In 1994 Stephen Young was convicted of a double murder. When it emerged that some jurors had used a Ouija board to inform their verdict, however, a retrial was ordered.

The philosophical (though not legal) justification for doing so has been described by Jason Brennan in his new book, When All Else Fails. He calls it the competence principle:

People have a right that certain types of high-stakes decisions be made by competent people, who make their decisions competently and in good faith. It is unjust, and violates a person's rights, to forcibly deprive a citizen of life, liberty or property, or significantly harm her life prospects, as a result of decisions made by an incompetent decision-making body (p85).

Obviously, using a Ouija board to decide a man's guilt is incompetent and thus unjust by this principle.

But here's the thing. Brennan thinks the principle should apply not just to juries but to major policies as well:

Political decisions are presumed legitimate and authoritative only [my emphasis - CD] when produced by competent political bodies in a competent way and good faith.

If this is correct the Brexit referendum was illegitimate not (just) because it excluded big stakeholders, or was founded upon lies, or was bought with Russian money, but because the vote was taken by an incompetent electorate - one that was ill-informed, swayed by cognitive biases, or motivated by racism.

You might object that this is true only of some voters. Irrelevant. Only a minority of jurors in the Young case used the Ouija board.

Or you might think this is the sour grapes of a Remainer. Again, irrelevant. Young was convicted again at his retrial. The Ouija board jury got the decision right. But it's not good enough to get it right. The competence principle says the decision must be taken in the right way.

Instead, the question is: is Brennan right? Does the competence principle actually hold?

Maybe not. In other cases of alleged incompetence by juries, law lords have refused appeals on the grounds that the need to preserve the secrecy of the jury room outweighs the competence principle. Likewise, maybe the principle is trumped in politics by another one - people's right to participate. Claiming this, of course, requires reasons and not just assertions. Why should an idiot's right to a say trump other people's rights to liberty, prosperity or good government? It's not obvious to me, or to Brennan.

Even if you accept the competence principle however it does not follow at all that we should have a second referendum. There's no reason to suppose this will be more honest or rational than the first.

Instead, the only chance of reconciling the competence principle with the right of participation (assuming one to exist, which Brennan denies) would be through some form of deliberative (pdf) democracy. This requires, among other things, tight rules on what evidence is admissible as well as safeguards against cognitive biases and inadmissible preferences.

It's possible that such a device would deliver Brexit: as Matthew Goodwin says, people care about "identity, community. Belonging and tradition" as well as GDP. Or if you believe in a form of proceduralism then you'll believe that the outcome of a truly good deliberative process will itself be legitimate.

It is of course obvious that our actually-existing media-political institutions are a trillion miles away from any ideal of deliberative democracy. This being so, the claims of both Leavers ("respect the will of the people") and Remainers ("let's have a second vote") are both dubious. They beg the questions: why is the competence principle wrong? Why should we abide by decisions taken incompetently? The answer is by no means as obvious as small-minded partisans think it is.