Why people behave justly? Is it because of the fear of punishment, or the fear of divine reprisal? Or because behaving just is good to do so? Or is it because the weaker one is scared into submission in the name of law, due to the fear of the stronger? Is justice good in itself, not considering its rewards and punishments? How can justice be defined? Plato sets out to answer these questions in his dialogue, Republic. He wants to define justice in such a way as to show that justice is valuable in and of itself. His objective was to show that justice is worthwhile. Just action is good in itself, and one must not engage in just activity only to gain immediate advantage from it. He succeeds to meet these two challenges by defining justice that appeals to human psychology.
Before he can prove that justice is a virtue in itself, Plato must first state what justice is, and instead of defining justice as a set of behavioral norms, Plato identifies justice as structural one.
According to him, political justice resides in the structure of the society and individual justice resides in the structure of the soul. The just structure of the society is summed up by the principle of specialization which means that each member of society must play the role for which his nature best suits him. He considers an ideal social order to be consists of three main classes of people; guardians (rulers), defenders (soldiers and warriors) and artisans (farmers, craftsmen, etc.). A society is just when relations between these three classes are balanced. Each one of the three classes must perform its appropriate, well defined function in the society and should not interfere in the roles of the other two classes. Guardians must rule,