Modeling Legal Rules

Common law rules admit of exceptions. When a court, especially a higher court, finds that the routine application of a rule would result in an injustice it is likely to distinguish: to concede that yes, the case does appear to fall under the rule as it is currently understood, but to insist that there are further factors, not mentioned in the rule (though perhaps acknowledged in other rules in other parts of the law) that distinguish this case from the cases that the existing rule was meant to cover, and that mean that the verdict that the existing rule suggests would be wrong. Nevertheless, the old rule does not die.1 When the writers of case books come to accommodate the new ruling it comes in as an amendment: the old rule was correct except under these new circumstances. So how should we understand the form of legal rules? A simple minded approach is to see them as universally quantified claims: whenever this holds, then this is the right verdict. But exceptions make it hard to maintain this. A universally quantified sentence cannot have exceptions, only counterexamples; and counterexamples show that the sentence is false. Of course, one could insist on the approach, maintaining that a simple legal rule is indeed false, and that the need to amend it shows this to be so. But since every common law rule, however amended, is very likely to admit of further amendment, this leaves us in the uncomfortable position of saying no rule is strictly true. We might try to soften the blow by saying that as they are amended the rules get closer to the truth, but proposals to explain such an idea have been signally unsuccessful.2 So we need an approach that allows rules to have exceptions. Elsewhere I have developed an account that sees legal rules as universals containing implicit unless-clauses; the idea is that the exceptions trigger the clauses.3 The challenge comes in doing this in such a way that the rules do not become trivial. In the first part of the paper I summarize that account (and correct what I now take to be some errors in my initial formulation). In the second part of the paper I ask how it fares against some alternatives. I see two. One treats legal rules not as universal generalizations at all, but as generics. I argue that whilst this has some plausibility for legal principles, it doesn’t do the job for legal rules. The second alternative is more radical still, treating legal rules as default rules within a nonmonotonic logic. Here I argue that the move to nonmonotonic logic does not really bring the advantages claimed for it, and further that it fails to explain something that is handled very nicely by the approach I favour: how it is that a legal decision can be criticizable, even though the court used the legal rules that were in force at the time.