We pick up where we left off in the Ethics. Aristotle has defined what the Good life consists of and argues that it is impossible to achieve it without the corresponding societal circumstances. This sets up the importance of Politics.
Aristotle proposes a genealogical method to examine political organization: “If one were to see how these things develop naturally from the beginning, one would, in this case as in others, get the best view of them." Starting from a state of nature, he provides an account of how the relationships between man and woman, master and slave, villagers, and citizens of a state came to be out of the desire for better lives.
He draws two important conclusions about politics from this genealogy. Firstly, "man is a political animal" because we are not self-sufficient without a community and nature endowed us with capacities to communicate topics such as justice and fairness; nature never provides capacities that are useless. Secondly, the city-state is natural, the first organization that truly provides humans a basis for happiness, and "It comes to be for the sake of life, and exists for the sake of the good life." In other words, since we construct states for the desire of the Good life, then we can measure and rank different types of states based on how well it can satisfy this goal.
The Best Constitution
So what is the best state that enables it's citizens to pursue the Good life?
Aristotle first describes six types of constitutions across two axis: the number of people who rule, and its ideal and perverted state. Ideal means that the ruler(s) make decisions on the behalf of everyone; perversion means that the ruler(s) only care about their own interests. Kingship and Tyranny are if one person rules ideally or perversely respectively. Aristocracy and Oligopoly are if a group of people rule ideally or perversely respectively. Polity and Democracy are if everyone ruled ideally or perversely respectively.
Since a constitution is an agreement on the rules of organizing society, we must examine how each constitution deals with justice. The first branch of justice, universal justice in the realm of politics manifests as a will to benefit the common good. This idea is taken up again by Acquinas, Rousseau, etc.
The second branch, particular justice, in the political realm, is defined by equality. But it is a specific form of equality: equality for equals and inequality for unequals. The problem with the oligarchic constitution is that because the oligarchs consider themselves unequal and superior in one domain (wealth), they considers themselves superior in all domains (political governance). The problem with the democratic constitution is that because the democrats consider themselves equal in one domain (freedom), they consider themselves equal in all domains (political governance).
Aristotle did not believe in an objectively "better person". We can only compare when we put them up against subjective standards of measurement. “According to this argument every good would have to be commensurable with every other. For if being a certain height counted more, height in general would be in competition with both wealth and freedom."
Thus, it would be arbitrary to distribute political power based on criteria irrelevant to one's ability to rule. "For among flute players equally proficient in the craft, those who are of better birth do not get more or better flutes, since they will not play the flute any better if they do." The state is neither a business enterprise not is its end to promote liberty and equality, thus both the oligarch's and the democrat's constitutions are wrong. The end of the state, as we have already discussed, is so that its citizens may pursue the Good life. Thus, the correct conception of justice is aristocratic. Political power should be given to those who make the greatest contribution to the community through their virtue, property, and freedom.
While monarchy and aristocracy may be theoretically desirable, it faces three practical problems. 1. There rarely comes one or a group of people that can be enlightened rulers. 2. Both are unstable. Monarchy specifically can descend into the worst type of government, Tyranny, incredibly fast. 3. The two constitutions are just too idealistic. For example, Plato's abolishment of private property sounds nice. But it will lead to less pleasure for people who can enjoy their property, and it will lessen each individual's incentive to cultivate the property.
So practically, Aristotle favors polity. It essentially is a blend of oligopoly and democracy, giving equal rights and voting power to certain people above a wealth cut-off. One of the best characteristics of the polity is that it gives power to the middle class. The middle class is in the best place to develop virtue (just as the middling human realm is the best place to obtain nirvana) because they are neither troubled by too much or too less wealth. Because of this, the middle class would elect the most virtuous leader who would act in the interests of all.
Furthermore, Aristotle was one of the first to observe the wisdom of the multitude. He reasoned that the masses were better judges of judicial matters or things like music and art because, much like how a feast provided by many is superior to that provided by one, people would bring in their own perspectives that would lead to a mean of sorts in the overall decision process. Of course this doesn't apply to all forms of decisions.
Thus the polity would have the benefits of the aristocratic constitution as the middle class elected virtuous leaders while benefiting from the wisdom of the multitude in certain decisions.
On Slavery and Freedom
Aristotle believed that certain people, those with no access to reason, are naturally born slaves. For these people, slavery was both advantageous for them and just. People who can't reason, Aristotle argued, are led by their desires and get further and further away from the Good.
This was also fundamental to his critique of democracy: they define freedom badly. Freedom is defined as the opportunity to do whatever one wants rather than to pursue what is truly good. As a result, one becomes enslaved to one's desires.