IT’S possible that Jeremy Corbyn is not “an anti-Semite and a racist”, as Dame Margaret Hodge is said to have called him last week, but…

Actually, let’s stop there. The problem is always what comes after the “but”. “I believe in free speech, but” is almost always followed by some statement which demonstrates that the speaker doesn’t, in fact. Similarly, no one ever says “I am a racist, but I think we should let them all in.”

True, the Labour leader uses a variant on the “I’m no racist, but” technique, reaching instead for the apparently more inclusive conjunction “and”. His preferred formulation is “I condemn anti-Semitism and all forms of racism”. It’s difficult to see why the first four words of that sentence wouldn’t do perfectly well on their own, were it not apparent that a great many of Mr Corbyn’s supporters see the next bit of the statement not as an amplification, but as a qualification.

If you doubt this, have a look at Twitter, where any suggestion that the Labour Party has any sort of an issue with anti-Semitism is met by howls of protest and abuse from Mr Corbyn’s fans. These protestations often prove what they are trying to deny, because they frequently move rapidly from specious distinctions between Jews and the political stance of the state of Israel into wild claims that political parties, the banks, the media, or the capitalist system in general are run by a cabal of Zionists.

Mr Corbyn keeps assuring us that he would obviously like to distance himself from these racists, but… There we are again, you see. He never does so without some addendum.

So let’s judge the party’s problem by its actions under his leadership. Labour appointed Shami Chakrabarti to look into whether anti-Semitism was a problem, and she very quickly said it wasn’t. Then Mr Corbyn very quickly gave her a peerage.

The National Executive Committee suspended Ken Livingstone for saying Hitler “was a Zionist until he went mad”, then decided not to expel him (until he spared their blushes by resigning).

Mr Corbyn makes regular statements condemning Israeli government action, but has never condemned Hamas for firing rockets at the country; less than a decade ago, as well as describing them as “friends”, he was calling for the group to be removed from Britain’s list of banned terrorist organisations, and appearing on Iranian state-funded TV after it had been censured by Ofcom.

A lot of Jewish Labour supporters, unimpressed by all this, staged a protest outside Parliament recently. And the party’s response last week? To draw up a policy that deliberately changes the usual definition of anti-Semitism drawn up by the International Holocaust Remembrance Alliance, which is accepted almost universally.

It deliberately omits or rewords such standard anti-Semitic tropes as claiming that the state of Israel is intrinsically racist, accusing Jews of being more loyal to Israel that their own country, comparing Israeli policy to the Nazis, and judging Israel (the Middle East’s only democracy) by different standards from other countries. Such as, say, the 18 Arab League countries which don’t recognise Israel’s right to exist at all.

For some, including the Chief Rabbi and many Labour members, that confirms the problem, and the failure to tackle it. It ought to strike every reasonable person as bizarre. Bluntly, if the Corbynite wing of the Labour Party isn’t racist, why does it keep weaseling away from the issue?

Part of the answer lies in the certainty of the Left that it is intrinsically virtuous. This belief is so deep-rooted that it persists in spite of any evidence to the contrary, which is what makes it so difficult to correct.

It’s hard to imagine people openly describing themselves as Nazis, and arguing that the Third Reich was a misapplication of fascism, which would otherwise have worked splendidly, yet Communists make this argument all the time. The fact that every one of the numerous attempts to implement Communism have led to economic failure, suppression of human rights and mass murder doesn’t seem to matter, because their intentions are pure, so it wouldn’t happen if only it were implemented properly.

This accounts for the hard Left’s notorious tendency to split into smaller and smaller factions, and readiness to accuse other groups of having strayed from the One True Way. It also explains the way in which Momentum activists, convinced of their own rectitude, can concentrate on attacking the perceived shortcomings of liberal democracies, while turning a blind eye to the actual racism and oppression of many non-Western regimes. Stairhead Marxists of the kind who think that Tony Blair is on the far right of the political spectrum are now in charge of the party.

But the assumption of virtue is pervasive even amongst less extreme figures who identify with Leftist causes. It has created the cult of the NHS, where what matters is its structure, rather than its results or wide provision of free care, the avoidance of “elitism” in schools and universities, and the insistence on public ownership of industries, even when it’s obviously counter-productive.

Liberal or “progressive” causes which suffer from the same problem include well-meaning measures intended for our own good, such as the sugar tax or bans on the advertising of junk food.

It’s at first sight paradoxical that what often calls itself liberalism should so often be illiberal, but it is the natural conclusion of a smug certainty in one’s own political virtue. The belief that you are morally in the right very seldom has anything to do with politics, which is mostly a matter of entertaining doubts, weighing up the practical evidence, and trying to see the other person’s point of view.

The attitude that you can’t be racist because you define yourself as an anti-racist, or that human rights and economic realities can be disregarded if your motives are pure is the opposite of the genuinely liberal virtue of tolerance. Tolerance implies judgment; it’s just that it doesn’t assume that the belief that someone is wrong creates a corresponding right – or worse still, a duty – to put them right.

It was CS Lewis who pointed out that “a tyranny sincerely exercised for the good of its victims may be the most oppressive”, because the “omnipotent moral busybodies” imposing it “do so with the approval of their own conscience”. Coincidentally, it was Susanna Centlivre’s play of 1709, The Busie Body, which was the first to produce the rebuke: “But me no buts.”