In Book 3, Aristotle proposes that studying the nature of both a city and its citizens will provide guidance into what type of political constitution will most benefit all. He argues that it is not enough to look at a city as a single entity, because a city is made up of many parts—citizens included. The philosopher carefully defines a citizen and determines that a citizen is someone who holds an office, excluding anyone else living in the same city as non-citizens. Citizens must also be born of other citizens from both their mother and father’s parentage. Aristotle defines the term based upon whether the individual’s ancestors were a part of developing the constitution for the city. If so, they are naturalized citizens. When a constitution is amended, potentially changing who qualifies for citizenship, it must be followed.

However, the question of citizenship leads Aristotle to another question: “When can a given act be considered an act of the city or polis?” (88). The philosopher asserts that a city’s credit is called into question when the government alters its original nature, thereby losing its political identity. He compares a city to a river which has a specific identity even though it is always changing.