Nature and convention are opposed. Convention deems that all things should distributed equally. This reflects a mentality of a slave majority. In order to curve the stronger gathering more riches they define injustice as the accumulation of wealth. Nature however deems inequality of wealth and power as right. Plato finds evidence for this in creatures, human communities and nations:
“By what right, for instance, did Xerxes make war on Greece or his father on Scythia, not to mention countless further cases of the same kind of behaviour? These people act, surely, in conformity with the natural essence of right and, yes, I’d even go so far as to say that they act in conformity with natural law, even they presumably contravene our man-made laws.”
My first instinct is to see this as a strange and somewhat un-thought out argument. Why should Xerxes aggression’s be justified by natural law? It is natural that a lion hunt and kill a wilder beast, or a spider trap a fly, but does this really equate to an act of aggressive warfare? However, the game here is to get to the bottom of Plato’s thinking. Is he perhaps suggesting that because the stronger agent is capable in reality of some conquest, it is entirely his prerogative to undertake that conquest? So-be-it for the conquered? In this sense, the sense that if it can be, then there be no natural reason why it shouldn’t be, Plato is defining justice. He is aligning right and wrong with the natural world rather than the conventional. Is this just for arguments sake or does Plato truly believe this? It is not clear in this particular passage. Also, what conventions of the time would have called for an equal distribution of wealth? These seem like ultra-modern terms. Perhaps a translation issue?