Abraham Lincoln once made this argument that white people have no right to enslave black people:
"You say A. is white, and B. is black. It is color, then; the lighter, having the right to enslave the darker? Take care. By this rule, you are to be slave to the first man you meet, with a fairer skin than your own."
If I understand Lincoln correctly, he is arguing that because some white people have darker skin than other white people, skin color is not sufficient justification for slavery. Isn't this a fallacious conceptual slippery slope argument?
Let's say we have three men. The first has only a few dollars, the second is a multi-millionaire, and the third is a billionaire. The third one is richer than the second. But that does not change the fact the the first and second are both rich and the first is not.
In the same way, it might be true that some white people have darker skin than others. But this doesn't change the fact that there are white people and black people (as well as borderline cases.) And it doesn't undermine the premise that white people have the right to enslave black people.
Am I misunderstanding Lincoln? Or is Lincoln's famous argument a bad one?