Great Britain banned slavery in 1833 because laborers when motivated by salary were working harder than slaves who were motivated by punishment. It wasn't a coincidence that Britain's Industrial Revolution has started right after that time. Slavery wasn't profitable any more. In the same period, slaves living under Ottoman rule had rights as this: No owner can free a disabled slave as compensation for his (owner's) legal faults. The protection and survival of such a disabled slave was ensured that way.
You can't grasp the truth in your hand, you may only touch it.