Plato wrote the Republic to give expression to an ideal of a city in which a perfect justice has been established in all of its parts. He was not naive. He did not suppose that his ideal city could be easily realized, if at all; in the end he concluded that it could not. Yet his pessimism never led him to conclude that the ideal was false, only that it was less accessible to thought.
Thus, the idea of justice, which is the central theme of the Republic, became transcendent.
Justice is a moral virtue, a ruling characteristic of a life well lived. Plato supposed that there are four cardinal virtues essential to being good: courage, temperance, prudence and justice—he occasionally added others, such as a quick wit, good memory, and greatness of soul or magnificence, which is an ability to imagine and achieve great things beyond the merely personal or parochial.
Justice is distinct from all of these. One may be brave, temperate, prudent and uniquely gifted and still live an immoral life. Indeed, without justice these qualities merely enhance corruption.
Justice makes the difference: it includes all the other virtues and outstanding qualities in itself and endows them with that special quality that it uniquely exemplifies, without which nothing can have moral value. What is this quality? What is justice?
It is the ruling virtue of civil society and the individual soul, of the city and its government. The answer lies here.
Among the key features of justice is that it does no harm: conflicts are mediated; punishment is restorative, never vengeful. Justice is also tolerant and impartial; differences in gender identity and sexual preference make no political or moral difference. The aim of just government is to provide gainful and useful employment for all its inhabitants and the material means to subsist and to flourish; free education for all, as much as anyone is able to receive, from pre-natal care through higher education. And there is an equitable redistribution of excess wealth. Finally, justice is impartial and equitably maintained.
 To achieve these ends is the duty of those who govern. Much of the Republic is devoted to their education. The rulers and guardians of a city must be persons, male or female, who are: gifted, capable of self-mastery, indifferent to their own personal interests, and who have renounced personal fame and fortune or any other variety of personal gain (even reputation). They are also selfless, caring only for the welfare of the people whom they have been trained to govern and protect; are never selfish; never wanting to get even, or desiring a reward for their accomplishments or approval or praise; who are so constant in securing the good of the governed that they are taken for granted or even despised. Yet they bear all with equanimity and grace.
Justice is that quality that makes them so; it is the form of their lives and the quality of all they do. Some commentators have noted a similarity of Plato’s just ruler with the suffering servant: “He was despised and rejected by men; a man of sorrows and acquainted with grief, and as one from whom men hide their faces, he was despised and we esteemed him not. Surely he has born our griefs…” (Isaiah 53:3–4).
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If Plato is correct about justice, one wonders why anyone would choose to be a just ruler? And if the task of just rulers is to create a truly just society, why would anyone want to be a part of it, for the selfish question “what’s in it for me” must always have a negative answer?
Plato’s revelation is that it is the only way to secure lasting happiness. Only the just can be happy. Seeking one’s own advantage before all else only causes misery to oneself most of all.
Plato’s defense of this claim is tied to his admittedly paradoxical claim that only philosophers should rule. He is not referring to the ordinary run-of-the-mill philosopher. He meant someone who has persisted in philosophical enquiry to its ultimate goal and glimpsed Being itself, which he equated with the very Idea of Goodness, which is not a mere idea, but the cause of existence.
He believed that truth of this sort is not arrived at by an empirical method, by experiment and induction, but by purely intellectual or logical pursuits, whereby the mind ascends to truth through a play of concepts. Its ascent is marked by a degrading of the particular and material—he called his method “dialectic,” whereby the mind leaves behind the everyday world of sensory experience and by pure reason ascends to the realm of forms; there it contemplates the archetypes of every being, chief among them forms of “pure value,” such as Justice itself, Beauty itself, and, finally, the Good all by itself — radiant, pure, infinitely vital and captivating. These are not mere abstractions, but potent realities, inexhaustible causes of being. Merely to contemplate them brings immeasurable joy without end, in contrast to which all other pleasures and delights are mere infatuations, passing fancies.
Plato was the first and perhaps the only genuine transcendentalist.
But true philosophers, knowing what justice is, are not allowed to continue to dwell in this paradise of forms. They have a duty to descend to the world of appearances and counterfeits. The very idea of justice impels them. In this respect, Plato’s Republic is a manifesto for elitism. The elite are persons perfected by justice, whose judgment is determined by truth, and whose moral nature assures that they always do the right thing.
A city ruled and protected by such paragons of justice hardly requires laws; if any are needed the good judgment of the ruler will provide them and ensure that they are fit to promote virtue in the city. Such is the ideal city that Plato envisions, but since it is an ideal never to be realized, it evokes a mood of deep pessimism. The nobility of impartial justice filled Plato’s mind with joy, but the political realities of Athens, the corruption and backbiting, the self-serving nastiness of politicians, the phony populist rhetoric of ambitious self-promoters, and the fickleness of the people, the hoi polloi, made him despair that true justice could ever be achieved.
He concludes the Republic not with a paean of praise, but a narrative of death and rebirth, a hypothesis of the migration of souls, and a faint promise of deliverance.
In the Laws, Plato is in search of a plausible alternative to the rule of the just; a model city that exemplifies the rule of law, and which approximates justice, but may sometimes be unjust, and in any case ambiguous. His depiction of the model city launched the political practice of constitutionalism.