And by the 1820s and 1830s the American South became what I think you could safely say was the fifth slave society in human history; maybe the sixth. This is debatable.

自十九世纪二三十年代起,美国南方,绝对成为了,人类历史上的第五个奴隶社会,也可能是第六个,这仍然处于争议中

耶鲁公开课 - 美国内战与重建课程节选

Why are we having a debate over Social Security?

为什么我们对社会保障存在争议呢

耶鲁公开课 - 美国内战与重建课程节选