In The Republic, Plato strives to display through the character and conversations of Socrates that justice is better than just the proper good for which men must strive for, regardless of whether they could receive equal benefit from choosing otherwise. His method is to use the dialogue from Socrates, questions which led the reader from one point to another, supposedly with convincing logic by obtaining agreement to each point before proceeding to the next, and so constructing an intriguing argument.
In the beginning, his two listeners ask the question of whether justice is stronger than injustice, the consequences of the two, and what makes the first right and the second wrong. As a response, Socrates deals directly with the concept of the individual's inner goodness and decency, but also ties it to his idea of the perfect state, which is a republic of three classes of people with a developed social structure and little in the way of recreation.
Although Socrates returns regularly to the concept of justice in his statements on the perfect city-state, much of it seems off topic. One of his main points, however, is that goodness is doing what is best for the greater good, rather than for individual happiness. There is a real sense in which his ideals turn on the concepts of virtue, and his belief that ultimately virtue is its own reward.
His first major point is that justice is an excellence of character. He then seeks agreement that no excellence is achieved through destructive means. The function of justice is to improve human nature, which is inherently constructive. Therefore, at a minimum, justice is a form of goodness that cannot be involved in injuring someone's character. Justice, in short, is a virtue, a human excellence.
His next point is that acting in peace with excellence brings happiness. Then he ties excellence to one's function. His examples are those of the senses -- each sensory organ is excellent if it performs its function, as the eye sees, the ear hears. Therefore, the just person is a happy person is a person who performs his function. Since these are tied together, injustice can never exceed these virtues and so justice is stronger and is the good.
However, Socrates does not stop there. He goes on to examine the question of the nature of justice and the just life. He identifies the three of the Athenian virtues: wisdom, courage, self discipline. For the bulk of the book, he looks at each virtue separately in terms of the perfect state, but our focus is on justice. But he makes the point that justice, resides in man's relations to other men, not just in man as an individual. So justice which must be connected to the function of efficient and healthful cooperation. Justice is in one sense the greatest virtue for it is key to making the other virtues work together for the common good. If all the parts are to work together as a whole, each must have on function to excel at. Like the organs of the body, all contribute to the whole, but the eyes only see, the ears only hear. They do not share functions. Using this analogy, justice would be something like the moral mind which guides the body in its activities. Justice, then is the head, at the top of the hierarchy in social terms. When the three virtues work together in orderly fashion within the state, justice is produced. But for justice to be produced, it must come from everyone doing his assigned function under the excellent guidance of the ruling class.
Despite his emphasis of justice as a function of the perfect state, Socrates also deals with justice as a personal issue. He finds that there is a parallel between the organization of the state and the organization of the individual. Just as there are three virtues, Socrates finds three parts in the individual soul -- sensation, emotion, intelligence. The just person, then must have balance between these aspects. Each must function in moderation to contribute to the health of the whole. Appetite and sensation are matters of desire. Desire must be subordinate to reason, or else they will throw the individual out of balance and