There is a very big difference between asserting, "I will be king someday", and asserting, "I'm the king now."
No doubt. In the sentence, "I am the true king of this land" the word "true" is doing the bridging work. A pretty common useage of "true" that is more-or-less synonymous: "I'm sure my true love is out there somewhere" - someone can be my "true love" in this sense even though I've never met them, may never meet them, and certainly am not currently in a loving relationship with them.
Anyway, in the case of true kingship you could read it as "I am the destined king", perhaps, or "By the lights of the law of nature, I am the king". And I'm sure there are other readings. Whether the possible differences of nuance between these reading matters can't be known before play starts.
For instance, you don't need to know exactly which interpretation (if any) the player has is in mind to know how to respond when s/he declares as his/her PC's action "I walk the streets explaining to anyone who cares to listen, plus plenty who don't, that "King" Tyrannus is an imposter, a traitor, a usurper, and that the true king must be one of the people, not above them. Someone like me!" Bring on the hostile NPCs, and start rolling the dice.
I don't think I would subvert a player's backstory without player permission.
A Beilef is not a statement about backstory. It's role, in play, is future-oriented.
"I'm the king now." when you aren't is already unhinged. Likewise there is a very big difference between leaving up in the air whether the character will in the course of play obtain the throne, and leaving up in the air whether the basic facts of the characters backstory are actually dangerous delusions.
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I'm rather skeptical that players even of BW are open eyed accepting of the idea that everything in their backstory is in an indeterminate state and could in fact be a delusion
The Belief "I am the true king" is not, in the first instance, nor perhaps at all, a statement about backstory. It's an attempt to shape the plot trajectory - "This campaign is going to be about, among other things, whether or not this guy is the true king".
What true kingship requires, and entails, can't be settled ahead of time. Otherwise there'd be nothing for play to do. The game would be over before it started.
If, in the course of play, it turns out that true kingship requires royal bloodline, it may be that - in the trait vote - the other players vote this particular PC the "Born to be King" trait (this trait can be acquired by starting with the Prince of the Blood lifepath, but that is not the only way to acquire it). Everything else being equal, this might well be a sign that this particular campaign is reaching its climax. But maybe not. Traits can be voted off, too - and maybe if it turns out in play that this PC is not really the true king after all, the trait gets voted off.
There is a very big difference between asserting that the answer to the question of a beliefs factuality could be 'true' or it could 'false', and asserting that it must always be 'null'.
You are missing the relevance of temporality. When the goal-oriented Belief is first framed by the player, it's truth value is not known. By the rules of the game it's truth value
can't be known. In particular, a GM who predetermined it's truth value (say via secret backstory), or who in the course of play set out to reveal it as true or as false, would not be doing the job that a BW GM is called up on to do.
Over the course of play the truth value of the Belief may come to be known. When it is known, that may be a sign to swap it out; or even that the campaign is finished (as per Eero Tuovinen, "The GM describes a situation that provokes choices on the part of the character. The player is ready for this, as he knows his character and the character’s needs, so he makes choices on the part of the character. This in turn leads to consequences as determined by the game’s rules. Story is an outcome of the process as choices lead to consequences which lead to further choices, until all outstanding issues have been resolved and the story naturally reaches an end.")
Or perhaps the campaign continues, and the Belief stays, but it no longer serves as a goal but as a "Fate mine" - every time the PC does something which expresses his Belief that he is the true king (eg perhaps the PC, having become king, diligently dispenses justice in his realm), the player earns a Fate point.
I believe a player which creates a peasant with the false belief "I am the rightful king", is wasting everyone's time if thinks that the resulting game is going to be inherently about affirming the truth of his false belief. Either we need to establish in his backstory that he is the rightful king, or at least has reasonable claim to it, or else the player needs to understand that the delusional belief is ridiculous in a peasant.
Once again, this remark evinces a failure to understand the role of the Belief in gameplay. It's not a statement about backstory. It's a future-oriented statement with the function of shaping the content of play (its story content, its thematic content).
Which also illustrates the relationship between No Myth techniques and this sort of narrativist play. For this particular BW campaign to work, there has to be at least one "fact" about the shared fiction which is not established when play starts, and that is open to being established in the course of play - namely, whether or not this PC really is the true king of the land. If that was already authored - already established as true or false within the fiction - then the game would not be playable.
A true king disposed of his throne may claim a right to redress this injustice, both for his own benefit and for the sake of his people. Even so, the rightful king could still be a figure of horror despite having legitimate grounds for his belief. A peasant who is not the true king, who attempts to redress the imagined injustice on the basis of his delusional rights is always an object of horror.
Forming this sort of moral or political judgement in advance is also at odds with BW and BW-style GMing.
In my 4e game, the wizard-invoker cultist of the Raven Queen and Erathis has a pathological hatred of hobgolins, orcs and the like. This is because the PC's home city was razed by humanoid hordes. In one episode of play, the PCs were fighting some hobgoblins who had attacked a refugee camp, where the human victims of hobgoblin raids had gathered. The fighting took place on either side of a low rise - and while the rest of the PCs went over the rise in pursuit of the main hobgoblin force, this one PC stayed on the camp side, to deal with the handful of hobgoblins who had grabbed children and were running off with them. The PC used a Colour Spray to knock the hogoblins unconscious without harming the children. He then - to the surprise and horror of those in the fiction who saw it, and to the surprise of everyone at the table too - drew a sword from one of the hobgoblin's scabbards and used it to slit the hobgoblins' throats.
Now, my best published work is on the ethics of warfare. I have strong, well-developed and (I like to think) deep views on the matter; and also on the matter of punishment. (Given board rules, I won't share them.) But as a GM in the BW-style (though GMing a different system, 4e) it is not my job to pass judgement on this action by the PC. By all means I can use story elements to place pressure on the PC, and the player - narrate the horror of some, perhaps the nods of others, and so on. And the other players can play their PCs as they see fit. But for instance, to have the Raven Queen or Erathis turn up and express a view one way or the other - in particular, say, to have them turn up and condemn the PC as a murderer - would be a GMing error by my lights. It would be to foreclose the very issue that the player, by playing this PC in this way, is putting forward as something to be worked out by and in the course of play.
This is a big part of what I was trying to get at in post 826 upthread, as well as some similar posts that preceded it. This is the heart of my narrativist play.