Can a rule be a rule if you can’t understand what it means? American law says that it can’t. If a law is written in a way that an “ordinary person exercising ordinary common sense” cannot sufficiently understand and comply with it, that law is “void for vagueness,” says the law.

Obviously, though, this isn’t true. I don’t really understand, for instance, what section 751(b) of the Internal Revenue Code means. My tax professor in law school speculated that even the IRS might not entirely understand how to comply with it. But no court would declare section 751(b) void for vagueness (see this article, however). If you read a bunch of the void-for-vagueness cases, you might rightly wonder whether this doctrine has anything to do with the “ordinary person.” Those laws that get struck down as being too vague aren’t the ones that are difficult to understand—in fact, they’re often the very laws that do appeal to common sense. Like the Chicago law that instructed police officers to disperse any group of “criminal street gang members” found loitering on the street.

But what if you’re not even allowed to look at the rule? If the rule itself is secret, can the “ordinary person exercising ordinary common sense” understand and comply with it? That is, can “a small circle of people on the playground secretly make up new rules for the basketball game going on nearby, then wait with grins for someone to do something wrong?” Yes, says the U.S. Court of Appeals for the Ninth Circuit, and the U.S. Supreme Court doesn’t have a big problem with that answer. This is the Gilmore v. Gonzalez case, a case about the sort-of-requirement that you show identification to board a commercial flight in the United States.

The rule at issue there is a “Security Directive” that the Transportation Security Administration issued. The TSA is permitted to classify its Security Directives as “Sensitive Security Information” and refuse to disclose them to the public. The requirements (or whatever) that force you to show your ID before boarding an airplane have been kept secret in this way. So John Gilmore challenged this, saying this kind of rule should be void for . . . vagueness? Unknowableness?

A federal appellate panel that included both of Idaho’s Ninth Circuit judges rejected Gilmore’s claims. In an opinion (168 KB PDF) written by Judge Richard Paez (a Latino, LDS Clinton-appointee who worked for many years as a legal aid attorney), the court said that all that matters is whether the public knows what to do. “Although Gilmore was not given the text of the identification policy due to the Security Directive’s classification as SSI, he was nonetheless accorded adequate notice of the policy and how to comply.” This morning, the Supreme Court announced that it will not hear Gilmore’s case.