I've been thinking about a crude distinction that one might want to draw between justice and ethics. Justice is obviously something different to ethics else it wouldn't have a separate name and be studied separately. But what is the difference between the two?
I take it that ethics is broader than justice, in that ethical principles concern a wider range of action than principles of justice. I also take it that justice is usually talked about in terms of laws, duty, obligation etc., whereas ethics is not necessarily couched in those terms.
So which areas does justice apply to if not all? This is a hugely controversial question, and is at the heart of the disputes between cosmopolitans and nationalists/statists. Traditionally justice has been conceived as a social and political issue - principles of justice are designed to regulate the interaction between states and their citizens. Cosmopolitans dispute this idea and argue that justice is a global issue.
An alternative way of making this distinction which would be more specific to distributive questions would be to draw a line between comparative and non-comparative justice. Comparative justice is concerned with relative well-being, whereas non-comparative justice is concerned with absolute well-being. Sometimes this is made in the language of justice versus charity or humanitarianism.
One thing that seems certain is that by labelling an issue a matter of justice it gains some normative weight that would not arise otherwise. Charity is supererogatory, justice is obligatory. Humanitarianism is desirable, but justice is duty. Given this evaluative element which the label carries, discussions of justice need to be clear about what they take it to refer to.