Ingvar, (01)
My position and Pat's position are compatible and for the same
reasons: we both have propositions, but we admit that their
truth values in different contexts can be different. (02)
IJ> Implicitly, I take it, it then also introduces a new notion
> of 'proposition-in-a-context' that ought not to be conflated
> with the traditional philosophical and logical term 'proposition'.
> A proposition-in-a-context can change truthvalue, a proposition
> cannot. Stated in this way, I have no objections. (03)
You can add the qualifier "-in-a-context", if you like, but then
you also need to add the qualifier "-in-a-world" to accommodate
propositions in Kripke or Montague semantics. (04)
Quine's position is consistent with his anti-modal stance: (05)
WVOQ> "The newly remarked purpose of propositions as surrogate
> truth vehicles demands that propositions resist variation in
> truth value, ..." (06)
If you only allow one world or context, then I would agree that
a proposition would be restricted to a single truth value. (07)
But in modal logics (with a semantics based on either Kripke's
possible worlds or Dunn's laws and facts), the same proposition
with the same referents can have different truth values in
different worlds. (08)
A question does arise about what "same referents" means, and
that is why I quoted Whitehead's point about the fallibility
of reference. But various theories of possible worlds have
rigid designators that allow names in different worlds to
designate the "same" individual. (09)
As I said, I have serious concerns about the ontology of
possible worlds, but I am happy with a model theory that
has contexts that can be interpreted either way: (010)
- Kripke style: Possible worlds with rigid designators. (011)
- Dunn style: Collections of propositions that describe
possible alternatives and with the equivalent of rigid
designators that can fix the referents of some or all
of the identifiers in a proposition. (012)
I outlined a version of such a model theory in my laws.htm
paper, but there are many other versions of modal models
with similar properties. (013)
In short, I would agree that if there is only one world or
context (as in Quine's logics), then a given proposition
cannot have more than one truth value. But any version of
modal logic with a semantics of multiple worlds or contexts
must admit that a given proposition may have different truth
values in different worlds or contexts. (014)
John (015)
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