsheba, Nathan's Veil
One of the earliest literary examples similar to this concept appears in 2 Samuel, Chapter 12, of the Bible, where the prophet Nathan rebukes King David for his treachery in stealing the wife of one of his soldiers, Uriah the Hittite, through the telling a seemingly unrelated story.
There were two men in a certain town, one rich and the other poor. The rich man had a very large number of sheep and cattle, but the poor man had nothing except one little ewe lamb he had bought. He raised it, and it grew up with him and his children. It shared his food, drank from his cup and even slept in his arms. It was like a daughter to him. Now a traveler came to the rich man, but the rich man refrained from taking one of his own sheep or cattle to prepare a meal for the traveler who had come to him. Instead, he took the ewe lamb that belonged to the poor man and prepared it for the one who had come to him.
Abstracted from his personal bias and self-interest, David sees the injustice more clearly and is incited to bring justice to the situation by means of Nathan's technique—a veil of ignorance.
Another application of the veil of ignorance towards a recognition of the worst-off members of society is captured in Mahatma Gandhi's Talisman, given in 1948.
I will give you a talisman. Whenever you are in doubt, or when the self becomes too much with you, apply the following test. Recall the face of the poorest and the weakest man whom you may have seen, and ask yourself, if the step you contemplate is going to be of any use to him. Will he gain anything by it? Will it restore him to a control over his own life and destiny? In other words, will it lead to swaraj [freedom] for the hungry and spiritually starving millions? Then you will find your doubts and your self melt away.
The veil of ignorance is part of the long tradition of thinking in terms of a social contract. The writings of Immanuel Kant, Thomas Hobbes, John Locke, Jean Jacques Rousseau, and Thomas Jefferson offer examples of this tradition.
- Harsanyi, J. C. (1953). "Cardinal Utility in Welfare Economics and in the Theory of Risk-taking". J. Polit. Economy 61 (5): 434–435. JSTOR 1827289.
- Harsanyi, J. C. (1955). "Cardinal Welfare, Individualistic Ethics, and Interpersonal Comparisons of Utility". J. Polit. Economy 63 (4): 309–321. JSTOR 1827128.
- Rawls, John (1971). A Theory of Justice. Cambridge, Massachusetts: Belknap Press. ISBN 0-674-00078-1.
- Rawls, John (2001). Justice as Fairness: A Restatement. Cambridge, Massachusetts: Belknap Press.
- Rawls, John (1999). A Theory of Justice. Harvard University Press. p. 118. ISBN 0-674-00078-1.
- 2 Samuel Chapter 12 of the Christian Bible. New International Version.
- Gandhi Quotes,