The nature of fiction

2 September, 2008 at 6:48 pm (definition, rpg theory) (, , , )

In this post I will muse about and offer a definition for fiction, as it exists in roleplay. First I will cover a mathematical description of possibility (or believability or acceptability or some better word). After that I will discuss how possibility works in play.

A possibility space

Take any person immersed in some work of fiction (reading a book, watching a movie, roleplaying,… ). That person has a mental notion of what is true and what is possible within that fiction.

So, I will define fiction as a set of propositions about some world (imagined or not) and possibilities assigned to these propositions.

(Proposition claims that something is true. I have previously discussed how to make truth a meaningful concept in the context of roleplaying.)

Assigning a possibility to a proposition means saying, for example, that a proposition “The princess is not married.” is certainly true, or impossible, or other similar descriptor. I found it useful to use numbers in place of descriptors. More precisely: Impossible proposition has possibility 0. Certain (or necessary) proposition has possibility 1. Any possibilities between these two are possible, with higher number meaning that the proposition is more possible, or more likely true.

The scale between 1 and 0 is used because both probability theory and fuzzy (multivalued) logic use the same scale. Fundamentally the scale does not matter. Note for the (even more) mathematically inclined: Possibility is a function from the realm of propositions to the closed interval [0, 1].

One should note that what was defined above was possibility space of a single individual.


So, take a number of individuals enjoying a work of fiction together. Playing an rpg or watching a movie, say, and maybe even talking about it. If one of them suggests something, the others might consider it a natural outgrowth of the fiction, or a sheer impossibility, or anything between.

Group possibility of a given proposition being true is a function of the personal possibilities regarding the proposition. The function must fulfill the following criteria (in the order they occur to me):

  1. The function’s value is 1 if and only if the personal possibilities are all 1.
  2. The function is continuous (given the normal way of measuring distance in n-dimensional and one-dimensional real number spaces).
  3. The function’s value is 0 if all personal possibilities are 0.
  4. Increasing the value of any personal possibility may not (while keeping the others fixed) descrease the function’s value.
  5. Decreasing the value of any personal possibility may not (while keeping the others fixed) increase the function’s value.

(4. and 5. can be combined by saying that the function is increasing with regards to every personal possibility.)

The above characterisation defines a number of functions. Nomenclature: F is used for the group function’s value, f(i) for ith person’s possibility. For example:

  • F = 1 if for all i f(i) = 1. Otherwise F = 0.
  • F = minimum [f(i)], i goes through all the players.
  • F = f(1)*f(2)*…*f(n), where n is the number of players.
  • nth root of the above, or geometric average.
  • F = (f(1) + f(2) + … + f(n))/n, where n is the number of players. In other words: The arithmetic average of f(i).

The group possiblity F is a measure of how readily the group will accept the given proposition into fiction or how certain they are of the piece of fiction being true.

One can interpret possibility as being the probability of the given thing being true within the fiction, though that definition is not exactly true and there are flaws. Another possibility is to consider possibility as the truth value of the proposition. For more on this, consult a random book about multivalued or fuzzy logic.


Roleplay is the process of creating shared fiction. There usually are other standards for good play, many not related to creating fiction, but all roleplay does hinge on shared fiction (if you personal definition of roleplay includes solo play, consider it to be shared among one person).

Bruce has previously discussed creating shared fiction and the role of anchors, so I will not repeat the stuff too elaborately. The key point is that even though players usually have similar fictions in mind, the details are very distinct. One significant part of the activity of roleplaying is constantly aligning the fictions of the participants so that they are reasonably similar. Generally, the more similar the functions are, the smoother the game goes. In some styles the differences do provoke people to add interesting details and force the others to scramble for it all to make sense. Games with a game master and mystery only they know are like this, as are games of the Mountain Witch where all player characters have a secret no other participant, not even the GM, know.

As far as the model goes, establishing an anchor (shared fact) fixes certain possibilities to certain propositions; typically, possibility 1 to the anchor and 0 to contradictions and other possibilities to whatever is implied by them. Any change to the fiction will alter this spread of possibilities, as will out-of-game hints and references, as will time as people forget things. The fiction is very ephemeral thing, constantly shifting around.

Shared imaginare space

The part of fiction that affects future play is called shared imaginary space, as coined by Fang and later used and altered on the Forge. Material in the SIS has high possibility, maybe even possibility 1, because it is used and recognised by the participants.


1 Comment

  1. Links: story hour and theory « Cogito, ergo ludo. said,

    […] first post references Huizinga’s Homo ludens and reminds me of something I’ve written before. Links: Första (and English translation for convenience), andra, tredje post. I might comment on […]

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