Is Brexit unconstitutional?
That's the key issue in a suit argued this week before the High Court in London. What makes the question especially piquant is that Britain doesn't have a single written Constitution, but rather a complex tradition of constitutional law made up of principles, precedents and practices accrued over generations.
Under those principles, the answer to whether the United Kingdom can leave the European Union without a parliamentary vote is… maybe.
It seems most likely that the High Court - or the Supreme Court, which will hear an appeal next - will say that Brexit can be accomplished without it. But to reach that conclusion, the British courts will have to expand existing principles and grapple with the meaning of the referendum as a political tool. It's a sad day for the country that arguably invented representative government.
The constitutional argument against Brexit is clever and powerful. It's being made by Mr David Pannick, a member of the House of Lords and a particularly skilled barrister. As articulated before the court - and in an op-ed article in The Times - it runs like this: A well-recognised principle of the UK Constitution is that one law enacted by Parliament may only be changed by another law. Parliament enacted the European Communities Act in 1972. That law, which opened the way for the UK to join the European Economic Community, says that European laws apply in the UK.
European law today includes lots of individual rights, including those found in the European Declaration of Human Rights. Once the UK leaves the EU, the rights will no longer apply to Britons. Hence, Mr Pannick says, the UK can't leave the EU without a law authorising withdrawal. And a law requires an Act of Parliament.
If accepted by the courts, Mr Pannick's argument would have the effect of blocking Prime Minister Theresa May's government from invoking Article 50 of the EU treaty, which allows member states to withdraw. The government would need to go to Parliament, which would say no - and Brexit would be blocked.
So is Mr Pannick right? There's a somewhat plausible technical response to his claims. It says essentially that since the 1972 law incorporates European law, and Article 50 of the EU treaty is European law, there's no need for another Act of Parliament, because the 1972 law isn't really being revoked but simply relied upon to withdraw.
This would be a pretty cheap way for courts to solve the deeper problem, and I suspect the judges won't want to reject Mr Pannick's claim on a technicality - or at least they shouldn't.
The better, more substantive answer to Mr Pannick's argument is that while Parliament hasn't enacted a new law for EU withdrawal, there has been a public pronouncement on the topic: the Brexit referendum. True, a referendum isn't a law. But it is the voice of the people, at least as expressed on the fateful day of the vote.
Here's where things get really interesting from a constitutional standpoint. If the UK had a written Constitution, it would probably say whether a referendum should be treated the same as a legislative vote. But because it doesn't, the courts will have to decide whether a referendum is just as good as a legislative vote, or better, or worse.
And that judgment will require some serious thought about the true nature of democracy.
If you think democracy is all about giving voice to the people themselves, then the referendum must be at least as good at making fundamental change as a parliamentary vote would be. After all, the people are voting directly, not having their voices mediated by representatives.
But if you think that democracy in a constitutional republic is about decision-making by the people's chosen representatives - as James Madison did when shaping the US Constitution, for example - then the referendum isn't good enough.
Historically, Britain has been the home territory of representative government. Referendums are rare, which is one reason this constitutional issue hasn't been fully addressed before.
So if the courts wanted to follow the logic of British constitutional tradition, they might well block Brexit using Mr Pannick's theory.
But the pressures of modern democratic theory are profound - and those pressures militate for direct democracy and against government that is limited only to elected representatives' actions. In particular, the modern approach treats referendums as designed precisely so the public can get around the potentially divergent interests of elected representatives.
In an especially perverse twist, British experience in the EU has exposed Britons to the European practice of holding referendums on major issues, especially those connected to membership in the EU itself. That, too, pushes in the direction of treating the referendum as sufficient.
It follows that the courts will probably feel the need to treat the voice of the people as sufficiently weighty to allow Brexit without a parliamentary vote. Through their judgment, the British Constitution will evolve toward greater direct democracy. Through leaving the EU, British constitutional law will become more European.