Does a Rule with Vague Boundaries Have to Be Unfair?

However carefully a rule is written, it can meet cases its wording did not settle in advance. H. L. A. Hart used a famous example: a park rule prohibiting “vehicles”. A car plainly falls within the core, but what about a bicycle, a toy car or a military truck displayed as a memorial? General terms handle familiar cases without exhausting every future fact. This is the open texture of legal language.

Lon Fuller replied that words alone are not enough, because a rule's purpose always enters interpretation. If the prohibition aims to protect safety and quiet, a car driven through the park and a memorial truck fixed to a plinth need not be treated alike, even though both are vehicles. The dispute is not about whether text matters. It asks whether borderline cases should be resolved chiefly through settled usage or also through the rule's purpose.

My view is that vagueness does not by itself make a rule unfair. Perfectly precise boundaries are often unavailable, and trying to specify every possibility can create new absurdities. Unfairness arises when interpretation is hidden: similar cases receive different reasons, standards change after the event, or affected people cannot understand and challenge the decision. Fairness requires a knowable core, reasons for borderline judgements and similar treatment for similar cases. Vagueness calls for constrained interpretation, not a licence for arbitrary discretion.

https://plato.stanford.edu/archives/spr2026/entries/vagueness/
https://nyulawreview.org/issues/volume-83-number-4/a-critical-guide-to-vehicles-in-the-park/