Additional thoughts on human obligation and natural law...

From Robert P. George's book, The Clash of Orthodoxies,

Two weeks before Justice Thurgood Marshall resigned from the Supreme Court, I sat in my office in Princeton chatting with then-Judge Clarence Thomas who was in town to address a judicial eductation seminar. I was, at the time, putting together the volume of essays that appeared under the title Natural Law Theory, and our discussion turned to the questions of natural law and civil rights. However much Judge Thomas's confirmation hearings left the public confused about his ideas of natural law and natural rights, he made his position on the issue crystal clear to me: "Those who deny natural law," he said, "cannot get me out of slavery." Of course, Justice Thomas was not suggesting that contemporary historicists or conventionalists - "those who deny natural law" - believe in slavery, and he well knows that some nineteenth-century believers in natural law argued for a natural right to own slaves. His point was that the moral relativism that informs historicist and conventionalist accounts of rights precludes the proponents of such accounts from offering a rational moral argument against slavery. All they can say is that once upon a time in this country white people had the legal right to own black people, and now black people (and, indeed, all people) have the legal right not to be enslaved. For the latter proposition they can cite the Thirteenth Amendment of the United States Constitution. Their historicism and conventionalism preclude them, however, from saying that the Thirteenth Amendment embodies or gives legal force to a moral or natural right not to be enslaved. Under their account, no one would have had objective moral reasons (though some could have had economic or other instrumental or nonmoral reasons) to support the abolition of slavery. Of course, people may have believed (and acted upon their belief) in a natural right not to be enslaved, which provided a moral reason for them to support abolition, but this subjective belief, under the historicist and conventionalist account, lacked a rational ground. That is to say, it was in no sense rationally superior to the belief of other people that no such right existed or, indeed, that they had a right to own slaves. It also follows that neither history nor convention could provide an adequate rational defense against the return in the future of some form of slavery. (emphasis in original)