When is guilt by association not a fallacy? I ask because of two things I've seen recently.

The first is Daniel Hannan's fear that we're heading for "absolutely the most harmful outcome imaginable" of Brexit, "namely leaving the Single Market while keeping the Customs Union." He tries to blame everybody but himself for this, to which Jonn Elledge replies: "it is not enough to blame your opponents for the world's failure to live up to your fantasies."

The second example is David Goodhart's claim that the treatment of Windrushers is an outrage. To this, Jonathan Portes accuses him of "astonishing hypocrisy" as Goodhart was "one of the most vocal cheerleaders for the "hostile environment" from the beginning, knowing full well what it would mean."

In both cases, their opponents accuse Hannan and Goodhart of a form of guilt by association. Hannan's support for Brexit, his opponents say, associates him with the fiasco we have, whilst Goodhart's anti-immigrationism, it is alleged, implicates him in the Windrush scandal; Goodhart denies this by saying he's not responsible for bad implementation.

Let's take an obvious example of the guilt by association fallacy: "how can you be a vegetarian? Hitler was a vegetarian!" This has exactly the same structure as: "how can you support Brexit when it is also supported by little Englanders and racists and will be implemented by buffoons?" Or: "how can you support the hostile environment policy when it's supported by racists and implemented by [insert derogatory adjective here]?"

So, if my first example is an obvious fallacy, why aren't by second and third examples?

The answer is that sometimes association has information value. When it does, guilt by association is not a fallacy.

"Hitler was a vegetarian" tells us nothing about the merits or demerits of vegetarianism. However, the fact that a policy is supported by racists and will be implemented by people who fall well short of angelhood does have information value: it alerts us to the type of policy we'll get.

In saying this, I'm not using hindsight. Just before the referendum I wrote:

Some of you have a vision of a Britain outside the EU that is a free, liberal, socialistic country. These are ideals with which I have sympathy. But we are kidding ourselves if we think a vote for Leave will be a move towards such a society. Instead, it'll be a mandate for Farage and the inward-looking, reactionary mean-spirited philistinism he embodies.

And later in 2016 I said of immigration targets that:

if you give power to the state it'll be misused, because the actually-existing state is a stupid bully. Just as "anti-terror" laws have been used to harass journalists and peaceful protestors, so immigration controls will hurt decent people. And for the same reason - because they are the softest targets.

If someone of my limited cognitive skills could see this, I'd expect others to do so.

Critics of Hannan and Goodhart, therefore, are right. The fact that their causes are associated with bad people was a strong clue that they were indeed bad ideas.

Many of you, I guess, will be with me so far.

But here's the thing. You can make exactly the same criticism of me. I support much of Labour's economic policy, especially anti-austerity and backing for coops. To this, some will ask: how can I do so, given that such policies are also supported by anti-Semites, big staters and various cranks and fanatics?

My answer is that these are a much smaller fraction of Labour supporters than were (say) little Englanders and neo-racists of Brexit, and so the information value of crankish support for such policies is low. But I'm not 100% confident in this answer.