So...if you write down in a book, "If you aren't sure what happens, the referee will tell you what happens," suddenly it's a rule and not "DM decides"?
I gave Manbearcat his well-earned laugh XP, but this reply is serious in tone and spirit.It’s both.
1) Its a rule…
2) …that GM decides.
I’ll leave the implications upon play of said rule as an exercise for the reader…
This reply is also about RPGing, the social and leisure activity. Not free kriegsspiel as used by the Prussian army as a training exercise.
The essence of RPGing is shared imagination, with the play of the game involving establishing the content of the shared imagining. And this shared imagining has a typical allocation of roles at any given moment of play: one participant manages setting and backstory and resulting situation, while another manages a character/protagonist who is in that situation. (These are the player and GM roles.)
For the game to work - ie so we're not all just sitting around the table looking dumbly at one another - stuff has to be said that adds new fiction. First, (i) the "GM" has to make a situation be part of the fiction. Then, (ii) the "player" has to declare actions for the protagonist. And then, (iii) there has to be some way for working out how the shared fiction changes as a result. And if that's not to be the end of the game, then (iv) there has to be a new situation presented that somehow follows from that change in the shared fiction.
Each of (i), (ii), (iii) and (iv) requires what Edwards calls system: "a means by which in-game events are determined to occur."
Sometimes that system involves "mechanics"; ie some sort of rules-governed reference to cues like dice or cards or tables. Just to give some examples:
* In classic D&D, (i) is determined by reference to a pre-authored map-and-key, or by the use of wandering monster dice and tables;
* In Burning Wheel, sometimes (ii) depends on the outcome of a Steel check;
* In Apocalypse World, (iii) is done by the GM saying something (Drama resolution, in terms of Tweet's schema; and a "soft move" unless the player hands the GM a golden opportunity) unless a move is triggered - in that case, "if you do it, you do it" kicks in and (iii) is done by rolling the dice and then following the instructions for that particular move;
* In classic D&D, (iv) is just the same as (i), but in DitV the escalation rules mean that (iv) is a bit different (I'm not an expert on DitV, but am using it as an example of a system that expressly feeds the result of (iii) into the new/re-framing that follows, and hoping not to get it too wrong in the process).
* In Burning Wheel, sometimes (ii) depends on the outcome of a Steel check;
* In Apocalypse World, (iii) is done by the GM saying something (Drama resolution, in terms of Tweet's schema; and a "soft move" unless the player hands the GM a golden opportunity) unless a move is triggered - in that case, "if you do it, you do it" kicks in and (iii) is done by rolling the dice and then following the instructions for that particular move;
* In classic D&D, (iv) is just the same as (i), but in DitV the escalation rules mean that (iv) is a bit different (I'm not an expert on DitV, but am using it as an example of a system that expressly feeds the result of (iii) into the new/re-framing that follows, and hoping not to get it too wrong in the process).
Now consider a game in which the only rule, at each of (i), (ii), (iii) and (iv), is "GM decides". That is not a RPG, as it lacks the player role altogether. The same person is both framing and declaring actions and resolving. It is just one person telling a story, albeit under a particular structure of storytelling.
So to get a RPG at all, we have to introduce some limit on GM decides at some place in the system.
Introducing "player decides" at (ii) is one possibility, but I think that is prone to cause issues: eg it encourages the player to declare things like "I pull my <insert game-winner here> out of my pocket and deploy it!", which then in turn encourages escalation by the GM at (iii) and/or (iv). Most school children are familiar with this sort of thing in shared imagination play.
I think that's why there are very few RPGs which really intend participant decides as the entirety of their system. Even if they don't always spell out what the rest of the system is meant to be!