But slavery is another matter -- the most vicious habit humans fall into and the hardest to break. It starts up in every new land and it's terribly hard to root out. After a culture falls ill of it, it gets rooted in the economic system and laws, in men's habits and attitudes. You abolish it; you drive it underground -- there it lurks, ready to spring up again, in the minds of people who think it is their 'natural' right to own other people. You can't reason with them; you can kill them but you can't change their minds.
Robert A. Heinlein
Citizen of the Galaxy