A knife is good, he said, when it cuts efficiently, that is, when it fulfills its function.
The assignment of all people to their respective classes would come only after extensive training and only those capable of doing so would progress to the higher levels. Although theoretically all people would have the opportunity to reach the highest level, they would stop in fact the level of their natural aptitudes.
This implied that by nature some would be rulers and others craftspeople, and would provide the basis for a perfectly stratified society. Whereas later societies in Europe assumed that children born into such a stratified society would stay at the level at which they were born, Plato recognized that children would not always have the same quality as their parents. He said, therefore, that among the injunctions laid by heaven upon the rulers there is none that needs to be so carefully watched as the mixture of metals in the souls of children.
If a child of their own is born with an alloy of iron or brass, they must, without the smallest pity, assign him the station proper to his nature and thrust him out among the farmers and craftspeople. Similarly, if a child with gold or silver is born to craftspeople “they will promote him according to his value.”
Most importantly, Plato thought that everyone should agree on who is to be the ruler and agree also on why the ruler should be obeyed.