Three days after the publication of Michael Waldman’s new book, “The Second Amendment: A Biography,” Elliot Rodger, 22, went on a killing spree, stabbing three people and then shooting another eight, killing four of them, including himself. This was only the latest mass shooting in recent memory, going back to Columbine.

In his rigorous, scholarly, but accessible book, Waldman notes such horrific events but doesn’t dwell on them. He is after something else. He wants to understand how it came to be that the Second Amendment, long assumed to mean one thing, has come to mean something else entirely. To put it another way: Why are we, as a society, willing to put up with mass shootings as the price we must pay for the right to carry a gun?

The Second Amendment begins, “A well-regulated Militia, being necessary to the security of a free State,” and that’s where Waldman, the president of the Brennan Center for Justice at the New York University School of Law, begins, too. He has gone back into the framers’ original arguments and made two essential discoveries, one surprising and the other not surprising at all.

The surprising discovery is that of all the amendments that comprise the Bill of Rights, the Second was probably the least debated. What we know is that the founders were deeply opposed to a standing army, which they viewed as the first step toward tyranny. Instead, their assumption was that the male citizenry would all belong to local militias. As Waldman writes, “They were not allowed to have a musket; they were required to. More than a right, being armed was a duty.”