Each culture has its own "region" within the Astral Plane, and subjectively experiences its own archetypes and deep structures of reality there.
Which is exactly the problem I'm talking about. There are only three possibilities:
1. Only one specific group is actually correct about the structures of reality.
2.
No specific group is actually correct about the structures of reality, but it's possible for someone to learn what is correct.
3.
All groups are
equally correct, no matter how contradictory their experiences may be.
#1 and #2 are universes that allow for facts to exist, and only differ on whether anyone yet knows what those facts are. #3 is a universe where facts cannot exist, because all perceptions are equally correct and thus equally meaningless. Every claim you could make, even two exactly contradictory claims, can be true, and thus you can prove
literally anything.
Pedantic's example above, of things conditioned on/by a particular person's hopes/dreams/fears/etc., still permits facts so long as either (a) at least one person can see past the superficial
appearance conditioned by those hopes/dreams/fears/etc. and down to whatever fundamental
stuff is acting upon those things (meaning universe #1), or (b) currently nobody can, but it's at least possible someone
could do that (meaning universe #2).
If two people can look at the exact same sources of information, and both
correctly claim that what they observe is real, even though both of them see incompatible things, then facts do not exist anymore. The person who wrote such a cosmology has ensured that truth has no meaning inside that cosmology. There is no problem in having something that
rearranges itself to suit any particular observer's beliefs, so long as there are still things that are inherently true about that thing prior to any such rearrangement--that's reality
deceiving you into thinking it is X way when it's really not.
Further, there is no problem if, working off your answer, each "region" of the Astral Plane is isolated from others, and thus someone in one region would be speaking
only about that region and not about the absolute, complete
totality of the Astral Plane. E.g., the World Tree could be correct for one Astral Plane region, while the World Axis could be true of PoLand's region, and the Great Wheel true of Toril's astral region, and likewise Athas and Eberron would each have their own
particularly isolated Astral Plane regions.
But you cannot have it be the case that the Great Wheel is true for those who perceive it,
and also the World Axis is true for those who perceive it,
and also the World Tree is true for those who perceive it, all at the same time, in the same "region" of the Astral Plane, all of them looking at exactly the same information, because these three models all make mutually-contradictory claims that can be observed.
Generally, he Astral Plane (including the alignment planes within it) behave like dreamscapes, albeit the dreams of a culture rather than an individual. That said, there are "dominions" that can be a manifestation of an individual person/persona.
Okay. Whether it's conditioned by the perceptions of an entire culture in aggregate or a single person makes no difference. There can only be three options: either exactly zero cultures correctly understand the structure and behavior of the planes; or exactly one does and all others are incorrect insofar as their perceptions conflict with the one correct perception; or multiple cultures (any number 2 or greater) all "correctly" understand the structure and behavior of the planes, and thus "facts" about the planes are nonexistent, only (cultural) perceptions thereof. A "fact" could be completely true for one culture and completely false for another, and
both perspectives are correct. Due to the principle of explosion, this now means you can "prove" literally anything is true--and thus "truth" ceases to have any meaning.