the man who is by nature not his own but another man’s is by nature a slave.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“The bulk of this discussion is concerned with slavery—for in antiquity the slaves were always reckoned as part of the family. Slavery is expedient and right, but the slave should be naturally inferior to the master. From birth, some are marked out for subjection, others for rule; the man who is by nature not his own but another man’s is by nature a slave. Slaves should not be Greeks, but of an inferior race with less spirit (1255a and 1330a) Tame animals are better off when ruled by man, and so are those who are naturally inferior when ruled by their superiors.” source