Even if terrorists were able to pull off one attack per year on the scale of the 9/11 atrocity, that would mean your one-year risk would be one in 100,000 and your lifetime risk would be about one in 1300. (300,000,000 ÷ 3,000 = 100,000 ÷ 78 years = 1282) In other words, your risk of dying in a plausible terrorist attack is much lower than your risk of dying in a car accident, by walking across the street, by drowning, in a fire, by falling, or by being murdered.

So do these numbers comfort you? If not, that's a problem. Already, security measures—pervasive ID checkpoints, metal detectors, and phalanxes of security guards—increasingly clot the pathways of our public lives. It's easy to overreact when an atrocity takes place—to heed those who promise safety if only we will give the authorities the "tools" they want by surrendering to them some of our liberty.