The English common law relies to a surprisingly large extent on a single magical word: "reasonableness". It appears everywhere. Businesses must make "reasonable adjustments" so that employees with disabilities can work without being at a disadvantage in comparison to other colleagues. To be recoverable, damages in negligence must be "reasonably foreseeable". Contract damages can only be awarded for losses "reasonably in the contemplation" of the parties as a likely result of breach. Exclusion clauses must pass the "reasonableness test". A public authority must not make a decision "so unreasonable that no reasonable authority" would impose it. I could go on.
The virtue of "reasonableness" is that while it sounds very definitive, what is "reasonable" of course varies according to the eye of the beholder. Once you get into the realm of what is "reasonable", you are basically in the zone of the judge's discretion (although, of course, previous cases will tend to influence his or her decision). But this isn't such a bad thing. HLA Hart, probably the most influential jurisprude of the 20th century, used "reasonableness" as his example of what he called a standard rather than a rule. Real life is too complicated and messy to make hard and fast rules that will apply in every case. That will rapidly result in injustice and contradiction. It's often better for judges to have some flexibility by applying a standard - like reasonableness - instead. Not always, because if everything came down to what was reasonable, judges would simply be deciding each case on its merits, and that would result in an unpredictable and arbitrary legal system. But there is space for a bit of vagueness.
The important point about "reasonableness", of course, is that it's not a floating signifier - it can't just mean anything. Its meaning is socially constructed, like that of all words, but that doesn't mean it lacks all objectivity. As Stanley Fish would say, there are a potentially infinite number of ways in which anybody could interpret "reasonableness" in any given context, but the great majority of these will be "ruled out" by social and cultural expectation. You could interpret a decision by a public authority to ban the use of all languages except Klingon in public buildings as a reasonable one. But nobody realistically would, because that wouldn't accord with the way society constructs the meaning of "reasonableness" in the main. The way English judges used to refer to this phenomenon was by talking about somebody called "the man on the Clapham omnibus". What a judge determines to be reasonable is what this archetypal figure (basically, a lower-middle class employee on the bus on his way to work) would consider to be reasonable on the basis of the facts presented. So whether or not, for example, a business has made reasonable adjustments for a disabled employee is not a total crapshoot. The judge is deciding that question in reference to what an ordinary, sensible person in possession of the facts would consider to be reasonable. That is socially constructed, but social understanding of words has an objectivity to it all the same.
Making a game of D&D work has a lot to do with the DM making decisions about what would be reasonable. Can my character do [x], where [x] refers to persuading somebody of something, telling a convincing lie, pulling off a neat combat move, reacting suddenly to an unexpected event, ducking behind that pile of crates, tugging a potion out of his backpack while simultaneously backing away from the dragon, or any of the other infinite number of things that a player will want to do in-game which aren't covered explicitly by the rules? Well, would it be reasonable for him or her to be able to do it? That isn't the doorway to arbitrariness. It is an invitation for the DM to make a sensible decision on the basis of the social expectation of what reasonableness entails. Far better this than the alternative, which is to try to make a rule to cover every eventuality - and a 20,000 page long rulebook, and an unplayable game, as a consequence.