's idea of justice for a Philosopher King
if the philosopher cannot find employment as a ruler.
Actually, in Plato's myth of Er, Plato illustrates this concept by recounting what life Odysseus chooses when he is at the lap of the Muses. He has been a great hero in his life and has spent it fighting monsters and villains. So what life does he choose when given the chance? The soul that has 'A quiet life doing one's own work'. His experience with the best and worst the world had to offer made him aware that the life of such a soul was truly more rewarding than any accolade or honour that could be bestowed.
The full explanation of this concept can be found in book VI of Plato's Republic, at 496a-e.
Plato says of true philosophers "Now the members of this small group have tasted how sweet and blessed a possession philosophy is, and at the same time they've also seen the madness of the majority and realized, in a word, that hardly anyone acts sanely in public affairs and that there is no ally with whom they might go to the aid of justice and survive, that instead they'd perish before they could profit either their city or their friends and be useless to both themselves and to others, just like a man who has fallen among wild animals and is neither willing to join them in doing injustice nor sufficiently strong enough to oppose the general savagery alone. Taking all this into account, they lead a quiet life and do their own work"