Hah, looks like someone went through and upvoted all your posts in the conversation while downvoting mine. Relativism has at least one anti-fan :P
I didn't understand your last reply, but I'd still like to ask you a favor: imagine what the universe would look like if there weren't any particular best morality, only moralities that were best by some individual's standard, which nobody else was under any particular cognitive necessity to accept. All the electrons would stay in their orbitals, things would look the same, but inside agents would just do what they did for their own reasons and not for others.
Okay, thanks.
My last post was a question (now edited). You were tacitly assuming that being able to predict is what matters, that non predictive theories can be disregarded. I was questioning that being able to predict matters more than morality (in fact, I was doubting that anything does). I think the does-it-predict test is flawed in that sense.
I also think the other tacit assumption, that morality is non predictive is false. If you act on your morality, it will predict what you observations...whether they are eventually of a death row cell, or a the receipt of a nob...
Derek Parfit has published his second book, "On What Matters". Here are reviews by Tyler Cowen and Peter Singer.