In Plato's Utopia, there are three classes: the common people, the soldiers, and the guardians chosen by the legislator. The main problem, as Plato perceives, is to insure that the guardians shall carry out the intention of the legislator. For this purpose the first thing he proposes is education. Education is divided into two parts, music and gymnastics. Each has a wider meaning than at present: 'music' means everything that is in the province of the muses, and 'gymnastics' means everything concerned with physical training fitness. 'Music' is almost as wide as what is now called 'culture', and 'gymnastics' is somewhat wider than what 'athletics' mean in the modern sense. Culture is to be devoted to making men gentlemen, in the sense which, largely owing to Plato, is familiar in England. The Athens of his day was, in one respect, analogous to England in the nineteenth century: there was in each an aristocracy enjoying wealth and social prestige, but having no monopoly of political power and in each the aristocracy had to secure as much power as it could by means of impressive behavior. In Plato's Utopia, however, the aristocracy rules were unchecked. Gravity, decorum and courage seem to be the qualities mainly to be cultivated in education. There is to be a rigid censorship from very early years over the literature to which the young have access and the music they are allowed to hear. Mothers and nurses are to tell their children only authorized stories. Also, there is a censorship of music. The Lydian and Ionian harmonies are to be forbidden, the first because it expresses sorrow, the second because it is relaxed. Only the Dorian (for courage) and the Phrygian (for temperance) are to be allowed. Permissible rhythms must be simple, and such as are expressive of a courageous and harmonious life. As for gymnastics, the training of the body is to be very austere. No one is to eat fish, or meat cooked otherwise than roasted, and there must be no sauces or candies. People brought up on his regimen, he says, will have no need of doctors. Gymnastics applies to the training of mind as well. Up to a certain age, the young are to see no ugliness or vice. But at a suitable moment, they must be exposed to 'enchantments', both in the shape of terrors that must not terrify, and of bad pleasures that must not seduce the will. Only after they have withstood these tests will they be judged fit to be guardians. What is the main topic of the passage?