“If the spirit of their intercourse were still the same after their coming together as it had been when they were living apart,' Aristotle writes, their association can't really be considered a polis, or political community.'A polis is not an association for residence on a common site, or for the sake of preventing mutual injustice and easing exchange.' While these conditions are necessary to a polis, they are not sufficient. 'The end and purpose of a polis is the good life, and the institutions of social life are means to that end.”