YPlato is an idealist thinker; in the Republic Plato said that "justice is the harmony and coordination existing in all nature." Therefore, according to Plato, each person should act on the basis of the qualities of his soul.
Plato said in his Republic that the knowledge of ideas is the best and the highest. Knowledge of thoughts is necessary to come out of ignorance.
Plato's justice is based on morality and duty, presenting the idea of the philosopher king; he said that a rational person would be the ruler, it is necessary to have discretion to rule.
Modern justice is based on law, based on the constitution. But Plato rejected the recognition of the rule of law.
The law is made by man, which man has made for his convenience, but it is absolutely necessary to have knowledge for making law.
A person with knowledge is not materialistic, for him all this is meaningless i.e. being materialistic for living can be okay to an extent, but a person with knowledge is not materialistic.
Law or ordinance is materialistic and Plato considered law to be synonymous with ignorance because laws keep changing 'according to time'. But knowledge lasts forever.
Plato said that the rule should be of wisdom and reason and not of law or ordinance. That's why Plato's idealistic principal idea is very meaningful.