Empathy, it works.

How do you know slavery is wrong?

- It is impossible to empathize being a slave: I was never a slave, how about you?

A rational person would not enter a contract where they are the property of another person. Therefore, slavery is wrong. And yes, I can empathize being a slave. The more you learn about a subject, the more you learn about what actually happened, the more you understand their circumstances and what they had to go through. Can you empathize being tortured? If you never have gone through it, how would you know its not a pleasurable experience? No offense, but your argument is dumb. I don't need to experience being dangled over a barrel of fire while being prodded with a hook to know that it causes suffering. The same could be said for slavery.