many people are meant to be slaves is also a fact -- it is not like everyone is gifted cerebrally, or even blessed to be in an environment where, by the time they are adults, have been shown a good formula for life; so consequently a section of the society is destined to be just slaves, it just needs the presence of a manipulator.
Aristotle, being an endless questioning guy he was, might have figured this out, although i'm not sure.