The texts of Plato are an intelligent exercise in normative philosophy. He offers a portrait of how society should be constructed and how society should act, which essentially makes his prescription utopian. His construction of justice and society is something that I am agreeable with, as Platos justice is a complex value that is founded on the individuals actions. Justice is indeed a virtue that is possessed and developed by the person, which is hence why the conceptions of justice in Books 1 and 2 would develop into the prescription of the rule of the wise the philosopher king towards the conclusion of the series.
Plato basically gives several ideas, not concrete definitions though, of justice in the first parts of the series. The first book for instance does not yet have any resolution on the problem of justice, but Socrates in the book has debunked several conceptions of justice, like for instance it is not justice to harm others. This is something that I am agreeable with as the idea of justice should not be imposed through coercion of others, as the perception of what is good or bad is ultimately dependent on the judgment of the person, who of course can not guarantee just decisions or can be subject to flaws.
Injustice was also argued to be feared more than committed, which is why it is condemned, hence making injustice in fact stronger than justice is. This idea is something that is enlightening indeed has wrongful actions establishes a form of social agreement on how it would be penalized and deterred. Justice therefore emerges out of the implicit contract agreed on by people to ensure that such injustices would not occur in the society.
It was similarly mentioned that just men are wiser and make better decisions, which is why the society should be developed from collective actions and cooperation from its citizens. Also, for Plato, justice is about the individual committing to play out the role endowed to them, as it therefore contributes to the advancement of the society. This would be the rationale for Platos construction of the utopian society with several classes who are tasked with different things.
I also consent to the idea that happiness is an important consideration as it would help fulfill the function of man, which is to live. Happiness can be acquired by the just man, as injustice is generally unhappy as it would bring miseries to individual. This is something that is indeed accurate, as even the modern society can find social order from the establishment of justice, as injustices can result to disorder or unhappiness of the citizens. Justice can be more profitable for the society and the individual than injustice is.
The second book goes on to argue that justice and happiness can be found in basic necessities like food and shelter, and that city-state requires an army to maintain welfare. Even our modern concept of government and society would rely on guardians as the group and institution to ensure that there is protection from internal and external coercion. Social order, which is constitutive of the idea of justice, is something that can be maintained if there is a guardian institution.
Platos conceptions in the first and second book, especially the idea of justice are something that until this contemporary period is a collective value. This value is indeed an arguable even at times inexplicable one that is subject to different interpretations; but at the end of the day, justice, like Plato mentioned, is essentially reliant on the just person, who acts according to what is good, and what is good is something following and being faithful to virtue. I agree with Plato in saying that justice is happiness and knowledge; it is indeed something that is difficult to define but satisfying to reflect on.
Plato. (1955). The Republic. Desmond Lee, Trans. USA: Penguin Classics.