A large number of slaves has different effects under various governments. It is no burden in despotic government : the political slavery established in the body of the state makes civil slavery hardly perceptible. Those who are called free men are hardly more free than those who have not that title ; and inasmuch as the latter, as eunuchs, emancipated men, or slaves, have almost all business in hand, the condition of a free man and that of a slave are very similar. It is therefore almost indifferent there whether few or many people live in slavery.
But in moderated states it is very important that there not be too many slaves. There political liberty makes civil liberty precious, and he who is deprived of the latter is also deprived of the former. He sees a happy society of which he is not even a part ; he finds security established for others, and not for himself ; he senses that his master has a mind that can expand, and that his own is constantly forced to humble itself. Nothing places a person closer to the condition of beasts than to see free men all the time and not be free oneself. Such persons are natural enemies of society and in quantity would be dangerous.
We should therefore not be surprised that in moderated governments the state has so often been perturbed by slave revolts, and that this has happened so rarely  in despotic states.