Bundle_Gerbe comments on The Useful Idea of Truth - Less Wrong
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A set of beliefs is not like a bag of sand, individual beliefs unconnected with each other, about individual things. They are connected to each other by logical reasoning, like a lump of sandstone. Not all beliefs need to have a direct connection with experience, but as long as pulling on the belief pulls, perhaps indirectly, on anticipated experience, the belief is meaningful.
When a pebble of beliefs is completely disconnected from experience, or when the connection is so loose that it can be pulled around arbitrarily without feeling the tug of experience, then we can pronounce it meaningless. The pebble may make an attractive paperweight, with an intricate structure made of elements that also occur in meaningful beliefs, but that's all it can be. Music of the mind, conveying a subjective impression of deep meaning, without having any.
For the hypothetical photon disappearing in the far-far-away, no observation can be made on that photon, but we have other observations leading to beliefs about photons in general, according to which they cannot decay. That makes it meaningful to say that the far away photon acts in the same way. If we discovered processes of photon decay, it would still be meaningful, but then we would believe it could be false.
Your view reminds me of Quine's "web of belief" view as expressed in "Two Dogmas of Empiricism" section 6:
Quine doesn't use Bayesian epistemology, unfortunately because I think it would have helped him clarify and refine his view.
One way to try to flesh this intuition out is to say that some beliefs are meaningful by virtue of being subject to revision by experience (i.e. they directly pay rent), while others are meaningful by virtue of being epistemically entangled with beliefs that pay rent (in the sense of not being independent beliefs in the probabilistic sense). But that seems to fail because any belief connected to a belief that directly pays rent must itself be subject to revision by experience, at least to some extent, since if A is entangled with B, an observation which revises P(A) typically revises P(B), however slightly.