One of the most elementary distinctions in economics (and in life) is that between instruments and objectives. Interest rates, for example, are only an instrument whereas the objectives are price and output stability. I fear, though, that this distinction is being overlooked by those Brexiters aroused by what Trump calls a "big and exciting" trade deal between the UK and US. Trade deals are only a means of achieving what really matters - prosperity. And they might be a weak one at that.

Liam Fox claims that bilateral US-UK trade could rise from £167bn a year to about £207bn by 2030 "if we're able to remove the barriers to trade that we have". This isn't as impressive as it seems.

The UK currently exports £100.3bn of goods and services to the US. Let's assume this rises as much as Fox claims. Over the 11 years from 2019 to 2030 that's a rise of £24bn. Which is 1.2% of GDP. Or 0.1% per year. And in fact, the rise would be much less than this simply because exports have a high import content*.

Granted, this might understate the benefit. Perhaps a trade deal would boost GDP not so much by raising exports but by giving us access to cheaper imports, thus raising real incomes. However, cheaper US imports would threaten to drive some UK producers out of the market - a fact they would of course resist. One reason why trade deals take such an infernally long time is simply that every goddamn lobbyist in the country sticks their neck in.

All this, though, assumes something that mightn't be the case - that Fox is right that a trade deal would raise exports a lot: that word "if" in his statement is doing a lot of work.

Research by Monique Ebell and Silvia Nenci shows that this mightn't be the case. Nenci, for example, shows that cutting tariffs has been of only "small significance" in boosting trade. A big reason for this is that trade deals often leave in place non-tariff barriers to trade such as differences in regulations. These are the sort of things removed not by trade deals but by a single market across nations. If only somebody had ever thought of such a thing.

What's more, there are countless non-governmental obstacles (pdf) to trade such as exchange rate volatility, consumers' preferences (pdf) for home-produced goods and services, uncertainty, credit constraints, a lack of entrepreneurial nous and of course distance. Two big facts tell us that these are important. One is that world trade has slowed in recent years without significant increases in legal obstacles. The other is that Germany exports much more than the UK does to non-EU countries - a fact which tells us that the barriers to UK exports are not primarily regulatory.

Herein lies my gripe with Brexit. It steals cognitive bandwidth. Instead of considering the countless possible ways** in which we might improve our economic performance without the omniuncertainty of Brexit, we're discussing how to clean a fucking chicken.

Why, then, are Brexiters making such a big thing of a UK-US trade deal? One reason lies in the old cliché that if your only tool is a hammer, everything looks like a nail. They're making the mistake the right has made for years - of assuming that dynamism and flexibility will be unleashed if only the state gets out of the way.

Also, there's the confusion of instruments and objectives. Those who regard Brexit as a good thing in itself are making a fetish of trade deals: they're saying "look at what we can achieve outside the EU" without asking what the payoff to that will be.

In this sense, Brexiters are like the worst sort of amateur DIYers who refuse to hire proper tradesmen. Their unwarranted belief in their competence and self-reliance is causing them to wreck the house.

* This is a specific example of the general policy made by Dietz Vollrath - that "you can't reform your way to rapid growth."

** There is of course a tension between this claim and Vollrath's scepticism about supply-side reform. My point is that economic stagnation requires a broad-spectrum response; we should throw as many sensible policies at it as possible, in the hope that one or two will work.