Many modes of play assign responsibility for
world to GM (MC, DM, etc.) and
character to player. For example, Harper wrote that
In Apocalypse World, the players are in charge of their characters. What they say, what they do; what they feel, think, and believe; what they did in their past. The MC is in charge of the world: the environment, the NPCs, the weather, the psychic maelstrom.
That sort of division could imply that if it's world it ought not to depend on player choices, but come from GM in response to those choices. Because
GM is world it's never a violation of causality and never casts doubt on membership in the diegetic set.
What Harper says only establishes an "ought" on the premise that it is accepted as true. It's not self-grouding.
Furthermore, as I've already pointed out in this thread, reading that passage of Harper's in disregard of the rest of what he says, and of the AW rulebook, is leading some posters into dogmatic assertions that are not true of Apocalypse World.
As one example, the psychic maelstrom - p 113 of the AW rulebook says that:
It’s especially important to ask, the first time each character opens her brain to the world’s psychic maelstrom, what that’s
like for her.
This feeds into an important point. Harper (together with Vincent Baker) is well aware of the tension between
X is in charge of the world and
Y is in charge of Z's thoughts, beliefs and memories of the world, given that veridical beliefs and memories imply truths about things other than the person whose memories they are.
Harper gives this example, and explains how it works within the Apocalypse World framework:
Sometimes, the players say things that get very close to the line. Usually this happens when the MC asks a leading question.
MC: "Nero, what do the slave traders use for barter?"
Player: "Oh man, those fuckers? They use human ears."
That's a case of the player authoring part of the world outside their character, however -- and this is critical -- they do it from within their character's experience and frame of reference. When Nero answers that question, he's telling something he knows about the world.
So it's actually not as simple as saying "The GM is in charge of the world". Rather, the GM gets to decide
when to bring in player contributions (by asking questions) and gets to decide
how to bring in player contributions (by asking about PC knowledge, experience, etc). This is consistent with what is said on p 109 of the rulebook:
The players’ job is to say what their characters say and undertake to do, first and exclusively; to say what their characters think, feel and remember, also exclusively; and to answer your questions about their characters’ lives and surroundings.
This shows how overly simplistic it is for someone to say that, in AW, "The GM is in charge of the world". And it's doubly simplistic, and absurd, to use that sort of wording to beat other posters over the head for playing RPGs which don't grant the GM exclusive authority over backstory.