Aristotle states that the type of government that is simultaneously most practical and most realistic is a politeia, or constitutional government, in which power rests in the hands of a strong middle class. Drawing on a major theme of the ##Nicomachean Ethics##, Aristotle asserts that a life of virtue consists of finding the mean between two extremes. In the case of politics, the middle class is the mean between the rich and the poor. In a city that consists only of rich and poor, the rich will feel contempt for the poor and the poor will feel hatred and envy for the rich. The spirit of friendship that is so essential to a healthy city is made possible only by a strong middle class that holds no grudges and is not prone to factionalism. Aristotle laments, however, that a strong middle class rarely develops: it is possible neither in small cities, nor in the superpowers of Athens and Sparta, which have encouraged democracy and oligarchy respectively.
Aristotle addresses the question of which type of constitution is best suited to which sort of state. The fundamental principle is that the part of the city that wants a certain constitution must be stronger than the part of the city that opposes it. Where the nobility, wealth, and culture of the rich outweigh the sheer numbers of the poor, an oligarchy is desirable, and where the numbers of the poor outweigh the trappings of the rich, a democracy is desirable. When the middle class outweighs both of these classes, a politeia, is desirable. The middle class serves as a good arbitrator and, so, should always be a party to the constitution.
Aristotle points out that oligarchies fine the rich for not participating in the assembly, public office, law courts, army, and athletics. The rich are thus encouraged to participate while the poor have no motivation to do so. Democracies practice the contrary, paying the poor but not the rich for their participation in civic activities. A mean between democracy and oligarchy would thus have to fine the rich and reward the poor in order to encourage both to participate. Aristotle recommends, however, that some minor property qualification, like the possession of arms, be required for those wishing to participate in government.
Aristotle considers the three elements of civic government: the deliberative, the executive, and the judicial. The deliberative element deals with public matters such as foreign policy, the enacting of laws, judicial cases in which a severe penalty is involved, and the appointment of public officials. The executive element holds public order and takes responsibility for governing and issuing commands. The judicial element passes rulings on matters of private and public interest. Generally, a democracy permits all people to be involved in these matters, an oligarchy permits only a select group to be involved, and both constitutional government and aristocracy permit all to be involved in some matters and only a select group in others.
Executive elements vary greatly from constitution to constitution, according primarily to four factors: the number of offices, the function of each office, the length of tenure in a given office, and the method by which officers are appointed. The method of appointment may vary depending on who does the appointing, who is eligible to be appointed, and what method is used to appoint (whether by election, by lot, or by a combination of the two.
Aristotle summarizes his Nicomachean Ethics: "the truly happy life is one of goodness lived in freedom from impediments and goodness consists in a mean," and he applies this concept to government. Just as the idea that everything requires moderation is crucial to Aristotle's ethics, so too is it integral to his politics, as he argues the merit of empowering the middle class. Rather than presenting a vague, theoretical suggestion, Aristotle backs up his argument with practical considerations: the middle class is the least susceptible to factionalism, to self-interest, and to hatred of other classes of society. The polis is fundamentally a koinonia, a shared venture in which everyone participates in order to achieve a common good. Moreover, Greek civic life greatly esteemed the virtue of friendship (and cooperative striving). Thus, the middle class, the least likely to feel resentment toward other classes, embodies this all-important virtue and is hence the best suited for government.