Well, it's wrong to the extent that something can be morally wrong. Saying "I believe X is wrong" is hardly meaningless. I wouldn't want to be disheartening. We'll... at least not TOO disheartening.
I don't mean to sound disheartened ;) My point was that even if slavery cannot be proven to be objectively wrong, the only thing that matters for me is that I think it is wrong due to a belief that follows from my assertion that all sentient beings are deserving of dignity, and that slavery is by definition an indignity. If circumstances were to change so that I no longer believed that every sentient being is deserving of dignity, my stance on slavery would follow suit... I don't see that happening anytime soon though.
Not having a sense of dignity=/= not deserving of dignity. A demented and incontinent elderly human is certainly not dignified, but they are certainly deserving of dignity.
No, that was not my intent. Sentient means (according to Merriam-Webster): "responsive to or conscious of sense impressions" Snails are sentient, but I'm not sure how much dignity we can meaningfully bestow on them. My wife makes a living assisting "demented and incontinent elderly human"s -- so I'm not inclined to disagree on that point. I used to use the word "sapient" for what I think you're trying to capture, but that term has problems too. I've enjoyed this exchange, BTW. Thanks!
Likewise! It's been a pleasure exploring this blurry line of ethics, upon which the snail balances :)