My earlier posts have been a little abstract, so maybe a stylized example will help us see the usefulness of separating law and language. There is a famous hypothetical scenario: a sign that says “no vehicles in the park.” But what exactly does it exclude? In his recent article, Richard Fallon gives this variation:

Suppose that an accident happens within the park. Someone falls and is badly injured or suffers a heart attack. A desperate phone call brings an ambulance to the gate. Does “No vehicles in the park” exclude rescue vehicles even in cases of life-threatening emergency? We might say that it does — perhaps the gatekeeper should admit the ambulance anyway, but if she does so, she will violate her instructions.
But would we necessarily say that? Suppose that the worst happens: The vehicle is excluded, and the victim of the accident dies. The owner returns. The gatekeeper tells her, “I did as you instructed.” How might the park owner respond?
She might say, “It was not my intended meaning to exclude ambulances,” but she might also say something such as this: “Ordinary principles of conversational interpretation call for us to ascribe a reasonable meaning to prescriptions and other utterances unless something about the context indicates otherwise.” … In ordinary conversation, we do not waste time and breath offering elaborations and qualifications of our utterances that ought to be obvious to any reasonable person. And, the park owner might insist, a reasonable person would not understand a prescription of “No vehicles in the park” (given the most plausibly imaginable circumstances of its utterance) to encompass an ambulance in a situation of life-threatening emergency.
Work in pragmatics may support the same conclusion, though, once again, I do not mean to rest my conclusion on any philosophical theory at this point. If I tell a group of friends “Everyone is invited to the party,” I do not mean — nor would my listeners reasonably understand me to mean — literally everyone in the world. The reference to “everyone” is impliedly limited. Analogously, the proscription “No vehicles in the park” may be subject to an implicit domain restriction excluding cases of life-threatening emergency.
Because I have described the notion of reasonable meaning as depending on the meaning that a reasonable person would ascribe to an utterance in a particular context, what I am calling “reasonable meaning” could, like the speaker’s intended meaning, plausibly be viewed as a subcategory of, or a feature bearing on, contextual meaning as framed by the shared presuppositions of speakers and listeners. Without denying that possibility, I offer a separate account of reasonable meaning to highlight the distinctive significance of moral or practical reasonableness in ascribing meaning to a prescription, especially when neither the speaker nor the speaker’s audience is initially likely to foresee — or thus to anticipate how the prescription would apply to — a situation in which the directive’s literal application would have jarring consequences. To be concrete, I am imagining that the speaker who says “No vehicles in the park” is thinking about automobiles driven for recreational purposes, not ambulances, and has not turned her attention to the proscription’s proper application beyond the paradigm cases that she has in mind. If the contextual meaning of “No vehicles in the park” embraces ambulances, it is not because this was a specifically expected application; and if it does not, it is not because the case of ambulances was specifically understood as not being included, either by the speaker or by her audience at the time that she uttered her directive. The moral reasonableness of a particular ascribed meaning possesses a distinctive importance.

We think Fallon gets to the right result (the ambulance should enter) but in the wrong way. We say:

As Fallon notes, that instruction, even given in a nonlegal context (say, by a private park owner to a gatekeeper), would never be relied on to exclude ambulances. Fallon explains this by suggesting that the phrase has a separate “reasonable meaning”—distinct from its semantic, intended, or contextual meanings—under which ambulances are exempt. But the reasons to let in the ambulance have little to do with the phrase’s meaning. (If the owner stopped by and asked, “any vehicles in the park today?,” the gatekeeper would be lying if she answered “no, boss, see you tomorrow,” instead of “yes, boss, but I thought an ambulance would be okay.”)
Letting ambulances into the park might be inconsistent with what the owner said, but it’s consistent with the rule those words established. The ambulance exception isn’t part of the linguistic practices of American English, but of our social practices of giving and receiving instructions. That’s why a similar emergency would be just as good an excuse to violate some other instruction with entirely different wording—such as “fetch some soupmeat.” The exception reflects a recognized social norm (of adjusting to emergencies) that can coexist with other, equally defeasible rules.
To return to the legal context, a statute forbidding “vehicles in the park” is similarly subject to defeat by recognized common-law defenses. For example, the public authority defense excuses officials for certain acts done pursuant to their duties, without incorporating those defenses as a matter of language. (That’s why ambulances can drive through red lights, fire trucks can blare sirens at night, police can seize contraband without “possessing drugs,” and so on.) We don’t need separate categories of meaning to explain this; all we need is defeasibility, and the recognition that statutes don’t lightly override the common law.