"What happened, happened," said philosopher Jean Améry, a survivor of Auschwitz. "But that it happened," he added, "cannot be so easily accepted." The Holocaust cannot be "accepted" because so many questions linger in its wake. How? Why? Those two small words ask the most persistent questions: How did the Holocaust happen? Why did it occur?

Holocaust scholar Raul Hilberg resisted "big questions" because he feared "small answers." Most Holocaust scholars find that "the devil is in the details." They do not jump to sweeping generalizations that "explain" the Holocaust. Instead, they gather, sift, and sort the evidence. They preserve and study documents, retrieve and evaluate testimony; they develop, analyze, and criticize the narratives that tell what happened.

As the 21st century begins, scholarly research focuses on questions such as: Precisely when did the Nazis decide to destroy European Jewry completely? What parts did Hitler, Himmler, Heydrich, and other key Nazi leaders play in the "Final Solution"? And were the perpetrators "ordinary" people or "willing executioners" of a specifically German kind? What is the significance of Jewish resistance during the Holocaust? How should oral histories from survivors be used in Holocaust research? How will memory of the Holocaust best be preserved as that event recedes further into the past and the survivors are no longer alive? Can the fundamental challenges that the Holocaust creates for Judaism and Christianity be credibly met? What can literature and art teach us about the Holocaust? In what ways is the Holocaust unique?

Although understanding emerges through inquiry about such issues, the resulting insights do not silence the questions How? and Why? How the Holocaust happened depends on why people acted as they did. Why people do the things they do is a question that historians can answer partly, but historical inquiry alone cannot master the range of human feeling, intention, reflection, and choice that enters into desires, motivations, yearnings, aims, hopes, and decisions. "Why did the Holocaust happen?" is a question that lingers even after our knowledge about how it happened has increased.

Why? also lingers because, as historian Saul Friedländer has pointed out, the smallest details raise the largest "whys." Those details--they involve the systematic murder of nearly six million Jews--show that Holocaust history is not history-as-usual. The Holocaust stretches comprehension beyond its limits. The more we learn about the Holocaust, the more devastating it turns out to be. Far from being satisfied, Why? lingers--silently if not openly--with unsatisfied intensity. Jean Améry was right: "What happened, happened. But that it happened cannot be so easily accepted."

What was lost includes more than a million Jewish children who were killed and, in addition, the countless girls and boys who might have been born if their potential mothers and fathers had not been murdered. Still, the "going on" is real, too. It includes children, grandchildren, and other descendants of the survivors. These flesh-and-blood survivor legacies already number in the millions. If the Holocaust is to be well remembered, their testimony will be important, too.