Aristotle defines the polis, or city, as a koinonia, or political association, and he asserts that all such associations, like all deliberate human acts, are formed with the aim of achieving some good. He adds that political association is the most sovereign form of association since it incorporates all other forms of association and aims at the highest good.
The different kinds of associations that exist are founded on different kinds of relationships. The basic unit of association is the household, the next is the village, and the ultimate association is the city, toward which end humans, seeking to attain the highest quality of life, naturally move. Aristotle concludes, "man is by nature a political animal." Only as part of a city can people fully realize their nature; separated from the city, they are worse than animals.
Aristotle identifies the three kinds of relationships that make up the household: master-slave; husband-wife; and parent-child. He also identifies a fourth element of the household, which he calls the "art of acquisition."
Aristotle views slaves as the means by which the master secures his livelihood. He defends slavery by noting that nature generally consists of ruling and ruled elements: some people are slaves by nature, while others are masters by nature. It is thus unjust to enslave, through war or other means, those who are not slaves by nature. Though being suited to mastery or slavery is generally inherited, slavery is just only when the rule of master over slave is beneficial for both parties.
Aristotle likens the relationship between master and slave to that between soul and body: the master possesses rational, commanding powers, while the slave, lacking these, is fit only to carry out menial duties. He also likens the relationship between master and slave to that between a monarch and his people and that between a statesman and free citizens.
Aristotle examines the art of acquisition, which pertains to the satisfaction of basic needs, distinguishing between natural and unnatural acquisition. Different people go about satisfying these needs in different ways, depending on their mode of life: some are farmers, some hunter-gatherers, and some pirates or freebooters, etc. This securing of food, shelter, and other necessities is called natural acquisition because it is an indispensable part of the management of a household.