The principles of justice so far looked at are for institutions. Rawls says that there are also principles of justice for individuals . To work out what they are we must again retire behind the veil of ignorance, and, assuming that principles of justice have been adopted for institutions, we consider what justice may require of individuals. Rawls distinguishes two kinds of principles of individual justice. Some have to do with obligations, others with duties.
Many people use the words 'duty' and 'obligation' as interchangeable.
There is a real distinction here, even if the distinction between 'duty' and 'obligation' is artificial. For example, the voluntary act of making a promise puts you under an obligation to do whatever you have promised. On the other hand you may have a duty to rescue someone from a burning car, even if you have never done anything to entitle that person to call on you for help. Rawls thinks that in the Original Position people will see the point of accepting principles requiring them to discharge various obligations and duties. For example, if promising is a social practice from which you have benefited, then if you make a promise you must keep it.