Concerning slavery,most slaves in Ancient Hellas were not in fact Hellens,except from Sparta,but Skythians.And China had an emperor,right?So,his nationals were not slaves to him?I mean,if a Chinese emperor had taken a decision,then all of his citizens were obligated to obey it,weren't they?
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There was an old Chinese saying "The emperor is so far away and the mountains between he and me are so high" (not very accurate translation). It is often said by the commons even in these days to shrug off the official power of the central government ( the royal court,if in the imperial China). Theretically, yes,all the people in the empire to some extend are emperor's slaves. However, comparing to an emperor lives hundreds of miles away, a Greek slave owner with a whip in his hand obviously more oppressive that the former. More important, the taxes that was imposed on the Chinese peasants by the central government in most periods of the history was relatively low (about 5%-15% out of their output) comparing with the almost-the-whole exploitation that Greek slave owners put upon to their slaves.
In fact, for many times in the ancient Chinese history, what led to the enslavment of the free peasants was the annex of their land by the local landlords or aristocats, who usually took chance of visitation of providence. Then it would finally caused revolution and the overthrow of the existing court. The new central govenment would distribute the land to the peasant as its predecessors had done. And the circle return to the starting point. This circulation happened many times in the Chinese history.