I said you bucked this person's assumption of "all people with morals believe a slave is bad"
Otherwise you could obviously moralize the more complex concept of slavery based on objective reality by observing "more slavery == more slaves"; slavery is bad QED
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Morality is concerned with examining if something is good or bad. So if all people with morals (regardless of what those are) find a concept to be either good or bad... then it might as well exist outside of the purview of morality for the purpose of debate. That's the metaphorical "floor of morality"
My point is normally you need nuanced examples to prove that things "on the floor" can still be moralized, like murder vs murder in self-defense.
What you proved is that there are also people willing to moralize concepts as simple as "slaves are bad" (vs "slavery is bad")
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In doing so you demonstrated why when someone tries to moralize things that I assumed to be "on the floor", or universally bad: I just discard their opinion without consideration.
If you ask me "why do you think slaves are bad", I'll mostly say "I just do, and I don't wish to entertain arguments otherwise". I see it as being pragmatic, but some may consider it fragile.
It's just, I'm a fairly introspective and have had a varied enough life that I trust my intuition that anyone who can't reach the floor of my morality is not worth convincing.