Our insistence on patterns and commonalities and some kind of understanding assumes coherence to the massacres, rationality. But the difference between the aimless, alienated young men who do not plant bombs or open fire on unsuspecting crowds — which is the vast majority of them — and those who do is less likely to be some discrete radicalization process that we can diagram and eradicate than a dose, sometimes a heavy one, of pure madness. And there’s no easy antidote to that. No amulet against it.

There’s also a danger built into the American experiment, the very nature of which leaves us exposed. Our rightly cherished diversity can make the challenge of belonging that much steeper. Our good fortune and leadership mean that we’ll be not just envied in the world, but also reviled.

The F.B.I. averted its gaze from the older Tsarnaev brother after it couldn’t find any conclusive alarms because that’s what the government is supposed to do, absent better information. We don’t want it to go too far in spying on us. That means it will fail to notice things.

While we can and will figure out small ways to be safer, we have to come to terms with the reality that we’ll never be safe, not with unrestricted travel through cyberspace. Not with the Second Amendment. Not with the privacy we expect. Not with the liberty we demand.