The answer to this question can be found early in chapter one of Uncle Tom's Cabin by Harriet Beecher Stowe. Mr. Arthur Shelby owns a farm in Kentucky and, though he does own slaves, he treats them better than most.
He has an aversion to the buying and selling of slaves, and he never thinks of himself as being a slave trader. Of course this is a kind of delusion, since he owns slaves and therefore does believe that humans can be bought and owned. We learn that humane, to him, means keeping families together, which is admittedly much more than most slave owners were willing to do. In fact, many slave owners preferred to separate families intentionally either as punishment or as a deterrent to problems of other kinds. Mr. Shelby does not have clean hands in the matter of slavery; however, he treats his slaves humanely compared to so many others, and that is something.
Unfortunately, Mr. Shelby is not a good manager and soon his farm is in financial trouble. Of course one of the first things he does to make some money is sell his most valuable assets (Eliza, Harry, and Uncle Tom) to his most significant creditor, though he does it reluctantly--or perhaps uncomfortably. He says:
"I would rather not sell him," said Mr. Shelby, thoughtfully; "the fact is, sir, I'm a humane man, and I hate to take the boy from his mother, sir."
The point is that Mr. Shelby, because he is a white slave owner, gets to decide what is humane and what is not. It is clear that he simply wants to avoid the unpleasantness of seeing the consequences of his actions and he calls that being humane.
Mr. Shelby's presence in the novel is short-lived and his claims and motives are not entirely pure, but he will serve as a sharp contrast to some of the other white slave owners in the novel who could not be considered humane by any definition. Watch for them.
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