“those who are as different [from other men] as the soul from the body or man from beast – and they are in this state if their work is the use of the body, and if this is the best that can come from them – are slaves by nature….For he is a slave by nature who is capable of belonging to another – which is also why he belongs to another”
The question is how does one know that one person is a “natural” slave. Firstly, Aristotle opposes slavery “by law” such as the slavery of the conquered, or the slavery of children of slaves. Which notably was a very progressive view at the time. I have held for a while that legalized slavery would obviate and make impossible true consensual slavery. This is so because only when slavery is always and only entered into freely can one truly know that the person entering that state of being feels it “natural” for him or herself to be in that state. By the same token, Mastery, as something not freely entered into is a responsibility that many if not most would refuse. Those who do freely feel that it is their appropriate state, and who want and thrive on the responsibility, can only choose to do so in a situation where they never and in no way are asked to take that responsibility as a matter of law, tradition or inheritance.