The Republic by Plato Part 8 Page 13

In the succeeding generation 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, which always and in all places are causes of hatred and war. This the Muses affirm to be the stock from which discord has sprung, wherever arising; and this is their answer to us.

Yes, and we may assume that they answer truly.

Why, yes, I said, of course they answer truly; how can the Muses speak falsely?

And what do the Muses say next?

When discord arose, then the two races were drawn