Parfit isn’t quite a non-naturalist (or rather, he’s a very unconventional kind of non-naturalist, not a Platonist) - he’s a ‘quietist’. Essentially, it’s the view that there are normative facts, they aren’t natural facts, but we don’t feel the need to say what category they fall into metaphysically, or that such a question is meaningless.
This is something substantive that can be said—out of every major attempt to get at a universal ethics that has in fact been attempted in history: what produces the best outcome, what can you will to be a universal law, what would we all agree on, seem to produce really similar answers.
The particular convergence arguments given by Parfit and Hare are a lot more complex, I can’t speak to their overall validity. If we thought they were valid then we’d be seeing the entire mountain precisely. Since they just seem quite persuasive, we’re seeing the vague outline of something through the fog, but that’s not the same as just spotting a few free-floating rocks.
Now, run through these same convergence arguments but for decision theory and utility theory, and you have a far stronger conclusion. there might be a bit of haze at the top of that mountain, but we can clearly see which way the slope is headed.
This is why I think that ethical realism should be seen as plausible and realism about some normative facts, like epistemic facts, should be seen as more plausible still. There is some regularity here in need of explanation, and it seems somewhat more natural on the realist framework.
Parfit isn’t quite a non-naturalist (or rather, he’s a very unconventional kind of non-naturalist, not a Platonist) - he’s a ‘quietist’. Essentially, it’s the view that there are normative facts, they aren’t natural facts, but we don’t feel the need to say what category they fall into metaphysically, or that such a question is meaningless.
I think a variant of that, where we say ‘we don’t currently have a clear idea what they are, just some hints that they exist because of normative convergence, and the internal contradictions of other views’ is plausible: