The Republic begins with the discussion of "individual justice", puts forward "city-state justice" and discusses what is "injustice". For individuals, justice is the greatest good.

Plato believes that justice is the highest good, it is not an external thing, but the state that the soul itself should have. The ideal country is a just country that conforms to the idea of supreme goodness. Justice is not only a personal virtue, but also a common virtue of the country and individuals, a two-way constraint and interaction between the country and individuals, and a requirement for individuals and even the country. Justice is the core idea of an ideal country.

The influence of the Republic:

The Republic embodies Plato's pursuit of perfection. Plato's ideal state form became a model of state system and political rule in later generations. The principles of many countries described in this book, such as the principle of division of labor and cooperation, and the principle that the purpose of the country is to pursue the best, have been valued in the national construction of later generations and absorbed by many regimes.

During the Middle Ages in Europe, many political and ethical thoughts of the Republic were re-discussed, and many thoughts penetrated into society and churches and became part of their thoughts. For example, the concepts of heaven and hell all come from The Republic.