Aristotle's mixed constitution springs from a recognition of sectional interests in the state. For the proper relation between the claims of "virtue," wealth, and numbers is to be based not upon their relative importance in the good life, but upon the strength of the parties which they represent. The mixed constitution is practicable in a state where the middle class is strong, as only the middle class can mediate between the rich and the poor. The mixed constitution will be stable if it represents the actual balance of power between different classes in the state. When we come to Aristotle's analysis of existing constitutions, we find that while he regards them as imperfect approximations to the ideal, he also thinks of them as the result of the struggle between classes. Democracy, he explains, is the government not of the many but of the poor; oligarchy a government not of the few but of the rich. And each class is thought of, not as trying to express an ideal, but as struggling to acquire power or maintain its position. If ever the class existed in unredeemed nakedness, it was in the Greek cities of the fourth century, and its existence is abundantly recognised by Aristotle. His account of the causes of revolutions in Book V. shows how far were the existing states of Greece from the ideal with which he starts. His analysis of the facts forces him to look upon them as the scene of struggling factions. The causes of revolutions are not described as primarily changes in the conception of the common good, but changes in the military or economic power of the several classes in the state. The aim which he sets before oligarchs or democracies is not the good life, but simple stability or permanence of the existing constitution.