John Rawls is widely considered as one of, if not the, most influential Political Philosophy thinkers of the 20th century.
In his main text," A Theory of Justice", Rawls conducts a thought experiment. In it people are asked to stand behind "a veil of ignorance" a theoretical device. In this position, they would have no information about their standing within society, such their gender, religious beliefs, socio-economic position etc. Deprived of this information ,individuals have to decide what kind of society they want and the levels of equality or inequality within it.