This past week, Harry Bains became something of an American hero when he, in his words, “saw something and said something.” The New Jersey bar owner spotted Ahmad Khan Rahami, the alleged terrorist charged with littering bombs across New York and New Jersey, sleeping in the doorway of his business. He immediately called the cops.

“If you see something, say something” has become the unofficial slogan of post-9/11 America. The mantra, posted on billboards and public transportation, turns us all into amateur anti-terrorism crusaders. Any of us, it suggests, could foil the next Osama bin Laden, as long as we stay alert.

That’s not always a good thing. The expression makes us vigilant, but it also makes us paranoid. It’s turned us into a country of people who see danger lurking inside every forgotten backpack, making an in­cred­ibly remote risk feel imminent. Americans shouldn’t be encouraged to live in unreasonable fear.

‘If you see something, say something” was born on Sept. 12, 2001. New Yorker and advertising executive Allen Kay came up with the phrase without a client in mind — he wanted to create something positive in the days after the attack on the twin towers. “The model that I had in my head was ‘Loose Lips Sink Ships,’ ” Kay told the New York Times. “I wasn’t born during World War II, but I sure knew the phrase and so did everybody else.”

He jotted the idea on an index card and kept it in his office. A few months later, when the MTA needed a safety slogan, he passed it on. In 2002, the phrase was one of several warnings the agency focus-grouped for a new ad campaign on city subways and buses. Others included “Be suspicious of things that look suspicious” and “If you see a package without a person, don’t keep it to yourself.”

“If you see something, say something” was the favorite, and the agency adopted it that December. It got attention. Reports of suspicious packages in New York grew from 814 in 2002 to 37,614 in 2006. Since then, the MTA has spent $2 million to $3 million a year on slogan-adorned placards for trains, subway cars and buses, as well as radio and TV ads. In 2007, the agency even trademarked the slogan.

“If you see something, say something” has since been adopted by the Department of Homeland Security, the Transportation Security Agency, Amtrak, and cities like Chicago, San Francisco and Melbourne, Australia. The MTA doesn’t charge other entities that want to use the slogan, though it will reject applicants that want to broaden the focus too much. According to the Times, the MTA refused to license “if you see something, say something” to a university that wanted to use it to tackle dorm burglaries. In recent months, a retooled campaign in New York featured the faces of locals who saw and said something.

Is this really such a good thing? Today, the New York Police Department receives roughly 100 suspicious-package calls a day (that number has surged since the Chelsea bombing last weekend). The vast majority of those tips generate no terrorism leads. In fact, it’s not clear that the tip line has ever prevented an attack; authorities refuse to say. According to a New York Times analysis, no terrorist has been stopped because of the tipline. Some people even use the hot line to call in phony bomb threats.

I worry, too, about a slogan that forces people to constantly imagine the worst. Today, 75 percent of Americans see terrorism as a “critical concern,” according to a Public Religion Research Institute (PRRI)/Religion News Service poll, and nearly half are worried that they or someone in their families will be a victim of terrorism. “The fear level seems terribly high given the actual likelihood of this happening to an individual. That speaks to the deep-seated feelings of anxiety that people have,” PRRI research director Dan Cox told USA Today.

This even though since Sept. 11, 2001, terrorism in the United States has been extremely limited. According to New America, just 94 people have been killed in America by violent jihadist attacks in the past 15 years; 48 have been killed in far-right-wing attacks. In the same period, more than 500,000 people have died in car accidents.

Identifying a real threat becomes even harder in a place like New York, where unaccompanied tanks of liquid nitrogen covered in warning signs are a regular sidewalk sight. Packages sit on doorsteps; should we really trust that it was FedEx that placed them there? Not to mention curbside piles of old luggage, discarded appliances and all those trash bags that could be hiding God knows what. It’s hard to look anywhere in New York and not see “exposed wiring or other irregularities” — on the MTA’s list of things that should prompt concern. Suggestions that we report suspicious behavior inadvertently encourage racial profiling. For example, college student Khairuldeen Makhzoomi, 26, was kicked off a Southwest Airlines flight in California after another passenger reported him for speaking Arabic. Texas ninth-grader Ahmed Mohamed was handcuffed at school after he brought in a homemade digital clock that looked, to some, like a homemade bomb.

And if everyone who saw something said something, New York would cease to function. Case in point: investigations of suspicious packages have at times been a leading cause of subway delays.

After 9/11, America vowed to never forget. But that doesn’t mean we should obsess constantly about the next attack.