rulers will be appointed who have lost the guardian power of testing the metal of your different races, which, like Hesiod’s, are of gold and silver and brass and iron. And so iron will be mingled with silver, and brass with gold, and hence there will arise dissimilarity and inequality and irregularity
Plato treats brass as one stratum in a psycho-political metallurgy of souls whose improper mixture produces civic discord, making it a marker of constitutional degeneration.
, Republic, -380thesis