Explaining other universes
In Inexploitability I described how Azkaban can happen in HPMOR only because it was Rowling’s invention rather than my own.
The phenomenon generalizes: There are authorial decisions that you cannot make yourself and that only another author can make for you.
Introducing the Deathly Hallows and their meaning into HPMOR is something I could only do because J. K. Rowling had already made the Potter’s family motto “The last enemy that shall be destroyed is death.” I could never have gotten away with revealing that sentence as the Potter family motto if it hadn’t been canon! Can you imagine the indignation? Can you imagine how loudly people would have screamed “Plot device!” and “Oh, come on, that is way too convenient!” and “That isn’t right at all, transhumanism isn’t something you get from your parents, it’s not something you inherit like a royal bloodline, it’s a decision we make for ourselves!” And they would have been right, if not for J. K. Rowling’s inscription on James and Lily’s grave.
Because that is what is there on the Potters’ tombstones, a moment that would be wrong if I’d decided it for myself, becomes a moment of sacredness, of unexpected echoes through time. (Which, in-story, has to involve magic or prophecy or destiny or something, because inside a story things like that can’t just be coincidences. Even when, in reality, J. K. Rowling put those words on that tombstone without ever having Methods of Rationality in mind.)
And deeper: To explain and reweave observations that are already there feels more wholesome, somehow. I don’t have to excuse the Potter family motto, or wonder if my decision about it has artistic integrity, because it is what it is. There’s something that feels right to me about having Harry guess the Dementors’ nature, which I wouldn’t have felt if Rowling hadn’t been the one to lay down the observations.
There’s something that feels natural about exploiting laws and explaining facts that someone else laid down, maybe because it bears a resemblance to the real challenge we face against Nature.
One way to get that feeling is the path of hard SF and hard fantasy: to have a small set of premises so lawful that by the time they turn into problems and puzzles, you don’t feel like you’re choosing them, because they were generated by the law.
But that path is hard, and doesn’t work for everything. There are simple laws that you can decide will govern time-travel and then everything else will follow from that; but there is no simple law that generates Dementors, or Azkaban, or the Potters’ tombstone.
So the other way is to write stories inside someone else’s universe, and stare at that universe’s given observations until you begin to imagine your own answers to its puzzles, and deduce what further facts your answers require as background truths.
Hence rationalist fanfiction.