Americans spend a lot of time in crowds. We enjoy exercising our constitutional right to assemble freely.

Nine times so far this month, for example, baseball fans have thrust themselves onto a Green Line train, like Spam in a can, heading toward the Navy Yard stop so that they could join an even-bigger crowd at Nationals Park. Washington this month also celebrated the National Cherry Blossom Festival, complete with a parade, and Tuesday there was another parade downtown, marking Emancipation Day.

Is there safety in numbers, or danger?

What do you do, day-to-day, in our increasingly urbanized world, when you know there are bad people out there for whom the killing and maiming of innocent people is not a tragedy but a successful outcome?

This is America’s psychic challenge after the horrific bombings Monday in Boston, a terrorist attack that was different from what we’d seen before — smaller in scale, on a city street, right on the sidewalk outside a LensCrafters and a candy store called Sugar Heaven. Such terrorist attacks have been part of the cultural landscape in other parts of the world for decades. Outdoor cafes, restaurants, subway stations and buses have all been blown up for a variety of political causes.

The United States hadn’t seen that until Monday. Terrorists have tended to hit military, financial or government targets, or major transportation systems.

We’ve seen a shoe bomber (foiled) and an underwear bomber (foiled) on jetliners. A terrorist struck at Fort Hood in Texas. At the millennium, a terrorist planned to hit Los Angeles International Airport. In 1993, terrorists detonated a truck bomb in the garage of the World Trade Center, hoping to cause the North Tower to fall and knock down the South Tower. And there was 9/11, of course, which struck again at the icon of international trade as well as the Pentagon.

The attack on the Boston Marathon is more akin to the 1996 Olympic Park bombing in Atlanta. The Boston bombs, reportedly inside pressure cookers packed with metal, were seemingly designed for maximum maiming, as well as media exposure.

No amount of security and intelligence-gathering is ever going to make us perfectly safe. This is an open society, not a police state, and so the best response to a tragedy such as the one in Boston is to go on with your life, eyes open. Alert, but not paranoid. Like the signs say: If you see something, say something.

So far, the Boston terrorist attack remains a mystery: Who did it? Why? To what conceivable purpose? What purpose, however twisted, would be served by setting the bomb that killed 8-year-old Martin Richard? The boy was there with his family at the marathon’s finish line. Now, the boy is gone, and his mother and one of his sisters are hospitalized with what news accounts refer to as “grievous” injuries — and Monday, we all saw, in those Internet images, what grievous looks like.

Suspicion can mislead us all, whether we’re amateurs or professionals. Repeatedly, in big stories such as this, misinformation goes viral. Speculation abounds. If there’s a link to an overseas terror network, then what happened in Boston is a continuation of the conflict that gave us 9/11, scaled down this time, and instead of four jetliners loaded with fuel, the weapons were small enough to fit into a couple of duffel bags. But maybe it was just one guy, a lone wolf with a malevolent agenda. The hardest thing to stop.

So now comes the question: How do we react, collectively? Culturally? More security? More checkpoints? More suspicion?

Risk analysis does not come naturally to us, as we’re shaped for better or worse by anecdotal evidence, our personal experiences, not by statistical trends. The hard facts tell us that terrorism is rare. And terrorists do not pose an existential threat to American society. At best, they can achieve a psychological goal, of getting into our heads, making us fearful, causing us to overreact individually and as a nation.

Strong countries are resilient. Life has to go on, terrorism or not. You can’t maneuver through life as if there’s an improvised explosive device around every corner.

“The American people refuse to be terrorized,” President Obama said Tuesday morning. “Because what the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love.”

Obama caused a ripple of controversy a few years ago when he told Bob Woodward that the United States could survive another 9/11.

“We can absorb a terrorist attack. We’ll do everything we can to prevent it, but even a 9/11, even the biggest attack ever . . . we absorbed it and we are stronger.”

At the very least, we have to get up off the pavement, like that 78-year-old Boston Marathon runner, Bill Iffrig, the one who crumpled when hit by the shock wave of the explosion just a few yards from the finish line. He’s the man in the now-iconic photograph of the three police officers springing into action with an older man — Iffrig — on the pavement, facing away from the camera. Iffrig was on TV on Monday night, describing what happened, his tone remarkably matter-of-fact:

“Everybody else is out there having fun, and you got one or two people trying to destroy the whole thing. It’s hard to figure out. Terrorists, whatever they are. . . . I don’t have much use for it.”

This is why terrorists won’t win: too many Americans like Iffrig, who, when they get knocked down, get right back up again.

And yes, he finished the race.