What is necessary for justice to exist in the Republic?

In Plato's "Republic," justice is fundamentally connected to the idea of harmony within the individual and society. The correct answer highlights the importance of balancing the three parts of the soul—reason, spirit, and appetite. When these elements are in harmony, an individual can act justly, with reason guiding actions and desires in proper alignment.

Plato argues that a just society mirrors a just individual. In the ideal city, harmony is achieved when the corresponding classes—rulers, auxiliaries, and producers—each fulfill their roles in accordance with their nature. Justice, therefore, is realized through the proper functioning and balance of these elements, both within the soul and within the structure of society.

This understanding emphasizes that justice is not merely about equality or wealth distribution but rather about embodying a deeper relationship between the different parts of the soul and the different classes within the society. Justice is about achieving a cohesive order wherein every aspect plays its appropriate role, leading to a well-ordered and harmonious society.