In the unbounded reaches of colonised outer space, Mr. X, as we all know, took Mamdami as his slave. 
from a chapter in Social Problems
Suppose, however, that instead of taking Mamdami as his slave, Mr. X had welcomed him as a man and a brother; had read him a Declaration of Independence, an Emancipation Proclamation and a Fifteenth Amendment, and informed him that he was a free and independent citizen, entitled to vote and hold office; but had at the same time also informed him that that particular space island was his (Mr. X's) private and exclusive property.
What would have been the difference?
Since without superluminal travel capability Mamdami could not fly up into space nor traverse to the next space island, since if he lived at all he must live on the space island, he would have been in one case as much a slave as in the other. Mr. X's ownership of the space island would be equivalent to his ownership of Mamdami.