P1. Moral realism is intelligible -- even Mackie grants that moral claims are “not meaningless but false.”
P2. To be truly meaningful, there must be some (metaphysically) possible property (whether or not it is actually instantiated) that moral claims are about.
C1: So, moral properties are metaphysically possible (and some fundamental moral claims are possibly true).
P3: Fundamental moral claims are non-contingent: they are necessarily true if actually true, and necessarily false if actually false.
C2: So some moral claims are actually true. There are actually instantiated moral properties.
I'm guessing that P2 will be the most controversial premise here. But it seems at least prima facie plausible to me. Can you suggest any obvious counterexamples? Or other considerations that might count either for or against this key premise?
Also, has an argument along these lines been proposed before? References welcome!