All one needed to do was ask the slaves whether they liked being slaves. Looks like some people did listen to the slaves and they found themselves on the right side of history. So you don't need to consult an Oracle. Listening helps.
Even today, we don't care that much for people's stated preferences. They only matter if they don't conflict with something we regard as wrong, if the person is capable of informed consent, if it doesn't create excessive positive obligations on other people, et cetera.
But Calhoun would have claimed, as implied in the quote upthread, that slaves were essentially incapable of 'informed consent'; they were like children, slavery was better for them regardless of their stated preferences. Calhoun would have considered that as ridiculous as asking a child whether or not they liked their vegetables. Obviously, given our values today and our knowledge of biology, that's not only wrong in every sense but absurd. But when you grow up in a culture of normalized racial inequality, that's what you get.
Not only that, he would have pointed out that many slaves liked their position in life.
I only wish right and wrong were so simple.
Incidentally, one of the proximate causes of the Civil War, and one of the most persuasive voices for abolition was John Brown, who died a martyr after being hanged for taking up arms to free slaves. It was his dedication to the cause even up to the gallows that convinced many people of its rightness. His speech at his trial should be read by everyone.
> we don't care that much for people's stated preferences
I strongly disagree. The foundation of democracy and liberty is self-determination. Civil rights protect their stated preferences and other limits to the law and government. The philosophical foundation of the U.S. is individual liberty.
Calhoun would make lots of claims; people always have justifications. The people doing evil don't stand up and say so; they give rationalizations to sway the gullible and lull others into confusion or not caring. Hitler, Stalin, Putin, and the rest did the same.
