Any Nihilists here?

[Belacqua]

That's not testing a moral claim, that's testing a physical claim???

Let's say that someone tells you causing harm is bad.  How can we test that?

Yes, you're right. If we prove that wanking causes hairy palms, that is a physical demonstration, not a moral one. It doesn't address whether wanking is bad. If you live in Siberia, hairy palms may be a positive good. 

There are a couple of ways to think of this, maybe. One is a language analysis: the word "harm" includes badness in its meaning. There is no good harm. So saying that harm is bad is tautological. This doesn't prove anything beyond the language, though. 

We could express these as "if/then" statements. So for example "when the guy broke my leg and put me in the hospital, it harmed my chances of making the soccer team." If your goal is making the soccer team, then getting your leg broken interferes with your chances. So it becomes a bit utilitarian and practical. 

There will be cases where the goal is more obviously moral than making the soccer team. If you want the starving villagers to be fed, then it's bad to blow up the grain shipment. I think it would require some elaborate arguments to demonstrate that feeding the villagers is bad, though I suppose it's possible. There is no scientific proof. 

Then in general cases there are if/then statements which we can probably all agree on. So for example, "If you want a baby to grow up healthy and strong and have a good chance at happiness in life, then it is bad to chop off its arms and legs." There is no scientific proof that it's good for a baby to grow up with a chance at happiness, but it's something I think sane people would agree on. 

So in extreme cases I think despite a lack of proof, we can still say that any sane person would agree. Is it bad to nuke Paris and kill everybody there simply for my entertainment? I see no way that a sane person could make an argument that it is good. 

Right at the beginning of the Nicomachean Ethics, Aristotle addresses the difficulty of having certainty in ethical questions. 

We must not expect more precision than the subject-matter admits of. The student should have reached years of discretion

Our discussion will be adequate if it has as much clearness as the subject-matter admits of, for precision is not to be sought for alike in all discussions, any more than in all the products of the crafts.

So to refer to your other thread, we can state the width of a table with confidence and precision. But we can't expect the same level of confidence and precision in moral statements. But this doesn't mean that moral statements are meaningless, only that in most cases we have to live with more fuzziness. 

We must be content, then, in speaking of such subjects and with such premisses to indicate the truth roughly and in outline, and in speaking about things which are only for the most part true, and with premisses of the same kind, to reach conclusions that are no better. In the same spirit, therefore, should each type of statement be received; for it is the mark of an educated man to look for precision in each class of things just so far as the nature of the subject admits...

emphasis added