Political justice takes two forms — natural and conventional. Natural justice has the same validity everywhere and is unaffected by the view men take of it at any given time or place. It is made up of rights and duties that are obligatory in any society (e.g., strictures against killing).
Conventional justice is composed of the rights and duties superimposed by the laws of particular states on the laws of natural justice. Justice is conventional where there is no special reason why it takes one form or another. The rules imposed by conventional justice are reached by common consent, after which they hold good until modified by consent (e.g., when several states decide that the ransom for a prisoner of war should be a certain amount, or when a religious code prescribes that a certain ritual requires a specifically enumerated number of sacrificial animals). Rules of justice established by convention on the grounds of convenience may be compared in some sense to the standard systems of weights and measures that are set up by states; they are not the same everywhere since governments are different, but are adhered to for the sake of stability.
Natural justice is absolute and cannot change, although in day to day affairs the rules under which it is administered tend to go through modifications and alterations. It is not always obvious which rules of justice are natural and which are conventional since variation is possible and both forms exist side by side.