Not long ago, a dissatisfied reader emailed that he had enough guns to stop people like me. I emailed back to ask whether he was threatening me.

The reply: “I'm not stupid enough to telegraph genuine ill intent.”

On Monday, the Supreme Court will hear a case involving the question of when a seemingly threatening communication (this one on Facebook, not email) can be a crime. Let’s clear up some confusion, shared by my correspondent above, about what threats are and why they can be punished.

The case is Elonis v. United States. Anthony Elonis lived in Lower Saucony Township, Pennsylvania. Until 2010, he was married with two children and worked at a nearby theme park. In May 2010, his wife left him, taking their two children. Not long after that, he was fired because of multiple complaints of on-the-job sexual harassment (for example, a female coworker alleged that he found her alone in the office at night and began to undress).

He turned to Facebook. About his former coworkers, he posted: “I have sinister plans for all my friends and must have taken home a couple [of keys].” About his ex-wife, he posted: “I’m not going to rest until your body is a mess, soaked in blood and dying from all the little cuts.” When she got a restraining order, he posted, “I’ve got enough explosives to take care of the state police and the sheriff's department” and “I’m checking out and making a name for myself Enough elementary schools in a ten mile radius to initiate the most heinous school shooting ever imagined ... The only question is ... which one?” FBI agents came to his door; he posted his fantasy of killing one female agent: “Pull my knife, flick my wrist, and slit her throat Leave her bleedin’ from her jugular in the arms of her partner.” He was convicted in federal district court of five counts of transmitting in interstate commerce (here, the Internet) “any threat to injure the person of another.”

Elonis argued that, under the First Amendment, the government had to prove that he had a “subjective” intent to threaten. He said he lacked that, in part because some of his posts echoed words by rapper Eminem. The court of appeals held instead that the statute only requires that “a reasonable person would foresee that the statement would be interpreted by those to whom the maker communicates the statement as a serious expression of an intention to inflict bodily harm.”

Let’s break that down carefully. Elonis argues that the government must prove beyond a reasonable doubt that he was actually thinking, This message will terrify the person it refers too, and I want that. The government says that it must only prove that “a reasonable person” would have thought it would terrify.

There’s a split in the circuits over this issue. The Supreme Court took the case in order to clarify what the government needs to prove. If the standard is “reasonable person,” then the government can largely rely on the words a defendant uses and the general context in which they are used (for example, if they are sent directly to the target or confided to a blog, etc.). If it is “subjective intent,” however, prosecutors must use a defendant’s specific acts and circumstances to convince a jury he meant to alarm or threaten the targets.

Even if Elonis wins at the high court, he can still be prosecuted and possibly convicted under the “subjective” standard. It wouldn’t make convictions impossible—juries assess “subjective intent” all the time without needing telepathy—but it would make them harder to get.

But many people don’t understand the first step: What is a threat and why is it not protected by the First Amendment?

Federal courts summarize this area by saying that “true threats” are a historic exception to “the freedom of speech” protected by the First Amendment. “True” is an unfortunate term, because it convinces some people that the implied statement—“I am going to harm or kill you”—must be true.

But that’s wrong. Threats are not illegal because they signal an impending crime. Threats themselves are the crime. As the Supreme Court explained in the 2003 “cross-burning” case, Virginia v. Black:

The speaker need not actually intend to carry out the threat. Rather, a prohibition on true threats “protect[s] individuals from the fear of violence” and “from the disruption that fear engenders,” in addition to protecting people “from the possibility that the threatened violence will occur.”

To understand why this is true, imagine the delinquent high-school kids who declare a holiday by phoning a bomb threat to the school. There is no bomb, and the kids wouldn’t set one off if they could. Nevertheless, school must be canceled. The building has to be evacuated. The bomb squad must don protective gear and sweep the classrooms. Students will suffer nightmares. And authorities may someday be tempted to ignore notice of a genuine planned bombing. The threat itself is the harm.

The category of “true threat” is narrow, though, because government has often used the idea of “threat” as an excuse to punish dissenters, whose speech is often exaggerated and inflammatory. The leading threat cases are Black, which concerned the Ku Klux Klan’s use of the cross to intimidate families and whole communities; Watts v. United States, which reversed the conviction of a civil-rights protester who warned that if he was drafted, “the first man I want to get in my sights is LBJ”; and NAACP v. Claiborne Hardware, which reversed a civil judgment against civil-rights leader Charles Evers, who had made a speech in which he told African Americans violating a boycott of local stores, “If we catch any of you going in any of them racist stores, we're gonna break your damn neck.”

All of these situations involve political protest that turned unruly. That’s why the “reasonable person” standard could be treacherous. Throughout history, judges have tended to assume that the powerful are reasonable and that those who assail them are dangerous.

This case, however, like most threat cases, does not involve political protest. And threats against co-workers, former spouses, school children, and law enforcement are a powerfully disruptive force in our heavily armed nation. Elonis posted his words on Facebook, surely knowing that they would be conveyed quickly to his ex-wife, his co-workers, and the FBI agents. The Facebook posts have attracted a lot of attention—many commentators see this as a case about rap lyrics and Internet speech—but really, it’s only relevant as part of a general question about context. If Elonis had sent the threats to the targets by mail, there would be little doubt of his intent. If he had written them in a diary that was discovered by accident, there’d be little doubt that they were protected. Where do Facebook postings fall?

Make it too easy to prove a threat, and government can muzzle those it dislikes; make it too hard, and the rest of us—on the job, on the streets, and in our homes—are at the mercy of men like Elonis.