In Book I of Plato's Republic, Socrates questions the value of justice and argues justice is for own sick rather than for deliberate personal benefits. In Apology, Plato suggests that justice cannot be reducible to people's opinions and in order for justice to be compelling in directing how we should act, we need a consistent concept of justice. Based on the ideas of Socrates and Plato, a philosopher king who is able understand and define the absolute just and govern the city only by means of justice is necessary to construct a just society. The city built in the harmonious way, which is considered as "true philosophy", is a perfect one. However, this city is an ideal one, the Utopia described in Republic will never become true because the nature of human being doesn't allow the existence of a Philosopher King, the idea of justice offered by Plato, as a good itself, cannot be defined, and a just society cannot be achieved by means of absolute justice.
Based on the requirement of the philosopher King provided by Plato's Republic Book VII, such a leader must be a prophet. He needs to have the knowledge of truth, goodness and beauty, which according to Plato, cannot be seen in the real form by normal people. He needs to see through life and have no pursuit for self-profits. This guarantees that he won't do things for his own good. At the same time, the prophet has to be a king who is able to secure monarchy and maintain his government. Such a combination is impossible because for a King, the priority is to guarantee his dominant status and when revolution happens, he must do so at all costs, even at the risk of hurting others when it is necessary, to secure his ruling. Since a king can never be a prophet at the same time, the philosopher king does not exist in reality.