I'm not sure whether he really is, but it sounds as though Mackie is implicitly conflating statements along the lines of 'this particular act was wrong' with statements along the lines of 'acts of this type are wrong'. Whether the former type of statement is true or false (if it has a truth value at all) depends on the characteristics of the act it refers to, a particular act that happens in the world. By contrast, the latter type's truth value (again, if it has one at all) is independent of whether any act of that type has ever happened or ever will. If you lump the two types together, and say that the lump has to either be about the world or not, then of course you get weird conclusions. But neither alone is weird, at least not in that way. -- I think that if there are things like Platonic forms of predicates at all, then there are equally-real, equally-Platonic forms of binary predicates. So you can perfectly well have a form of Rudeness. The pair (keeping your shoes on indoors, Japan as of 2023) partakes in the form -- again, this is if partaking in a form makes sense in the first place -- even if the pair (keeping your shoes on indoors, the US as of 2023) does not. Why would anyone try to insist that the rudeness-or-not of a partially-described action must be the same for all actions that fit the partial description, when the partial description leaves out all of the features that could make an action rude? That makes exactly as much sense as insisting that a moral evaluation of "swinging your hand" must always be the same, regardless of whether your neighbor's face is in its path. If we can't apply some mode of evaluation to rudeness, then we certainly can't apply it to assault. -- If you get into the TARDIS and examine some aliens, and try to figure out whether one of them is acting out of cruelty, you have to know enough about them to be able to tell whether they're doing surgery with some alien medical technology. So the "without knowing anything about them" criterion is unreasonable, unless we're willing to condemn surgeons as cruel.
