What does aristotle mean in saying that the city state is natural?

Aristotle lays the foundations for his political theory in Politics book I by arguing that the city-state and political rule are “natural.” The argument begins with a schematic, quasi-historical account of the development of the city-state out of simpler communities. First, individual human beings combined in pairs because they could not exist apart. The male and female joined in order to reproduce, and the master and slave came together for self-preservation. The natural master used his intellect to rule, and the natural slave employed his body to labor. Second, the household arose naturally from these primitive communities in order to serve everyday needs. Third, when several households combined for further needs a village emerged also according to nature. Finally, “the complete community, formed from several villages, is a city-state, which at once attains the limit of self-sufficiency, roughly speaking. It comes to be for the sake of life, and exists for the sake of the good life” 

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