This is why, Plato argues, the philosopher is best qualified to become actively engaged in the government of the state, because through the application of a balanced system of justice and sound moral principles grounded in the supremacy of human reason his education and training will have equipped him to do so .
Their discipline and their reverence for the harmony of the Forms will also effectively guarantee that they no longer possess the desires impelling them to act unjustly or antisocially, or to seek worldly advantages over other human beings.
It is therefore a theory or concept which leads Plato implicitly, if not explicitly, into basic considerations concerning the nature of the moral life, which must coincide with the good life, because the good life is essentially a just life.
We may therefore act in accordance with only the appearance of justice unless we have a clear and distinct understanding of the reality and actuality of justice that is the Form of justice .
Hence, we may conclude, from Plato, that the goodness of a thing the civic community, the soul or the health of the body consists in a kind of order or proportion which is appropriate to the thing it is.
Justice is, for Plato, at once a part of human virtue and the bond, which joins man together in society. It is the identical quality that makes good and social . Justice is an order and duty of the parts of the soul, it is to the soul as health is to the body. Plato says that justice is not mere strength, but it is a harmonious strength. Justice is not the right of the stronger but the effective harmony of the whole. All moral conceptions revolve about the good of the whole-individual as well as social.
Corresponding to these three elements in human nature there are three classes in the social organism-Philosopher class or the ruling class which is the representative of reason; auxiliaries, a class of warriors and defenders of the country is the representative of spirit; and the appetite instinct of the community which consists of farmers, artisans and are the lowest rung of the ladder. Thus, weaving a web between the human organism and the social organism, Plato asserts that functional specialization demands from every social class to specialize itself in the station of life allotted to it. Justice, therefore to Plato is like a manuscript which exists in two copies, and one of these is larger than the other. It exists both in the individual and the society. But it exists on a larger scale and in more visible form in the society. Individually "justice is a 'human virtue' that makes a man self consistent and good: Socially, justice is a social consciousness that makes a society internally harmonious and good."
Thus, after criticising the conventional ideas of justice presented differently by Cephalus, Polymarchus, Thrasymachus and Glaucon, Plato now gives us his own theory of justice. Plato strikes an analogy between the human organism on the one hand and social organism on the other. Human organism according to Plato contains three elements-Reason, Spirit and Appetite. An individual is just when each part of his or her soul performs its functions without interfering with those of other elements. For example, the reason should rule on behalf of the entire soul with wisdom and forethought. The element of spirit will sub-ordinate itself to the rule of reason. Those two elements are brought into harmony by combination of mental and bodily training. They are set in command over the appetites which form the greater part of man's soul. Therefore, the reason and spirit have to control these appetites which are likely to grow on the bodily pleasures. These appetites should not be allowed, to enslave the other elements and usurp the dominion to which they have no right. When all the three agree that among them the reason alone should rule, there is justice within the individual.
Plato realises that all theories propounded by Cephalus, Thrasymachus and Glaucon, contained one common element. That one common element was that all the them treated justice as something external "an accomplishment, an importation, or a convention, they have, none of them carried it into the soul or considered it in the place of its habitation." Plato prove that justice does not depend upon a chance, convention or upon external force. It is the right condition of the human soul by the very nature of man when seen in the fullness of his environment. It is in this way that Plato condemned the position taken by Glaucon that justice is something which is external. According to Plato, it is internal as it resides in the human soul. "It is now regarded as an inward grace and its understanding is shown to involve a study of the inner man." It is, therefore, natural and no artificial. It is therefore, not born of fear of the weak but of the longing of the human soul to do a duty according to its nature.
At this juncture the new point of view is stated by Glaucon and he put Forward a form of what was later to be known as a social contract theory, arguing we are only moral because, it pays us or we have to be. Glaucon describes the historical evolution of the society where justice as a necessity had become the shield of the weaker. In the primitive stage of society without law and government, man was free to do whatever he likes. So the stronger few enjoyed the life at the sufferance of the weaker many. The weaker, however, realised that they suffered more injustice. Faced with this situation they came to an agreement and instituted law and government through a sort of social contract and preached the philosophy of just. Therefore, justice in this way something artificial and unnatural. It is the "product of convention". It is through this artificial rule of justice and law that the natural selfishness of man is chained. A dictate of the weaker many, for the interest of the weaker many, as against the natural and superior power of the stronger few.
Justice implies superior character and intelligence while injustice means deficiency in both respects. Therefore, just men are superior in character and intelligence and are more effective in action. As injustice implies ignorance, stupidity and badness, It cannot be superior in character and intelligence. A just man is wiser because he acknowledges the principle of limit.