Hm, how about: "[...] of any observer which our best current theory of how minds work says could exist".

So for example, a statement along the lines of "a ghost watches and sees whether or not Mars continues to exist when it passes behind the Sun from Earth's perspective" would have been meaningful a long time ago, but is not meaningful for people today who know a little about brains.

This also means that a proposition may be meaningful only because the proposer is ignorant.