“If Congress had passed some common-sense gun safety reforms after Newtown,” President Obama said on Friday, “we don’t know if it would have prevented what happened in Charleston.” Actually, we do know: Had the bill to which Obama was referring been enacted, it would not have stopped Dylann Roof from murdering nine people at Charleston’s Emanuel AME Church last week.

Obama’s comment reflects the magical thinking that horrific crimes like Roof’s seem to invite: If only we had adopted Policy X, this might not have happened. That tendency — driven by the understandable desire to “do something,” as Obama put it — is most conspicuous among supporters of gun control but is not limited to them.

Obama was talking about legislation proposed following the December 2012 massacre at Sandy Hook Elementary School in Newtown, Conn. — in particular, “reforms that 90 percent of the American people supported,” meaning expansion of the background check requirement for gun buyers to include sales that do not involve federally licensed dealers. But as CNN reported the day of Obama’s remarks, Roof bought the .45-caliber Glock Model 41 pistol he used in the church attack from a Charleston gun store, which means he passed a background check.

There is no reason to think he wouldn’t, because Roof apparently did not have a criminal or psychiatric record that would have disqualified him from owning a gun under federal law. Although he was arrested for illegal possession of Suboxone, a schedule III narcotic, in February, the misdemeanor charge was not enough to bar him from buying a firearm.

If Roof used Suboxone without a prescription, that would have legally disqualified him from owning a gun, because federal law excludes anyone who is “an unlawful user” of a controlled substance. But a gun dealer would have had no way of knowing about Roof’s tastes in psychoactive chemicals, because the FBI’s National Instant Criminal Background Check System does not track people’s drug habits — a good thing, because otherwise anyone who has recently consumed cannabis (even in states where it’s legal) or used someone else’s prescription medication would be stripped of the right to own a gun in practice as well as theory.

In short, “universal background checks” make no sense as a response to the Charleston attack. Then again, universal background checks made no sense as a response to the Sandy Hook massacre, the perpetrator of which used a rifle legally purchased by his mother and in any case did not have the sort of criminal or psychiatric record that would have legally barred him from buying a gun on his own.

Gun controllers like Obama were not the only ones tempted by magical thinking after the Charleston massacre. Sen. Lindsey Graham, R-S.C., whose voting record earned him an A from the National Rifle Association, suggested such crimes could be prevented by “just being able to track people — put them into systems where they can be deterred or stopped.”

Surely Roof would have been stopped by this imaginary surveillance system, Graham said, because “I bet there were some indicators early on that this guy was not quite there.” But odd people with eccentric or extreme ideas, even odd people who drunkenly spout off about starting a race war, far outnumber mass murderers.

Monitoring everyone who is “not quite there,” therefore, is not a very practical approach, even leaving aside the implications for civil liberties in a free society where holding abhorrent beliefs is not supposed to be a crime. As Graham conceded, “it’s very complicated in a nation of 300 million people where you have freedom of movement and freedom of thought.”

Similarly, Obama fantasized in an interview last week about “some common-sense stuff” that respects the right to keep and bear arms yet “prevents a 21-year-old who is angry about something, or confused about something, or is racist” from “going into a gun store” and buying a weapon. Don’t worry: The details will be worked out later.

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