But what, in the real world, would create a reasonable suspicion that someone is not American? It can’t be appearance or language. Americans come in all shapes and sizes, and there is no legal obligation for Americans to speak or understand English. In the old war movies, knowing who had won the World Series was proof of being a real American. But that is not information you wear on your sleeve.

This isn’t some theoretical question. If I can be forced to prove my citizenship on pain of arrest, I need something to convince the police that I’m a citizen. If I’m a legal immigrant, I will own the papers to prove it — and I will have to carry with them with me everywhere.

But what about citizens who are born in the U.S. but do something that makes the police reasonably suspect that they are illegal? What they need is a piece of paper declaring that they are citizens. A state-issued driver’s license won’t do, because noncitizens can drive. Anyway, states may have no record of who is a U.S. citizen. The ID card has to be national, as when we travel abroad — which means a passport.