A city is self-sufficient in that it contains all that is necessary to lead a good life. The city therefore exists by nature, because it is the natural end of human partnerhips. The city is prior to the individual in importance because the individual apart from the city is not self-sufficient. One who does not need the city and is self-sufficient is either a beast or a god. Man has potential for great good, but without law and virtue can be the worst of all the animals.
Justice belongs to the city.
Some think that mastery is similar to political rule, and some think that slavery is unjust. A slave by nature is one who does not belong to himself by nature. Now we must investigate whether such a person exists by nature or not, and whether or not it advantageous for anyone to be a slave. First it is necessary to examine the proper relationship between body and soul as analogous to different types of rule. The soul rules the body as a master rules a slave and the intellect rule the appetites like a king rules a city.