For those of us who were alive in 2001, forgetting the day itself is nearly impossible. I remember the shock and the fear, the replays of the second plane hitting the World Trade Center, and the moment the second tower fell. I remember watching on television as missing-person photographs covered New York, and the awful recognition that very few of those people were likely to be found alive. I remember the bravery of the New York firefighters and police.

My college students have no such memories or feelings. Most were born years after Sept. 11, 2001. They encounter what happened through photographs, documentaries, memorials, classroom lessons, and stories told by people who were alive that day. The National September 11 Memorial & Museum estimates that about 100 million Americans have been born since the attacks, and the institution has launched campaigns aimed at educating this generation with no firsthand memory of the day. That generational handoff is the one every historical trauma eventually undergoes — as with the attack on Pearl Harbor, whose living witnesses have dwindled to a handful — but experiencing it happen to one's own defining event is something else again.

The shift is ordinary and yet somehow startling. An event that once felt as though it had split time into a permanent before and after is fading into history itself.

In my work as a professor of politics and public affairs, I pay close attention to the stories Americans tell about their past and the ways those stories shape civic identity in the present. The 25th anniversary of 9/11 interests me especially because it marks a moment when an event once experienced as immediate and world-changing is increasingly carried forward through memory, ritual, and interpretation.

Understanding history includes forgetting

Scholars of collective memory, also called public memory, have long argued that societies do not remember the past simply by preserving it. Public memory describes how collective meaning is made and remade through stories, institutions, rituals, and political arguments. For example, the Department of Homeland Security was created in response to 9/11. Many Americans understand DHS as a structural and cultural reform made to fix the intelligence failures the attacks made evident.

Forgetting is part of the process, too. As memory scholar Paul Connerton has argued, some forms of forgetting are not failures but necessary parts of social life. No individual, school, or nation can keep everything equally present forever. History accumulates faster than collective memory can accommodate it.

The question, then, is not whether Americans will forget parts of 9/11. They will. The more interesting question is what remains after that forgetting takes place.

From the first anniversary onward, commemorating the attacks has carried a careful choreography: the annual reading of the names of those who died, the memorial lights, and the stories of first responders — each keeping the human loss and courage of the day in view. Those rituals matter, because for the growing share of Americans with no living memory of 2001, they are increasingly the memory itself.

In a 2016 Pew Research Center survey, 76% of Americans named 9/11 as one of the 10 most consequential events of their lifetimes. Nearly 1 in 5 said the nation's immediate response to the attacks was the moment they felt most proud of the United States.

I understand those points of view. Some of what I most want preserved about Sept. 11 is the shared grief of those first days and the ways ordinary people revealed their humanity under unimaginable circumstances.

Firefighters climbed stairs and ran toward mortal danger while a cascade of people tried as hard as they could to get down. Office workers helped colleagues and strangers. Families searched for missing loved ones long after hope had become terribly thin. And passengers on United Flight 93 fought back and forced the plane they were on to crash in a field in the middle of Pennsylvania, sacrificing themselves in the hope of saving others.

Before 9/11 became a justification, a policy, a war, or a political symbol, it was an experience of shared human vulnerability.

Politics and its consequences

No event remains sealed inside its original meaning. Sept. 11 became the starting point for a series of political choices that reshaped American life and, in turn, reshaped the memory of the day itself.

The attacks led to the war in Afghanistan, which lasted for 20 years after the invasion's initial success in dismantling al-Qaida and removing the Taliban government. Next came the invasion of Iraq, even though Saddam Hussein's regime had no role in 9/11. And the aftermath of the attacks helped create an atmosphere that allowed for expanding government surveillance, enlarging presidential power, and narrowing the space for dissent. Muslim Americans found themselves subjected to new suspicion and hostility.

Those consequences now compete with the original experience of Sept. 11 for a place in its meaning.

For some Americans, the attacks have become inseparable from the wars and the broader official willingness to sacrifice civil liberties in the name of security that followed. From this perspective, 9/11 is remembered not only as a national trauma but also as a warning about how fear and grief can make policies that once seemed unthinkable appear necessary.

For others, the day — and its aftermath — represent restored American pride and military perseverance. For still others, 9/11 remains most powerfully a story about innocent lives lost and ordinary people behaving with extraordinary courage.

Each way of remembering draws a boundary around the terrorism the nation experienced, deciding how far beyond that September morning its meaning should extend.

Even events once regarded as world-defining eventually have to share the historical landscape with everything that follows. Since 2001, Americans have experienced Hurricane Katrina, the 2008 financial crisis, the election of the first Black president, the rise of Donald Trump, the violence on Jan. 6, 2021, and a global pandemic.

Sept. 11 has not become unimportant because those things happened. Its place in the story has changed because life — because the world — kept going.

For people who were alive during the attacks, there is something sad about watching that distance and diminishment happen. When an event once remembered in minute detail becomes a date others know only from history, the passage of one's own life becomes newly visible. There is a strange nostalgia in realizing that something once so immediate now belongs increasingly to people who know it only as the past.

What are we to remember?

Maybe this is why "never forget" feels both moving and impossible.

What is supposed to be the subject of our remembering? The people who died? The courage? The grief? The fear? The wars that followed? The mistakes? The unexpected decency of people thrown together on an unbearable day?

A society cannot keep all of those things equally vivid forever. Nor should its members confuse remembering everything with understanding anything. The task of collective memory is inevitably one of selection.

For me, one part of the memory seems especially important to preserve. Before Sept. 11 became an argument about war, security, patriotism, or political allegiance, it was an encounter with human vulnerability and then human resilience. The solidarity of Sept. 11 and the political choices made in its aftermath were not the same thing, even if they have become intertwined in our memory of the period.

If the ongoing march of history means Sept. 11 cannot always carry the emotional weight it does for people who were alive when it happened, perhaps the best outcome would be to focus on the collective grief, along with the extraordinary human response that grief engendered. Doing so would not erase any of what followed. But it might help preserve the understanding that the day was one of unimaginable horror and tragedy before it became a political inheritance.

This article has been corrected with the location where UA93 crashed.

Stephanie A. (Sam) Martin, Frank and Bethine Church Endowed Chair of Public Affairs, Boise State University. This article is republished from The Conversation under a Creative Commons license.