"Guns don't kill people," says the old NRA slogan. "People kill people." Yesterday, while responding to the mass shootings that killed 22 people in El Paso and nine in Dayton over the weekend, Donald Trump offered a dubious variation on that theme. "Mental illness and hatred pulls the trigger, not the gun," he said.

Although the president's speech was scripted, that line was illogical as well as ungrammatical. For one thing, I've never heard anyone claim that "the gun" pulls "the trigger," and I'm not sure how that would work in a non-cartoon universe. For another, it is plainly not true that "mental illness and hatred" pull the trigger—or do anything else, for that matter. We should not lose sight of that fact that mass murder is volitional behavior, as tempting as it may be to speculate about the killer's motives or mental state.

The president offered two theories. "We must reform our mental health laws to better identify mentally disturbed individuals who may commit acts of violence and make sure those people not only get treatment, but, when necessary, involuntary confinement," he said. But Trump also decried the "racist hate" that seems to have inspired the El Paso shooter.

In practice, those two explanations may be hard to disentangle. "These are sick people," Trump's chief of staff, Mick Mulvaney, told ABC News on Sunday. He added, referring to the El Paso shooter, "You cannot be a white supremacist and be normal in the head."

Mulvaney thereby demonstrated the malleability of psychiatric diagnoses and the danger of letting them determine who shall be free and who shall be subjected to "involuntary confinement." Under current law, shaped by civil libertarian concerns about that danger, states generally have to show by "clear and convincing evidence" that a candidate for commitment, because of mental illness, poses a danger to himself or others.

A belief in white supremacy, or any other abhorrent ideology, is not enough to meet that test, even though it might occasionally motivate murderous violence. The risks of going down that road should be obvious to anyone who values freedom of conscience and freedom of speech.

While Trump seems to think it should be easier to lock people up and forcibly subject them to psychiatric treatment, the "clear and convincing" standard of proof is, according to the Supreme Court, required by the Fourth Amendment. The thing to be proven, however, varies from state to state, so there is some room for adjustment there.

Under Florida's Baker Act, for example, the state has to show by clear and convincing evidence that there is a "substantial likelihood" a candidate for commitment will, because of mental illness, "inflict serious bodily harm" on himself or others "in the near future." Alabama similarly requires "a real and present threat of substantial harm to self and/or others." Massachusetts, by comparison, requires "a substantial risk of physical harm" to self or others, without reference to timing. New York's standard is similar.

When Trump says we should "reform our mental health laws to better identify mentally disturbed individuals who may commit acts of violence," he may have in mind loosening the standards for commitment by reducing the requisite level of risk or imminence of harm. But that would not really "better identify" potential killers; it would just cast a wider net, ensnaring more harmless people as well as (perhaps) more people with homicidal tendencies.

For what it's worth, Columbia psychiatrist Michael Stone estimates that "one in five mass murderers shows evidence of psychosis," which does not necessarily mean mental health specialists would have deemed them dangerous enough for commitment. "Over thirty years of commentary, judicial opinion, and scientific review argue that predictions of danger lack scientific rigor," University of Georgia law professor Alexander Scherr noted in a 2003 Hastings Law Journal article. "Scientific studies indicate that some predictions do little better than chance or lay speculation, and even the best predictions leave substantial room for error about individual cases. The sharpest critique finds that mental health professionals perform no better than chance at predicting violence, and perhaps perform even worse." Since psychiatrists are terrible at predicting violent behavior, the potential public safety payoff from making involuntary psychiatric treatment a bit easier does not seem worth the cost, especially if you are one of the innocent people caught in the wider web.