[Abraham] Lincoln provided another example of principled moral reasoning in assessing the sorts of arguments that his contemporaries put forth to defend the enslavement of black people by white 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.
You do not mean color exactly?—You mean the whites are intellectually the superiors of the blacks, and therefore, have the right to enslave them? Take care again. By this rule, you are to be slave to the first man you meet, with an intellect superior to your own.
But, you say, it is a question of interest; and, if you can make it your interest, you have the right to enslave another. Very well. And if he can make it his interest, he has the right to enslave you.
Lincoln was making the point that if one were to apply the arguments for slavery to the prospective and current slave-owners, whites, then one has put in place premises that may be employed by the government to undermine the rights of all human beings under its authority. For the premises of the pro-slavery arguments contain propositions that appeal to degreed properties that carry no moral weight—color, intellect, and interest—when it comes to the question of human equality. Lincoln’s assessment of these arguments is an impressive example of moral reasoning.
— Francis Beckwith, from A Summit Reader, Michael Bauman and Francis Beckwith, editors, p. 76