The theory of three classes and three souls by Plato is an idea borrowed from Pythagoras. According to this theory, every human soul has three qualities- rational, spirit, and appetite. Justice is considered to be the fourth virtue which is architectonic in nature binding all the other qualities.
Plato explains that individuals who have the rational faculty would constitute the ruling class, and wisdom would be the virtue of such soul. Only this soul has the power to comprehend the Idea of Good.
Individuals in whom the spirit is the predominant quality are the auxiliaries or soldiers, and the virtue of such soul is courage. The rulers and soldiers will together constitute the guardian class.
On the other hand, individuals having the appetite faculty are the artisans or the producing class. The quality of such soul is temperance.
Plato states that any interchange in jobs between the three social classes will bring harm to the State and is the worst evil. On the contrary, if they perform their appointed task, then such a State would be just.
Here, Plato clearly points out that a child with the potential of becoming an artisan could be born to a philosopher and vice versa.
The Myth of Metals
The philosopher ruler in Plato’s Republic is supposed to spread a “noble or royal lie” among people and that is the myth of metals. According to this myth, all human beings were born of earth, and as such they were born with some metallic component in their bodies. The philosophic-rational ones were born with gold, the spirited-courageous ones with silver, and the appetitive ones with bronze.
The myth is justified by Plato saying that this myth is necessary to sustain the ideal state. It would make everyone believe that he is part of a bigger family in the society, and it would also lead everyone to accept their station in life as natural, as suited to the qualities they were born with.
The myth and the way of specialisation, according to Plato, are ways of unification.
Bertrand Russell, History of Western Philosophy
Plato, Republic (Translated by Allan Bloom)