The problem with Putnam's theory is that you can't get humans, capable of *forming* models, without a pre-existing reality to create them. So internalism doesn't strike me as being very self-consistent. In my philosophy, which is called "Externalism" for a very good reason, external reality predated humans, who evolved to model that reality. You can't "prove" reference, of course, because if that were true our thoughts would define reality. (And of course, I don't believe in instantiation, especially of Turing-computable behaviors, but that's not the point.)
To me, this sounds like another version of that old dilemna of "How do you prove that rational thought works without assuming the rational processes you use for the proof?" Or in other words, "How do you explain your theories to a rock?" And the answer... is complicated, but I discuss it in the new triple-size page on "Logic" in the latest version of the TMOL FAQ released two days ago.
email@example.com Eliezer S. Yudkowsky http://pobox.com/~sentience/tmol-faq/meaningoflife.html Running on BeOS Typing in Dvorak Programming with PatternsVoting for Libertarians Heading for Singularity There Is A Better Way