I have a good bit of experience doing one or the other. Objective truth is a fairly standard presence in D&D, particularly in the Gygaxian spirit in which characters know and talk about their alignments, but also generally as in the spirit of players develop a sense of the DM's approach to certain questions. Orcs always being evil is a good example of an objective truth situation, but so is the inverse that orcs aren't always evil--in either case, one of these is defined as true by the DM and players can figure out which one.
|Good? Bad? I'm just the lady with the|
shadow that cuts people to ribbons.
It's also possible to approach subjective truth from a position of not knowing the answer to a question *yet*. Looking to a video game example, whatever became of the Dwemer in the Elder Scrolls setting is an example. It's likely that the setting's writers don't know what happened exactly--but it's also likely that they will eventually decide on an answer and reveal it. In this case, an objective truth is born from a theory within a subjective module being selected.
My conundrum though, is that I want to achieve both of these at once, not merely transition from one to the other. Specifically, my game is going to begin in a Celt-Irish milieu which, over time and as a result of player exploration, will open other cultural milieus both as things to interact with as visiting explorers *and* as sources from which to draw future characters. To achieve the latter in such a way that the systems which govern character options, magic systems, etc, can be ready-to-go upon being discovered, certain concrete facts have to be there. I have to answer a lot of questions about these places so that I can populate the hexes before the players walk into them.
The initial area in which our Celt-Irish PCs will explore is most closely bordered by a small civilization of elves, which is initially great because elves feature prominently in Celtic mythology and have an important place in setting the experiential tone of the setting in play. Except... the elves that Celts know about are Sidhe--nature spirits of dubious motives, full of mystery and pregnant with implications of what is and isn't true about other planes (Tir nan Og, for example). The Viking milieu on the far side of the elves have their own notions about the Alfr and their mystical realm (Alfheim). And the elves themselves are ideally to be of a Tolkien model--aloof and wise beings who are now retreating from the mortal realm (to the campaign's analogue of the Gray Havens or Valinor).
Are these the same creatures, or are Sidhe, Alfr, and Elves different? I think it's more elegant and interesting if these are three views of the same beings, and that leads to an objective conclusion that they are Elves in the Tolkien sense, and our Celts and Vikings are merely wrong about them. But then what is a beansidhe (banshee)? Do the Leanansidhe steal men's souls and wither them to husks, or are these the fanciful stories of broken-hearted Celts whose Elven lovers abandoned them? Are the Firbolg and Jotnar likewise just different names for the Giants, about whom there is also an objective, concrete answer?
Each of these questions is pretty trivial to answer on its own (frex: the banshee is an undead spirit that the Celt people wrongly consider to be a type of elf, as they do any supernatural being), but it's pretty easy to see that these answers satisfy our pre-game narrative considerations while being imminently difficult to keep mysterious during actual play, provided player agency and a DM who doesn't want to arbitrarily lie about what the characters experience, misleading them to the culturally relevant but factually wrong conclusions. Players stand, in other words, to immediately puncture the milieu, which can hardly be expected to survive the very first encounter with an elf. The Sidhe immediately become a myth, preventing right away many of the sorts of adventures that evoke the experience of the culture.
It's a difficult question, and I've been wrestling with it for awhile. I will likely continue to wrestle with it going forward, but I eventually need to have an answer. Have you ever dealt with this in a way that was satisfying, either as a player or DM? How did it work? How long were you able to maintain the mystery?
There's also the matter of having both polytheism and monotheism present in a world where priests can cast spells, although I have come up with an answer for that one that, if I might pat myself on the back, is worthy of its own future post.