  The Republic is unquestionably Plato's most elaborate defense of his central ethical doctrines in the middle period. It explores the question what is Justice over the course of ten books, with the aim of demonstrating that the best life for a human is the life devoted to virtue and knowledge, for such a life will result in happiness for the individual. The virtuous person will be one who has all three parts of her soul working in a harmonious fashion, i.e., her reason, cognizant of the Form of the Good and the other Forms, will with the support and consent of her appetite and spirit govern all of her pursuits. The analogue of the virtuous individual with her tripartite soul is the ideal state, the Republic, with its three parts or classes, rulers, warriors and laborers, all working in harmony with one another under the auspices of the ruler, i.e., reason. Having established that justice is psychic harmony at the end of Book Four, Plato next turns to show what it is that the philosopher ruler (or reason in the individual) knows that licenses his claim that they will rule for the benefit of the respective parts and the whole state or person