Nick,

Fair enough, but consider the counterfactual case: suppose we believed that there were some fact about a person that would permit enslaving that person, but learned that the set of people to whom those facts applied was the null set. It seems like that would still represent moral progress in some sense.

Perhaps not the sort that Eliezer is talking about, though. But I'm not sure that the two can be cleanly separated. Consider slavery again, or the equality of humanity in general. Much of the moral movement there can be seen as changing interpretations of Christianity -- that is, people thought the Bible justified slavery, then they stopped thinking that. Is that a purely moral change? Or is that a better interpretation of a body of religious thought?