Books II–IV of The Republic are concerned with Plato addressing the makeup of an ideal polis. In The Republic, Socrates is concerned with the two underlying principles of any society: mutual needs and differences in aptitude. Starting from these two principles, Socrates deals with the economic structure of an ideal polis. According to Plato, there are five main economic classes of any polis: producers, merchants, sailors/shipowners, retail traders, and wage earners. Along with the two principles and five economic classes, there are four virtues. The four virtues of a "just city" include, wisdom, courage, moderation, and justice. With all of these principles, classes, and virtues, it was believed that a "just city" (polis) would exist.