Why did the Greeks consider it to be a unique institution

The polis was what they called the small, independent Greek city-states during the Classical period. It was a unique institution in that governance was not ruled by a monarchy as was usual with traditional states of the time, but a group of men who were either elected or chosen from among the nobility and later on, from among all free citizens. Membership was determined by birth although citizenship by naturalization was oftentimes admitted. Slaves, foreigners and women who live within the polis were not involved in government and not allowed to own land. In fact, slave labor was encouraged because they formed the backbone of the agricultural economy. The polis was usually bounded by walls and citadels and within was an agora or marketplace, the center of the city trade, and numerous temples. Most citizens live within but some members resided in the countryside.The modern-day equivalent of the polis would be any city in Western Europe or America. Like the classical polis, modern cities are political states, the citizens register with city hall or pays residential taxes, and there would be foreigners or residents coming from other places who would be subjected to the laws and regulations of the city. Its leaders are also elected or appointed. Unlike the polis, however, affiliation with a modern city is only political and not religious. Also, the hierarchies in modern cities are not ascribed by birth but by one’s economic status. Most importantly, women and residents coming from other cities already have political rights and could even join in the government. There are open borders now instead of walled fortification between cities. Slavery has been banished and the basis of the economy has become as complex as the social structure of the city. Finally, although the city government can make its own ordinances and budget, the city itself is not dependent in that it is governed and part of a larger unit, the national government.