So Roman slavery and the freedom of that was actually one way that a few people in the Ancient World recognized some kind of social mobility, which was very rare in the Roman Empire.

所以罗马奴隶制以及奴隶释放制,被古代部分人认为是,某种社会阶级流动，在罗马帝国很少见。

耶鲁公开课 - 新约课程节选

That a free laborer is a mobile laborer, especially in a society like the United States that had this thing called The West, the limitless apparently to them anyway--boundless West.

自由劳动者是流动的,尤其处在美国这样的社会,有他们所说的广袤的西部,特别对他们来说,无边际的西部

耶鲁公开课 - 美国内战与重建课程节选