Jeremy Corbyn and John McDonnell never tire of saying that they condemn racism “wherever it comes from”. But where does it come from? Surely – in Western culture at least – its original, most enduring and venomous form is anti-Semitism.

It is true that hating people because of their race or nationality is evil whatever that race or nationality is. If someone hates – to take an improbable example – all New Zealanders, he does them as much wrong as he does to all Jews if he hates them. If Turks hate Armenians, or Hutus hate Tutsis, or Serbs hate Albanians, the deaths of hundreds of thousands can result – indeed, have resulted. But anti-Semitism is more than any of these things. As well as being...