Your understanding of Kaplan is excellent. But for him propositions areOn the Russell-Kaplan view (although Russell himself probably never consistently held such a view), the semantics of singular thought is ‘object dependent’. A thought about Mont Blanc actually contains Mont Blanc – with all its snow fields. That seems absurd for a number of reasons. How can a thought contain a physical object? How can a thought even be internally related to an object when the thought remains the same whether or not the object exists? Does the proposition ‘Etna is a volcano’ cease to be meaningful even if Etna is completely destroyed in an eruption? Surely not. Is the proposition ‘Caesar was a man’ meaningful even though Caesar does not exist? Surely it is.
Russellian, not Fregean. If 'Mt Blanc is snow-covered' expresses a Russellian
proposition, then Mt Blanc itself, that massive physical object, together with
its snow fields and subterranean gopher tunnels, etc. etc. is a constituent of
the proposition. But I can't swallow the Russellian view; how could a finite
mind wrap itself around such a monstrous object?
The problem is to explain individuation. The name ‘Mont Blanc’ individuates. It tells us which large object is the subject of the proposition 'Mt Blanc is snow-covered'. It distinguishes that object from other similar objects such as Ben Nevis, Kilimanjaro, Everest and the rest. How do we give an object-independent account of the semantics of individuation?
The key, as I shall argue, is to explain the semantics of fictional names. The name ‘Aeneas’ distinguishes a character in Virgil’s epic from all the other characters mentioned in the Aeneid. If we can explain how that is possible, it will be possible to explain individuation in general, I believe.