Aristotle on Slavery

But is there any one thus intended by nature to be a slave, and for whom such a condition is expedient and right,
or rather is not all slavery a violation of nature?
There is no difficulty in answering this question, on grounds both of reason and of fact.
For that some should rule and others be ruled is a thing not only necessary, but expedient;
from the hour of their birth, some are marked out for subjection, others for rule.

– Aristotle, Politics, 350 B.C.E.

http://classics.mit.edu/Aristotle/politics.1.one.html