The Hierarchy of Impossibilities[]
Impossible-0 or "Expressibly Impossible"[]
Impossible in prosaic terms. You can express the results of the act as a mundane state, but you cannot reach that state through any causal chain of events.
One achieves an expressive impossibility by altering the underlying rules by which the mundane world operates.
Example: Miracles.
Impossible-1 or "Syntactically Impossible"[]
Impossible in miraculous terms.
You cannot express the results of the act as a mundane state.
You can express the results of the act as the state of a world that does not exist, but you cannot mutate the existing world into that world through any series of miracles.
One achieves a syntactic impossibility by altering the underlying rules by which the mundane world is defined, followed by an alteration of the rules by which it actually operates.
Example: The Excrucian flower rite.
Impossible-2 or "Comprehensibly Impossible"[]
Impossible in Imperial terms. You cannot express the results of the act within any syntactically valid world, by which I mean, a world that abides by the fundamental tenets of Nobilis cosmology and can therefore be defined in Nobilis terms.
You can express the results of the act as a world that is coherent with regards to some other syntactic definition.
One achieves a comprehensible impossibility by altering the underlying structure through which one performs miraculous actions in the Nobilis universe.
Example: Ambrolam's ambition, or, more generally, True Gods spreading their Estates permanently to other worlds.
Impossible-3 or "Meaningfully Impossible"[]
Impossible for transcendent entities.
You cannot express the results of the act within any well-defined roleplaying universe.
You can express the results of the act in some notation. It has meaning; it simply does not yield a well-defined universe.
One achieves a definitional impossibility by reconstructing the cosmos in a fashion that refers to external factors and forces.
Example: Ianthe's integration of her thoughts into the Nobilis manuscript.
In Nobilis, this translates as:
Impossible-0: Significant. Impossible-1: Difficult (major character goal). Impossible-2: Very difficult (campaign goal, character lifework, major plot). Impossible-3: Insanely difficult (but why bother?)
Note that while Ambrolam's purpose falls under Impossible-2, many entities in the setting have Impossible-2 goals. Some of them will achieve them. Leave them alive long enough, and maybe all of them will.
It gets one big jump harder if the change represents a significant deviation. In terms of estimating viability, I figure that you can get fairly close to the next class.
Impossible-0 tasks include ideas that require one simple miracle to implement. That's still significant. They range up to plans requiring many miracles, some reactive---a good character goal for a set of stories.
Impossible-1 tasks are still feasible for Nobilis to dump their side energies into. I mean, sure, it's impossible to free all the souls from Hell. But it's a fairly low-grade impossibility, just like Aleph-1 is a low-grade infinity. You can't get there within the rules, but you've got access to things that break the rules.
Impossible-2 tasks . . . it gets iffy. Hard to be sure you'll succeed even if the HG gives it spotlight time! Note, however, that I'm not talking about 'change the Nobilis world into the World of Darkness or Star Trek' here. That's a very *involved* and *unappealing* Impossible-2 task. One is much more likely to succeed at making small changes to the metaphysics. The key tools for achieving Impossible-2 tasks: cooperation from hypothetical transcendent entities like Cneph; messing around with the Lands Beyond Creation and High Summoning; and hacking Creation by figuring out part of how Cneph put it together and what the bugs are in the code.
Impossible-3 tasks . . . hm. Mostly, this involves breaking the fourth wall, and I'm not very interested in that for actual Nobilis play. :) Ianthe Falls-Short doesn't have to go around in your game talking about Nobilis. Even if she does, if a PC bribes her to conk me over the head and slip a rule favoring that PC into the next supplement, the HG does *not* have to pretend that rule's in the book even long enough to house-rule it away. It's a device that I understand another game---don't spoil which one for the audience!---used to good effect, but it's not actually very Nobilis. Particularly once the PC's player argues that Ianthe can write rules so persuasively that the HG wouldn't house-rule them out. *giggle*
Basically, if the world can still be described reasonably by a book---not even my book, just a RPG---then you haven't really made an Impossible-3 alteration. So what you have to have is a world that includes literally indescribable elements, or a world contingent upon circumstances external to its description such as player state. (I'm speaking loosely here, since "events track those in our world" is *not* an ill-defined world element, and "this happens based on player mood" fits in a book. Hm. But maybe my meaning is apparent.)
I have no point here. Mostly, I just started to write that what Ambrolam was doing was "impossible, by which I mean, of course, very hard", and decided to go into more depth.