A trend seems to be emerging among their advocates to describe both Brexit and Universal Credit as good policies badly implemented. For me, this raises a question: how big is the "good policy, bad implementation" set? I suspect it's small, at least for major policies.

Take, for example, the biggest failures of post-war policy: Suez, Iraq, the poll tax, ERM membership. The fault with these is that they were bad ideas in principle. Few believe they'd have been redeemed by feasibly better implementation.

Yet another set of policies are those that are still controversial such as many of Thatcher's reforms: weakening the trades unions, cutting taxes on the rich and privatization. Not even the most abject triangulator would argue that these were good ideas that were badly implemented. The debate is about the merits of them in principle.

So, what does fit into the set? Robert Harris suggested one possibility when he tweeted:

Brexiteers are sounding increasingly like Marxists: the theory is perfect, it just hasn't been implemented properly...

But I don't think this works. Marxism is like liberalism or conservatism - a loose set of principles. The failure of a few Marxist governments does not discredit Marxism, any more than the failure of conservative or liberal governments discredits conservatism or liberalism. Of course, central economic planning is a bad policy. But it's bad in principle; the problem is not that the USSR had the right idea but screwed up the implementation.

So what are we left with? There are contenders: numerous government IT projects, the Child Support Agency or individual learning accounts for example*.

All these are, however, second-line policies - the equivalent of squad players rather than stars. Nobody saw ILAs as their lifelong political project or identified emotionally with them in the way they do with Brexit. These tell us that bad administration is common - that there's a great deal of ruin in a nation. Routine incompetence is commonplace - in fact, the norm.

I'm left, then, with my question: how many major, government-defining policies can we describe as good policies badly executed?

There's a reason why there have been so few. It's that implementation is policy. Policy-making is not like writing newspaper columns. It's all about the hard yards and grunt work of grinding through the detail. A failure of implementation is therefore often a sign that the detail hasn't been thought through, which means the policy itself is badly conceived. Reality is complex, messy and hard to control or change. Failing to see this is not simply a matter of not grasping detail; it is to fundamentally misunderstand the world. If you are surprised that pigs don't fly, it's because you had mistaken ideas about the nature of pigs.

It is, therefore, often unacceptable to hide behind the "good policy, badly implemented" line. Bad implementation is at least sometimes a big clue that the policy was itself bad.

All this, though, raises another question. What about the mirror image of "good policy, badly implemented"? It's vanishingly rare that someone says of modern-day British politics that something was a terrible policy but beautifully implemented**. Why might this be?

* King and Crewe add tax credits to this list, but I'm not sure about that; given the trade-off between support for the poor and high withdrawal rates, and the difficulty of supporting people's whose circumstances often change, administering the system was always going to be tricky. And I don't think it's been terribly bad given the scale of the challenge.

** I suspect that free marketeers could say this about the minimum wage, but I'm struggling to think of other examples.