There are two kinds of absence, and the tension between them is part of the memory of the Holocaust. One is a mere void, a nothing formed by amnesia and denial.
The other is a potent force, and I experienced it by accident last year when I was in the charming German city of Heidelberg, whose university has been for centuries a great centre of European culture. I was strolling along the narrow, perfectly-preserved streets, when I found myself in a small, oddly-empty square.
It took me a few moments to realise that it was not empty at all. What it contained was the ghost of the synagogue that stood there from 1887 to the night of November 10th, 1938 - Kristallnacht - when Hitler's thugs went on the rampage and announced, more eloquently than in words, their intention to annihilate the Jewish people.
The Heidelberg synagogue was burned down that night. Four years ago, the city redesigned the square with exquisite and devastating delicacy. The outlines of the vanished synagogue are now marked in paving stones of white marble.
Grey granite indicates the windows and entrance. Twelve sandstone cubes, some of them illuminated, symbolise the pews and recall the 12 tribes of Israel. A small stone, inscribed in German and Hebrew, starkly records the destruction of the synagogue. It is the perfect Holocaust monument. Its silence and virtual invisibility chill the bones. By saying almost nothing, it conveys almost everything.
By contrast, one of the most depressing moments of recent times came when Germaine Greer had run away from the circus that was Celebrity Big Brother. Explaining why she had finally snapped, she complained that the other housemates had refused to join her protest against Big Brother's "fascist" bullying.
"Persecution is what happens," she said, "holocausts are what happen when good people do nothing." The holocaust in question was BB's refusal to give John McCririck a Diet Coke.
That a deeply-cultured, highly-intelligent and supremely-articulate woman such as Greer could connect a trivial incident in a TV show to the greatest act of organised genocide that the world has seen reminds us of how the Holocaust can be everywhere and nowhere at the same time.
Ubiquitous though it may be in literary, cinematic and televisual imagery, the memory of Auschwitz and the other camps has become increasingly empty. It may be that the more it is talked about, the more banal it becomes.
There is, I think, a reason for this that goes deeper than the general debasement of language and history in our culture. The memory of the Holocaust is squeezed between two contradictory but equally compelling imperatives.
One is the need to preserve a sense of uniqueness; the other is to make connections. On the one hand, we have to hold on to an awareness that the systematic attempt to eliminate an entire people holds a special place in the history of human depravity. On the other, this very sense of uniqueness can prevent us from recalling the Holocaust, not just as a passive and pious memory, but as an active tool for human survival.
When the great and the good gather at Auschwitz this week, for example, how many of them will choose to acknowledge that their exposure to the sickness of the Nazi mentality failed to inoculate them against the same disease?
Within 20 years of the liberation of Auschwitz, each of the victorious Allied powers had recreated concentration camps of one kind of another. The Soviet gulags were filled up again after the war. The British incarcerated around 80,000 Kikuyus, including children, in concentration camps in Kenya in the 1950s.
The French pushed around two million people into camps or exile in Algeria. The Americans forced tens of thousands of Vietnamese peasants into so-called "strategic hamlets". China still has its "re-education camps".
The idea that the Holocaust was a unique event helped to preserve the victors from awkward comparisons. But Nazi-style tactics - mass executions, collective punishments, concentration camps - have been used again and again by supposedly civilised nations since 1945.
Even the heir of the Holocaust itself, the state of Israel, has used its own collective memory of victimhood to justify brutal repression of Palestinian civilians. And, more passively, the world that swore never to tolerate genocide after Auschwitz has done precisely that, both in Rwanda and in Bosnia.
Perhaps, after 60 years, the time has come to break the cordon of difference around the Holocaust and to acknowledge that its uniqueness is largely a matter of scale. Without trivialising its horror, can we not recognise that the impulse behind it was not unique to Germany or the Nazis?
Once it is possible for someone cold-bloodedly to murder a neighbour, rape a child, or burn a place of worship and not feel guilty because the act was "justified" by political or religious necessity, it is also possible to murder a million people, establish organised rape camps, or exterminate an entire nation.
All the Holocaust tells us that we might not have known from other episodes of collective sadism is that there is no limit to the consequences of such attitudes. That knowledge must be the absent synagogue that occupies a permanent space in our minds and hearts.