I just wanted to elaborate some thoughts I had at the end:
1. If R is correct, then the SC of a proper name is just its reference.
2. If SC of pn is just its referent, then for all S, if S contains a pn with no reference, S is meaningless.
3. It’s not the case that they are all meaningless.
4. So R is false.
So we can consider: "Pegasus is make-believe". Doesn't this turn out to be as meaningless as "Blart mook tuk ne oonto"?
A. I guess that (2) should be denied automatically. "is make-believe" seems to be a fine bit of language. "Pegasus" seems to be referentless, so "Pegasus is make-believe" seems to have a gap:
Pegasus is make-believe.
So I think we can resist that is meaningless. It's just gappy. It seems that a sentence like: "My (said by Wes) son will be a boy" is like this too. I don't think we have a semantic content of my son. I think it would be queer to call this sentence meaningless. Maybe the phrase / name 'My son' and 'Pegasus' is meaningless, in some sense, but this just leaves a gap in the otherwise fine proposition.
B. The less sensible view I was pushing was that we have to object to (2) on the grounds of an ambiguity in 'existence'. The quantification-existence, and the predicate-existence. So if someone claims:
Pegasus doesn't exist
we can ask: Do you mean that we cannot quantify over Pegasus? This seems false. Perhaps you mean that there is nothing such that it meets our criteria for being a concrete, extended thing.
So we might have reason to think that we can quantify over numbers, but they don't exist. Tables and chairs exist. Or maybe particles exist. Or whatever. We just quantify over tables and chairs like we quantify over numbers and Pegasus. We just say of tables and chairs that they exist, while numbers and Pegasus don't.
So why should a sentence with a non-existing thing named in it be meaningless? We can still quantify over the thing, so it is still meaningful. "Blart mook tuk ne oonto" is meaningless. "Pegasus is make-believe" just contains a non-existing-but-quantifiable term which lacks semantic content.
I guess that (B) is like (A), but (A) seems less weird.