John Staurt Mill thought that speech should always be free except when it directly incited a crime. Mill explained what he meant in On Liberty by citing the example of corn dealers, who were widely loathed as profiteers in 19th century Britain. If agitators claimed that corn dealers starve the poor, the law had no right to punish them, he said. Only if agitators say the same to an angry crowd ready to burn the corn dealer's house down can the state intervene.
Notice that Mill does not say that the law can punish the incitement of hatred against corn dealers – for it is not a crime to hate people, any more than it is to envy them or lust after them. Free speech is such an essential right, the state can only silence speakers who rouse violent mobs.
On these grounds, the Home Secretary should not ban Islamist hate preachers from coming to Britain unless she has evidence that they are planning to visit a synagogue and incite the murder of Jews.
No more should she ban Geert Wilders or American shock jocks unless she can prove that they are planning to urge a crowd to storm a mosque. The answer to hate speech is better speech, as the old saying has it.
The excusers of hate preachers never make anything approaching Mill's principled argument. Instead, they reveal the British Left's worst side as they defend the perpetrators of hate speech by denying that there is anything hateful about him.
When you watch Baroness Jenny Tonge for the Liberal Democrats, Jeremy Corbyn for Labour or the Muslim Council of Britain, Cage Prisoners or – and I am ashamed to say this – the "liberal" press where I make my living, you see them deploy two tactics. The first is a determined refusal to admit the nature of radical Islam. They never discuss the misogyny, homophobia and antisemitism, let alone stir themselves to confront it.
Second, they pretend that anyone who does describe and condemn it is a part of the supernaturally powerful "Israel Lobby" – or the "International Jewish Conspiracy," as previous generations called it.
And they get away with it. Ed Miliband and Nick Clegg do not withdraw the whip. Polite society does shun them. I have never forgotten the deference with which the production team of Any Questions treated Baronnes Tonge. They had invited me on to the programme with her, perhaps because they thought it a bit of laugh to sit someone called 'Cohen' next to someone who had updated the medieval blood libel and suggested that an inquiry was needed to ascertain whether Jews were stealing the vital organs of Christian earthquake victims. Certainly, they did not see anything bizarre or shameful in the spectacle of self-declared liberal leftists endorsing and indulging reactionaries.
I wonder how long this toleration of hypocrisy will last. It seems a permanent feature of leftish British life that has been with us for so long no one can imagine a new Britain where the alliance between the white left and the Islamist religious right is challenged and exposed.
Yet Britain can change with incredible speed. We have seen the festering scandals of MPs expenses and Rupert Murdoch's excessive power explode like lightning from a clear sky. Issues that small groups shouted about for years went mainstream almost overnight. The same will happen with the liberal-left's support for the Islamism. It won't be the issue of Israel that provokes a change – the actions of the Likud government deserve no sympathy – but the issue of Iran, where antisemitic hatred is a part of the theocracy's official ideology.
In my last column I wrote that Labour is taking a huge risk in running the tired and seedy figure of Ken Livingstone as its candidate in next year's London mayoral elections. No sooner had I filed the copy than a delegation of Iranian leftists came to see me. They had been sickened by the sight of Livingstone taking the money of Press TV, the propaganda station of the Iranian government that has murdered thousands of their comrades, and tortured and raped tens of thousands more. The Iranians intend to picket him at every stage of the way in the campaign and force the British media to ask two questions which few apart from Martin Bright, Andrew Gilligan and your humble correspondent have raised before.
How can the British Left justify fellow travelling with regimes and movements whose attitudes to gays, women, democrats, liberals and Jews are rooted in the foul traditions of clerical fascism? And what precisely makes them think that they can get away with it?