In addition to combining elements from the institutions of democracy and oligarchy, the person wishing to create a lasting polity must pay attention to the economic situation in the city. In Book II of the EthicsAristotle famously establishes the principle that virtue is a mean between two extremes. For example, a soldier who flees before a battle is guilty of the vice of cowardice, while one who charges the enemy singlehandedly, breaking ranks and getting himself killed for no reason, is guilty of the vice of foolhardiness. The soldier who practices the virtue of courage is the one who faces the enemy, moves forward with the rest of the troops in good order, and fights bravely. Courage, then, is a mean between the extremes of cowardice and foolhardiness. The person who has it neither flees from the enemy nor engages in a suicidal and pointless attack but faces the enemy bravely and attacks in the right way.
Aristotle draws a parallel between virtue in individuals and virtue in cities. The city, he says, has three parts: the rich, the poor, and the middle class. Today we would probably believe that it is the rich people who are the most fortunate of those three groups, but this is not Aristotle’s position. He says: “[I]t is evident that in the case of the goods of fortune as well a middling possession is the best of all. For [a man of moderate wealth] is readiest to obey reason, while for one who is [very wealthy or very poor] it is difficult to follow reason. The former sort tend to become arrogant and base on a grand scale, the latter malicious and base in petty ways; and acts of injustice are committed either through arrogance or through malice” (1295b4). A political community that has extremes of wealth and poverty “is a city not of free persons but of slaves and masters, the ones consumed by envy, the others by contempt. Nothing is further removed from affection and from a political partnership” (1295b22). People in the middle class are free from the arrogance that characterizes the rich and the envy that characterizes the poor. And, since members of this class are similar and equal in wealth, they are likely to regard one another as similar and equal generally, and to be willing to rule and be ruled in turn, neither demanding to rule at all times as the wealthy do or trying to avoid ruling as the poor do from their lack of resources. “Thus it is the greatest good fortune for those who are engaged in politics to have a middling and sufficient property, because where some possess very many things and others nothing, either [rule of] the people in its extreme form must come into being, or unmixed oligarchy, or – as a result of both of these excesses – tyranny. For tyranny arises from the most headstrong sort of democracy and from oligarchy, but much less often from the middling sorts [of regime] and those close to them” (1295b39).
There can be an enduring polity only when the middle class is able either to rule on its own or in conjunction with either of the other two groups, for in this way it can moderate their excesses: “Where the multitude of middling persons predominates either over both of the extremities together or over one alone, there a lasting polity is capable of existing” (1296b38). Unfortunately, Aristotle says, this state of affairs almost never exists. Instead, whichever group, rich or poor, is able to achieve power conducts affairs to suit itself rather than considering the interests of the other group: “whichever of the two succeeds in dominating its opponents does not establish a regime that is common or equal, but they grasp for preeminence in the regime as the prize of victory” (1296a29). And as a result, neither group seeks equality but instead each tries to dominate the other, believing that it is the only way to avoid being dominated in turn. This is a recipe for instability, conflict, and ultimately civil war, rather than a lasting regime. For the polity (or any other regime) to last, “the part of the city that wants the regime to continue must be superior to the part not wanting this” in quality and quantity (1296b16). He repeats this in Book V, calling it the “great principle”: “keep watch to ensure that that the multitude wanting the regime is superior to that not wanting it” (1309b16), and in Book VI he discusses how this can be arranged procedurally (VI.3).
The remainder of Book IV focuses on the kinds of authority and offices in the city and how these can be distributed in democratic or oligarchic fashion. We do not need to concern ourselves with these details, but it does show that Aristotle is concerned with particular kinds of flawed regimes and how they can best operate and function in addition to his interest in the best practical government and the best government generally.