How should we debate immigration? Here's my advice: don't.

Debates don't work, at least not as they should. They don't favour the truth, but plausible liars and for those who can best appeal to prejudice and cognitive bias. Jonathan Portes, who is doing great work in trying to bring facts and rationality to the issue, describes his opponents' strategy: "Flood the zone with a mixture of lies, half-truths, misleading claims and statistics taken out of context." Against this, rational discussion is bringing a knife to a gunfight.

We needn't look far for a parallel here. It's now obvious that Brexit was a bad idea: only 31% of voters say it was the right decision. But have Brexiteers retired in shame? No. Dan Hannan - who gave us the most absurdly cretinous fantasies - still has a seat in the House Of Lords and a newspaper column. The marketplace in ideas is broken; peddlars of crap do not exit as they would in a well-functioning market. Even if immigration scaremongerers could be proven wrong, they'd not shut up. And why would they when they have much of the media on their side? As the old(ish) saying goes, "never argue with a man who buys ink by the barrel."

Put it this way. Imagine that net immigration were to fall to (say) under 200,000 by the next election, a drop of three-quarters from its 2023 peak. Would Reform 2025 Ltd then say "Labour's making great progress; we'll therefore give up"? Of course they wouldn't.

Or to put it another way, the UK has been receiving immigrants for decades - be it Jews in the late 1800s, Pakistanis and the Windrush generation in the 40s and 50s, Ugandan Asians in the 70s and so on: as one who remembers the latter, talk about immigration seems as relevant as Slade or power cuts. If the "debate" about immigration hasn't been resolved by now it's not going to be, because the evidence isn't decisive.

A second reason not to debate immigration (or at least to be very careful about doing so) lies in a distinction between two types of politics - that based on lived experience versus that based on abstractions often gleaned from the media. Some us us think of immigrants as our friends, uncles or the local shopkeeper. The right by contrast invites us to see them as an abstract threat. Hence the fact excellently described by Ben Ansell; people who live in areas with more non-UK born residents tend to be more likely to think immigration is a good thing. One reason, I suspect, why support for Trump's immigration policies has declined is that "immigrants" are changing from the demons mythologized by Fox News into actual real people.

Unless we're careful, debating immigration risks us being like a therapist who indulges his patient's fantasy thus threatening to make the psychosis worse. If we must do so, we must speak in concrete terms, about real people not abstractions.

So, what should we do instead?

Matt Goodwin inadvertently gave us the answer recently on Twitter. "London is over -it's so over" he said, pointing to unreliable and overpriced trains, homelessness, crime, and struggling restaurants.

Which blurts out the truth. Immigration is not merely about immigration. "Concerns" about immigration are linked to a wider sense of national decline, of economic stagnation and government failure.

Sure, Goodwin might be seeing what he wants. But we now have abundant evidence that economic stagnation fuels the far right. Back in 2006, Ben Friedman showed that, historically, slower economic growth led to increased racism and intolerance. Markus Brueckner and Hans Peter Gruener have found that, across Europe "lower growth rates are associated with a significant increase in right-wing extremism." Thiemo Fetzer showed how areas hardest hit by austerity were more likely to vote for Brexit. He and his colleagues have shown that shop closures are correlated with support for rightist parties. And Diane Bolet has found that areas seeing more pub closures are more likely to support Ukip (as was).

A place in decline invites the post hoc ergo propter hoc fallacy: "since the immigrants arrived, this place has gone to the dogs". We could reverse this: if the cost of living were declining and places thriving, it would be harder for people to see immigrants as a problem - unless of course they were mere racists.

Which points to the way to fight the immigration battle: stop places going to the dogs. We should be saying to voters: "we get that many areas seem in decline, but this isn't because of immigration: here's what we should be doing instead." The better Labour MPs see this. Jeevun Sandher recently wrote:

If we want to win the next election, we need to make life affordable for our voters. Investing to get energy and housing costs down while creating good non-graduate jobs for Reform-curious voters will put more money in these voters' pockets. If we get cash in peoples' pockets, we win the next election. If we don't, we lose.

This is easier said than done. One problem is that these aims are undermined by the government's plan to raise military spending; doing so requires lower spending than would otherwise be the case elsewhere - on either public services or private consumption. And cuts in the latter would exacerbate the pub and shop closures which fuel the far right.

A second obstacle is the large number of vested interests blocking growth. I'm thinking not just of nimbys opposed to infrastructure spending and housebuilding but: incumbent companies opposing tougher competition policy; lawyers, accountants and financiers opposed to simpler taxes; landlords opposing a shift from taxing incomes to taxing land; utility companies opposed to fairer prices; Brexiteers opposed to rejoining the single market; and a financial system generally that likes the low real interest rates that accompany stagnation.

A further problem is the quality of British management. It has become so accustomed to rent-seeking and bullying that it has largely lost the ability to raise productivity.

And all this is before we mention the possibility that capitalism itself might be a barrier to growth. The problem isn't just that low profitability deters investment-enhancing growth: why do so many industries from steel to nuclear require government subsidies? It's also that some investments don't happen because capitalists fear that they will get only a "minuscule fraction" (William Nordhaus's words) of the payoffs from them, whilst others are undertaken out of misplaced optimism rather than a rational assessment of their returns.

Perhaps my pessimism is misplaced. But that's the debate we should be having. And the right don't want it because they don't want to discuss the failure of British capitalism. If they do, they only show the vacuity of their own thinking, be it about Brexit or "Britannia cards". They'd prefer that we demonized migrants than looked at the root causes of stagnation - and certainly prefer that we divide people along ethnic rather than class lines.

We should ask Reform: "why should we talk about your hobby horse when we know that your last pet obsession (Brexit) was a failure, and when we have more important things to think about?"

You might object that we should stand up for the rights of migrants and indeed - given that immigration controls also reduce their liberty - the rights of citizens too. Morally yes, but we have to choose our battles. When we discuss immigration, we are fighting the battle the right wants to have. That ignores Sun Tzu's advice: "he will win who knows when to fight and when not to fight." Setting the agenda matters, and we should not concede this to the right but rather set our own.

Discussing immigration steals cognitive bandwidth. The opportunity cost of doing so is that we don't discuss more important matters - matters that, if we could at least partly resolve them, would boost economic growth and so defeat the far right.