In my view, justice is entirely conventional; indeed, all of morality consists in conventions that are the result of continual tacit bargaining and adjustment. This is not to say social arrangements are just whenever they are in accordance with the principles of justice accepted in that society. We can use our own principles of justice in judging the institutions of another society, and we can appeal to some principles we accept in order to criticize other principles we accept.
To use David Hume's model of the relevant sort of convention, two people rowing a boat each continually adjusts his or her rate of rowing to the other so that they come to row at the same rate, a rate that is normally somewhere between the rate at which each would prefer to row. In the same way the basic principles of justice accepted by people of different powers and resources are the result of a continually changing compromise affecting such things as the relative importance attached to helping others as compared with the importance attached to not harming others.
Hume's rowers provide an example of a “convention” that is normally completely tacit. There are other models in which the bargaining can be more explicit, for example when a seller comes to set prices that are acceptable to customers, when employers reach understandings with employees concerning wages, or when political groups influence legislation.
I want eventually to consider the implications for moral reasoning and argument of the thesis that principles of justice are entirely the result of implicit bargaining and convention of this sort.