The veil of ignorance and the original position are concepts introduced by John Harsanyi and later appropriated 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”. The idea then, is to render moot 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.