WASHINGTON — As the museum of human nature, aka the United States Holocaust Memorial Museum, marks its 25th anniversary, it continues to receive artifacts, such as a letter handwritten on a yellow scrap of paper. It was donated to the museum by Frank Grunwald, 85, who lives in Indianapolis.

He was the younger of two Czechoslovakian boys who sit smiling on their mother’s lap in a photograph the museum has. It was taken before this Jewish family was swept into the Nazi murder machinery. Frank, then 11 and known as Misa, is alive because unlike his brother John, then 16, Frank did not limp. In July 1944, their father was segregated with male prisoners who were working in an Auschwitz factory. The boys were with their mother in the Czech family section of the camp when a Nazi noticed John’s limp and selected him for gassing. Unwilling to have John face death alone, on July 11, Vilma went with him, leaving behind this letter to her husband:

“You, my only one, dearest, in isolation we are waiting for darkness. We considered the possibility of hiding but decided not to do it since we felt it would be hopeless. The famous trucks are already here and we are waiting for it to begin. I am completely calm. You — my only and dearest one, do not blame yourself for what happened, it was our destiny. We did what we could. Stay healthy and remember my words that time will heal — if not completely — then — at least partially. Take care of the little golden boy and don’t spoil him too much with your love. Both of you — stay healthy, my dear ones. I will be thinking of you and Misa. Have a fabulous life, we must board the trucks.

Into eternity, Vilma.”

So, the museum presents human nature’s noblest as well as vilest manifestations. It has received 43 million visitors, 90 percent non-Jewish, many of whom have had opportunities to talk to survivors, such as Fanny Aizenberg, who in her 102nd year still comes most Sundays. Located just off the Mall, one of the world’s most pleasant urban spaces and the epicenter of American politics, the museum inflicts an assaultive, excruciating knowing: Nothing — (BEG ITAL)nothing(END ITAL) — is unthinkable, and political institutions by themselves provide no permanent safety from barbarism, which permanently lurks beneath civilization’s thin, brittle crust.

This is why the Holocaust is the dark sun into which this democracy should peer. Calling the Holocaust unfathomable is a moral flinch from facts that demand scholarship, which the museum enables. It has, for example, more than 900 video interviews with witnesses and collaborators. And perpetrators, such as Juozas Aleksynas, a member of a Lithuanian police battalion that committed genocide in Belarus in 1941:

“We were issued Russian guns and bullets … some were exploding bullets. … A person’s skull opens up so fast. … They would carry children — the little ones — they’d take the others by the hand. They lie down, lay the child next to them. … First you shoot the father. … How would the father feel if the child was shot by his side?”

An album found long ago in an abandoned SS barracks contains photos of Auschwitz guards and administrators at leisure — singing, picnicking. It includes some of the few pictures of a short, dark-haired man — Dr. Josef Mengele, who escaped prosecution for his “medical” experiments, drowning in 1979 while swimming in Brazil.

In his mind-opening 2017 book “Why? Explaining the Holocaust,” Peter Hayes says the subject “continues to resist comprehension.” Resist, but not defy. His many conclusions include the awesome — for better or worse — power of individual agency: No Hitler, no Holocaust. But Hitler began tentatively, with small measures. Hayes concludes his book with a German proverb: (BEG ITAL)Wehret den Anfangen(END ITAL) — beware the beginnings.

Today, there is an essentially fascist government in Hungary. Anti-Semitism is coming out of the closet: The Labour Party, which might form Britain’s next government, is riddled with it, from the top down. Blood-and-soil tribalism — degenerate successor to throne-and-altar conservatism — is fermenting across Europe. And there is a name for what is happening to the Rohingya in Myanmar: genocide. The museum of human nature remains what it would prefer not to be: pertinent to understanding not only the past but the present.

How do those who work at the museum, immersed in the task of making us remember the unspeakable, maintain their emotional equilibrium? By also remembering Vilma.

George Will’s email address is georgewill@washpost.com.