We're always on a slippery slope. Life is a slippery slope to death. Law is a slippery slope to its misuse. Thursday cannot be held responsible for Friday. But that's not the dangerous misapprehension I speak of. What's dangerous is supposing that David Irving is simply putting forward a point of view, however cranky, to history.

This is to misunderstand the nature of the errand Irving and other Holocaust deniers are on. Let's be plain . . .

Holocaust denial is Nazism in another guise.

The paradox at the heart of Holocaust denial is that it sets out to refute what it wished had happened.

Search the literature and you will not discover a Holocaust denier who is troubled by the idea of extermination itself, who finds Auschwitz inconceivable morally rather than mathematically, who is not at least half in love with Hitler, and who does not believe that, however much we scale down the numbers, the cause of whatever took place was the inherent odiousness of Jews.

Common to them is the conviction that the 'Auschwitz Lie' is a fabrication by the Jews themselves to extort guilt and reparation, to justify the crimes of Zionism, and to further their ambition of world domination. The Holocaust denier, in other words, repeats the identical slander which fuelled the crime he maintains never happened. The Holocaust denier does not reframe the Holocaust to spare the Jews, but to give himself the satisfaction of killing them all over again in his imagination.

Brooded over in the quiet of the study and the heated privacy of the mind, the word for this is paranoia, and its characteristics, above the usual delusional ones, are emotional violence and hate.

Proclaimed in public, at neoNazi rallies and the like, the word for it is rabble-rousing: an incitement to emotional, and ultimately to actual, violence and hate.

The one word which does not describe it is history.

Holocaust denial is hate ideology in action; its aim is not the propagation of a truth but the propagandising of that ideology. How far, then, we should accord it the respect owing to free speech is moot. I am not deaf to the concerns of those who regret Irving's sentence. The last time Irving was taken to the cleaners, I wrote of the emptiness I was surprised to feel. Not because his humiliation upset me on a personal level but because it is not good when the field of conflict falls silent. You need to keep argument alive; you need an interlocutor of mettle.

Naive of me. Far from hardening into unquestionable official truth with his disgrace, the Holocaust is now more subject to scepticism and fatigue than ever. Those British Muslims who boycott Holocaust Memorial Day on the grounds that it is "exclusivist", and the elected president of Iran who proposes turning his entire nation into a Holocaust Denial Centre, have seen to that. In so far as challenges to the authenticity and deployment of the Holocaust are the issue, who needs David Irving?

But should he be in prison, we go on asking? Should he not be allowed his say, no matter that what he says is malignant and stimulates malignancy in others?

Twenty-five years ago, Noam Chomsky was at the centre of a row because he prefaced a book by Robert Faurisson, a well-known and far more virulent Holocaust denier than Irving. Chomsky's argument was that the viler the speech, the more we had to defend its right to be heard. Why then he had to defend Faurisson from the charge of vileness, calling him a "relatively apolitical sort of liberal" while insisting he had scarcely read a word he'd written, is a bit of a mystery. But then, so is Chomsky's own position on the Holocaust. Time and time again, he has trotted out the same description of it, not changing a syllable, as "the most fantastic outburst of collective insanity in human history". A formulation I find eerie by virtue of its coldness, its lazily incurious vocabulary ("outburst"? "insanity"? ), and its use of the word "fantastic", which appears to deny what it describes, leaving one to wonder just whose the fantasy was.

Yet, consider another way in which Holocaust denial is murder. We forget, when we scratch our heads over Irving's sentence, that in those countries where it is called a crime, Holocaust denial is understood not only to lie but to defame. The defamed, in this instance, being the dead. Primo Levi wrote of a terrible dream shared by all survivors of the death camps, of never being listened to, of those they love turning from them in suspicion and disbelief.

Mankind's constant enemy, Levi wrote, are the "negators of truth". Those who should have been confuting those negators . . . the true witnesses . . . were precisely those who couldn't, for they had been annihilated.

Survivors "spoke in their stead, by proxy", but their words were at a remove, and subject to mistrust. Thus, through the callous logic of destruction, does terror wear away the memory of its crimes. Holocaust deniers are defamers of its victims.

They accelerate the process of forgetting, murdering a second time those already murdered. In the name of the living and the dead, may they rot in prison, and rot in hell thereafter.