According to the Brady Campaign to Prevent Gun Violence, the deadly August 5 attack on a Sikh temple near Milwaukee showed “our elected officials” need to “do something.” Slightly more specifically, it said we should “Demand Congress Stop Arming Dangerous People.”

But how do we know who is dangerous? The Brady Campaign mentioned “convicted felons,” “convicted domestic abusers,” “terrorists,” and “people found to be dangerously mentally ill.” Jacob Sullum says it overlooked a crucial category: people with dangerous ideas.