The most insidious effect of Roman slavery, however, is that through Roman law, it has come to play havoc with our idea of human freedom. The meaning of the Roman word libertas itself changed dramatically over time. As everywhere in the ancient world, to be "free" meant, first and foremost not to be a slave. Since slavery means above all the annihilation of social ties and the ability to form them, freedom meant the capacity to make and maintain moral commitments to others. The English word for "free" for instance, is derived from a German root meaning "friend", since to be free meant to be able to make friends, to keep promises, to live within a community of equals. This is why freed slaves in Rome became citizens: to be free, by definition meant to be anchored in a civic community, with all the rights and responsibilities that this entailed. By the second century AD, however, this had begun to change. The jurists gradually redefined libertas until it became almost indistinguishable from the power of the master. It was the right to do absolutely anything, with the exception, again, of all those things one could not do.

If history shows anything, it is that there's no better way to justify relations founded on violence, to make such relations seem moral, than by reframing them in the language of debt - above all, because it immediately makes it seem that it's the victim who's doing something wrong.