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Ian,
you solicited thoughts on " ... the Quine-Putnam indispensability argument which basically says that if we believe in the concreteness of the physical theories described by mathematical objects then we also ought to believe in the concreteness of those mathematical objects themselves."
"Concrete" as opposed to what? Abstract? Then is there such a thing as "concrete" language? That is, do symbols stand for themselves only, or are they independent of the objects for which they stand?
My view: language is independent of meaning. It follows therefore that physical (mathematical) theories are no more concrete than the language (mathematics) that describes them. There is a distinction to be made between objects and meaning; there is no such distinction between a (physical) theory that maps symbols to symbols and a (mathematical) theory that maps symbols to objects. In other words, physical objects and mathematical objects are both symbolic representations independent of phenomenological observation. Thus, physical science is necessarily an open system, progressing toward what Popper called verisimilitude, an asymptotic approach to truth, and never offering a completely closed judgment of truth, a proof of its conclusions. Mathematical science necessarily offers closed judgments based on axiomatic deduction, and theorems (true mathematical statements) are proven in the domains to which they apply. The physical domain--being the whole observed, and even perhaps the unobserved, universe--may not be subject to such axiomatization. How would we know in any case?--Goedel taught us that no set of axioms is sufficient to prove its own self-consistency; there always exist true statements that cannot be deduced from the axioms.
That being said, there are serious attempts to recast mathematics as an experimental science. Chaitin, Wolfram, et al, may really lead us to a common point of closure between what we say about the world and what the world says back.
Tom