I think it?s entirely natural for history to progress from a period when slavery is considered legitimate to a period where it isn?t. But I think it would be surprising if history went in the other direction. It seems to me that, throughout history, it is quite common to find things that were regarded as entirely reasonable, ethical, and acceptable in earlier periods regarded with great contempt and disgust in later periods. This is very true of our own traditions. If you read the Bible, say, you find that it is one of the most genocidal texts in our literature. It?s God who orders his chosen people to wipe out the Amalekites down to the last man, woman, and child. People wouldn?t be enjoined to do that sort of thing today; they wouldn?t want to attribute that to their God today. That?s the mark of some sort of moral progress.