The Core and Penumbra of Meaning

To bastardise things slightly, the English legal philosoper HLA Hart described rules as having a "core" and "penumbra" of meaning. At the core, everyone can agree what the meaning of the words are. At the penumbra, they can't, and will have reasonable disagreement. Hence, consider the rule, "No vehicles in the park."

Everybody knows this means that cars, trucks, buses etc. aren't allowed in the park. But does it include bicycles? Skateboards? Aeroplanes? Toy cars? Ambulances? What about (to use Lon Fuller's famous counterexample) a military truck set up in the park as a war memorial?

Different rules vary, of course, in terms of how much "core" they are, and how much "penumbra". Consider:

A: "All applications must be recieved by 5pm on Friday 29th February."

B: "An exclusion clause is invalid unless the party seeking to rely on the clause can prove it is reasonable."

Rule A is mostly core - there is little to disagree about. Rule B is mostly penumbra. What is "reasonable"? It depends. There are probably examples of exclusion clauses which any sane person would agree are reasonable or unreasonable, but the list is small and there are many more examples where universal agreement is not possible. 

The rules of D&D are mostly almost entirely "core" and contain little "penumbra". ("You need to roll your THAC0, subtracting your opponent's AC, on 1d20 in order to hit.") This is why there seems to be something wrong with fudging. In a system which has clear rules without much penumbra, we can all identify very easily when the rules are being broken. If you need to get a 16 to hit, there is no way that reasonable people can disagree and suggest a 15 is good enough. If you need to get a 12 on a save vs death roll, there is no way that a roll of 5 can be interpreted as being sufficient to save you.

A lot of people who play D&D evade this issue with concealed sub-rules which might be said to broaden the penumbra of meaning in an underhand way. A lot of DMs run with rules like, "If a monster needs a particular score on 1d20 to hit, then I need to get that number on my d20 roll unless I consider it preferable that the monster should not hit." Or, "If a player needs to get a particular score on 1d20 for his PC to save ve death, then this shall apply unless it wouldn't suit the dramatic requirements of the narrative in the given moment."

The problem here is that when one enters the realm of the "penumbra" of meaning, one finds oneself in the zone of discretion. If I need a 16 to hit, there is no discretion in the application of the rule. If I need a 16 to hit unless the DM thinks that it wouldn't suit the dramatic requirements of the narrative at that given moment, I am at the whim of his discretion and the application of the rule thus appears arbitrary. As a consequence, I start to become suspicious (perhaps justifiably, perhaps not) about the DM's motives, just as people become suspicious of a legal system in which judges appear to be founding their decisions not on clear rules, but on political or other extra-legal considerations. This, for those of you interested in legal philosophy, is the window into what the legal realists (the subject of some of Hart's criticisms) were talking about, and thereby the critical legal theorists who descended from them: are judges making decisions on the basis of formal rules, or considerations and biases that have nothing to do with law at all? 

In summary, I suppose one can say that fudging dice rolls is a bad idea for all kinds of reasons, but one of the main ones is that it makes the DM appear capricious and biased even if this is not in fact the case.