Adam Smith. An inquiry into the nature and causes of the wealth of Nations (№ 347138)

The experience of all ages and Nations, I think, shows that the work done by slaves, though she seemed to consume only their maintenance, is more expensive elsewhere. People who are not eligible to acquire absolutely no ownership, may be interested only in the fact that there are perhaps more and work less possible. Only violent measures, not his own interest, you can get it to work more than what is sufficient to pay for its own existence.