Yeah. I thought about this a lot in the context of the Hanson/Yudkowsky debate about the unmentionable event. As was frequently pointed out, both parties aspired to rationality and were debating in good faith, with the goal of getting closer to the truth.

Their belief was that two rationalists should be able to assign roughly the same probability to the same sequence of events X. That is, if the event X is objectively defined, then the problem of estimating p(X) is an objective one and all rational persons should obtain roughly the same value.

The problem is that we don't - maybe can't - estimate probabilities in isolation of other data. All estimates we make are really of conditional probabilities p(X|D), where D is a person's unique huge background dataaset. The background dataset primes our compression/inference system. To use the Solomonoff idea, our brains construct a reasonably short code for D, and then use the same set of modules that were helpful in compressing D to compress X.