Plato’s strategy in The Republic is to first explicate the primary notion of societal, or political, justice, and then to derive an analogous concept of individual justice. In Books II, III, and IV, Plato identifies political justice as harmony in a structured political body. An ideal society consists of three main classes of people—producers, auxiliaries, and guardians; a society is just when relations between these three classes are right. Each group must perform its appropriate function, and only that function, and each must be in the right position of power in relation to the others. Rulers must rule, auxiliaries must uphold rulers’ convictions, and producers must limit themselves to exercising whatever skills nature granted them. Justice is a principle of specialization: a principle that requires that each person fulfill the societal role to which nature fitted him and not interfere in any other business.
At the end of Book IV, Plato tries to show that individual justice mirrors political justice. He claims that the soul of every individual has a three part structure analogous to the three classes of a society. There is a rational part of the soul, which seeks after truth and is responsible for our philosophical inclinations; a spirited part of the soul, which desires honor and is responsible for our feelings of anger and indignation; and an appetitive part of the soul, which lusts after all sorts of things, but money most of all. The just individual can be defined in analogy with the just society; the three parts of his soul achieve the requisite relationships of power and influence in regard to one another. In a just individual, the rational part of the soul rules, the spirited part of the soul supports this rule, and the appetitive part of the soul submits and follows wherever reason leads. Put more plainly: in a just individual, the entire soul aims at fulfilling the desires of the rational part, much as in the just society the entire community aims at fulfilling whatever the rulers will.
The parallels between the just society and the just individual run deep. Each of the three classes of society, in fact, is dominated by one of the three parts of the soul. Producers are dominated by their appetites—their urges for money, luxury, and pleasure. Warriors are dominated by their spirits, which make them courageous. Rulers are dominated by their rational faculties and strive for wisdom. Books V through VII focus on the rulers as the philosopher kings.
In a series of three analogies — the allegories of the sun, the line, and the cave — Plato explains who these individuals are while hammering out his theory of the Forms. Plato explains that the world is divided into two realms, the visible and the intelligible. The visible world is the universe we see around us. The intelligible world is comprised of the Forms — abstract, changeless absolutes such as Goodness, Beauty, Redness, and Sweetness that exist in permanent relation to the visible realm and make it possible. Only the Forms are objects of knowledge, because only they possess the eternal unchanging truth that the mind — not the senses — must apprehend.
Only those whose minds are trained to grasp the Forms — the philosophers — can know anything at all. In particular, what the philosophers must know in order to become able rulers is the Form of the Good — the source of all other Forms, and of knowledge, truth, and beauty. Plato cannot describe this Form directly, but he claims that it is to the intelligible realm what the sun is to the visible realm. Using the allegory of the cave, Plato paints an evocative portrait of the philosopher’s soul moving through various stages of cognition through the visible realm into the intelligible, and finally grasping the Form of the Good. The aim of education is not to put knowledge into the soul, but to put the right desires into the soul — to fill the soul with a lust for truth, so that it desires to move past the visible world, into the intelligible, ultimately to the Form of the Good.
Philosophers form the only class of men to possess knowledge and are also the most just men. Their souls, more than others, aim to fulfil the desires of the rational part. After comparing the philosopher king to the most unjust type of man — represented by the tyrant, who is ruled entirely by his non -rational appetites — Plato claims that justice is worthwhile for its own sake. In Book IX he presents three arguments for the conclusion that it is desirable to be just. By sketching a psychological portrait of the tyrant, he attempts to prove that injustice tortures a man’s psyche, whereas a just soul is a healthy, happy one, untroubled and calm. Next he argues that, though each of the three main character types — money-loving, honor-loving, and truth-loving — have their own conceptions of pleasure and of the corresponding good life — each choosing his own life as the most pleasant — only the philosopher can judge because only he has experienced all three types of pleasure. The others should accept the philosopher’s judgement and conclude that the pleasures associated with the philosophical are most pleasant and thus that the just life is also most pleasant. He tries to demonstrate that only philosophical pleasure is really pleasure at all; all other pleasure is nothing more than cessation of pain.
One might notice that none of these arguments actually prove that justice is desirable apart from its consequences — instead, they establish that justice is always accompanied by true pleasure. In all probability, none of these is actually supposed to serve as the main reason why justice is desirable. Instead, the desirability of justice is likely connected to the intimate relationship between the just life and the Forms. The just life is good in and of itself because it involves grasping these ultimate goods, and imitating their order and harmony, thus incorporating them into one’s own life. Justice is good, in other words, because it is connected to the greatest good, the Form of the Good.
Plato ends The Republic on a surprising note. Having defined justice and established it as the greatest good, he banishes poets from his city. Poets, he claims, appeal to the basest part of the soul by imitating unjust inclinations. By encouraging us to indulge ignoble emotions in sympathy with the characters we hear about, poetry encourages us to indulge these emotions in life. Poetry, in sum, makes us unjust. In closing, Plato relates the myth of Er, which describes the trajectory of a soul after death. Just souls are rewarded for one thousand years, while unjust ones are punished for the same amount of time. Each soul then must choose its next life.