John Rawls' A Theory of Justice was an influential book on the topic of social justice. Rawls thought that social justice happens when we try to compensate for the lucky or unlucky circumstances that we are born into. We can create principles of justice only when we set aside social and economic circumstances that "seem arbitrary from a moral point of view". Rawls notes that there are strong hierarchies that systematically shape our socio-economic status. Where we are born into the hierarchy is a matter of luck. A theory of justice should negate the unlucky circumstances that people are born into.
According to Rawls, justice is a kind of fairness. Fairness is here means the same kind of fairness as we have when we play games. For example, when you play pictionary with a group of friends, it will not be fair to put all of the good artists on one team. Outcomes of social practices are fair if and only if we agree to the rules before we know whether the rules will benefit us more than they benefit others. A social practice is just an activity that people perform together as a group for mutual benefit. For example, if we play poker with a wild card, we will not choose the wild card based on what cards someones has in his or her hand. We choose the wild card before the cards have been dealt. In other words, we have to choose the terms of society in ignorance of our position of relative advantage. Choosing the terms of a social practice without knowledge of how we benefit from those terms is a matter of pure procedural justice.
Fairness in the "basic structure" of society involves two things. First, we must imagine that we are ignorant of our social position, class, gender or race. This is what Rawls calls a "veil of ignorance". Second, we then ask what rules or terms of society we would want (assuming that we don't know where in a social hierarchy we fall). Because we don't know where we fall in the society, we will want to minimize risks about the rules of our society. Rawls thinks we would want to follow the maximin rule, which means that we try to maximize the minimal position. The thought is that you want to provide the best possible worst position. Because it is possible that you will end up in the worst position, you will want that worst position to be as good as possible. Such a society is a fair because the way of choosing rules and terms of society was fair. Each person is represented as "free and equal" because principles of justice were chosen with everyone's interests being treated equally. When we consider the interests of everyone equally from behind this veil of ignorance, this is what Rawls calls the "original position".
Now that we know how we can choose principles for a just society, we have to consider what principles we actually would choose when behind a veil of ignorance.
Rawls thinks that we would not accept utilitarianism. Utilitarianism is the view that we should maximize collective welfare. In other words, utilitarians Rawls thinks that utilitarianism will be rejected because it allows individuals to be sacrificed for the benefit of the majority. For example, some people might lose freedom and liberty by being enslaved by other members of the society. So long as slavery creates maximum pleasures for the most people, it can be justified by a utilitarian principle. Another problem is that there is no limit to inequality in a society. As long as the majority of people are happy and doing well, it will not matter if a few people are stuck with extreme poverty. As long as the overall welfare is improved, a few individuals can be destitute or enslaved. Because utilitarianism allows for the sacrifice of individuals for the benefit of the majority, Rawls does not think that we will accept utilitarian principles when behind a veil of ignorance.