What was Plato's political theories?

The Republic, Plato's major political work, is concerned with the question of justice and therefore with the questions "what is a just state" and "who is a just individual?"

The ideal state, according to Plato, is composed of three classes. The economic structure of the state is maintained by the merchant class. Security needs are met by the military class, and political leadership is provided by the philosopher-kings. A particular person's class is determined by an educational process that begins at birth and proceeds until that person has reached the maximum level of education compatible with interest and ability. Those who complete the entire educational process become philosopher-kings. They are the ones whose minds have been so developed that they are able to grasp the Forms and, therefore, to make the wisest decisions. Indeed, Plato's ideal educational system is primarily structured so as to produce philosopher-kings.

Plato associates the traditional Greek virtues with the class structure of the ideal state. Temperance is the unique virtue of the artisan class; courage is the virtue peculiar to the military class; and wisdom characterizes the rulers. Justice, the fourth virtue, characterizes society as a whole. The just state is one in which each class performs its own function well without infringing on the activities of the other classes.

Plato divides the human soul into three parts: the rational part, the will, and the appetites. The just person is the one in whom the rational element, supported by the will, controls the appetites. An obvious analogy exists here with the threefold class structure of the state, in which the enlightened philosopher-kings, supported by the soldiers, govern the rest of society.