Sticks and stones may break your bones, but words will never hurt you. Or so we were told by our mothers. But events on both sides of continent in recent weeks seem to belie that old adage. A new generation of protesters has come to the conclusion that words do hurt — and that therefore, extreme measures, up to and including physical force, are justified to keep them from being spoken.

At Berkeley last month, a riot broke out over a speech planned by Milo Yiannopoulos, a sort of professional conservative troll who worked for Breitbart until a scandal over some remarks on pedophilia cost him his job and his book contract. This was not simply setting things on fire or breaking a few windows (though those would have been quite bad enough); multiple people seem to have been beaten by the "antifas" (anti-fascists). In the videos that have been released so far, the anti-fascists look a lot closer Nazi brownshirts than the people they're trying to stop. There was further violence over the weekend in Berkeley at a pro-Trump march.

Then a few days ago, a speech by Charles Murray at Middlebury College in Vermont also turned violent, and a professor was injured as she walked with Murray after his speech. Murray has given his own personal account of what occurred, and a lengthy video of the proceedings is available on the web. They are not as frightening as what happened at Berkeley, but they are plenty horrifying enough: They shouted him down, refusing to allow him to speak, then banged on the building and pulled fire alarms when he was transferred to a private room to do a streaming talk they were unable to disrupt. Finally, they tried to physically prevent him from leaving.

The fact that two different speeches triggered violence at two different campuses within the space of a month suggests that we may be entering into a new and more dangerous phase of the anti-free-speech movement. Free-speech advocates, particularly the Foundation for Individual Rights in Education, have done a great job pushing back against overweening college administrations that try to curtail the speech of students and professors. But these are actions coming from the students. Who do you sue to keep a mob of students from resorting to the heckler's veto, or their fists, to combat ideas they don't like?

I asked that of my friend Greg Lukianoff, the president of FIRE, who agrees that while it's early to call this a trend, there are definitely some warning signs. Hitting people with sticks and starting fires "does seem to us to be a new and scary thing," he says.

Why is it happening? He points to one possible contributing factor. "One thing we really noticed that things had changed was the progression of 'safety' into meaning 'perfectly comfortable,' " he said. Once you've defined words as being equivalent to assault, then you're plausibly justified in using violence to repel the threat.

That's basically the logic of the editorials that the Berkeley student newspaper published in defense of the rioters. "A peaceful protest was not going to cancel that event," wrote student Juan Prieto, "just like numerous letters from faculty, staff, Free Speech Movement veterans and even donors did not cancel the event. Only the destruction of glass and shooting of fireworks did that. The so-called 'violence' against private property that the media seems so concerned with stopped white supremacy from organizing itself against my community."

The implicit assumption here is that their protest movement is not merely entitled to be heard, but to win — win with a victory so total that no voice is ever even raised in opposition. And if they cannot win by raising their voices, then they must move on to more aggressive means. This makes sense only if, as Lukianoff says, you define Yiannopoulos's outrageous statements as equivalent to violence, or worse than violence.

I will admit that this is a coherent worldview. Indeed, it cohered for decades in the old Soviet Union. But most of us don't want to live in the world it leads to, if for no other reason than because we aren't so confident that we'll get to be the ones choosing who needs to be violently silenced.

Lukianoff notes that the illiberal left often argues that the distinction between violence and speech is open to challenge by those who think it is harmful. "When people say that the distinction between speech and violence is a social invention," says Lukianoff, "I say: Yes! It is a social invention, and it's one of the most important things we ever came up with."

In a pluralistic society where many questions are contested, the alternative to letting your enemies have their say is not that they shut up and you get to live in peace. It's that both sides arm up. Though the investigation is still ongoing, it appears that one of Yiannopoulos's fans may have turned up at an event looking forward to mixing it up with protesters (a video allegedly shows him telling someone in the crowd "they have to start it!"). He later shot a 34-year-old anarchist who was there to protest. When people show up at demonstrations looking for trouble, they rarely have difficulty finding it.

As I've written before, there's a real danger that the illiberal left and right are going to heterodyne each other into escalating rounds of violence, each side justifying what it does by whatever the other guys did last. If they do, they'll drag the rest of us into disaster. The majority of Americans want to live in a liberal order, where disputes are settled with words — sometimes angry words, wrong words, mean words, but still just words, allowing everyone to walk away with their bones intact. But if we want to keep living there, we need to act now to preserve it, because Lukianoff argues that when it comes to speech, the slippery slope is real, and perilously steep.

But when the censors are the students, what can we do? Well, when it comes to physical violence, however noble the cause, that's assault, not speech, and the perpetrators should be arrested. But when it comes to lesser but still illiberal mob tactics like using a "heckler's veto" to "no platform" speakers they don't like, the questions are harder.

Lukianoff has wrestled with this: "What can the rest of us do? Investigate. Get real stories. Talk to the people who were harmed. Talk to the people who were involved and find out whether they think they're helping their cause."

I wish he had a more satisfying answer, one that guaranteed we could reach an equilibrium where both protesters, and the people they're protesting, have the right to express themselves. But that's the thing about speech: it doesn't guarantee we'll find satisfying answers. What it does is make sure that we all have the freedom to keep asking questions that matter.

Megan McArdle is a Bloomberg View columnist. She wrote for the Daily Beast, Newsweek, the Atlantic and the Economist and founded the blog Asymmetrical Information. She is the author of "The Up Side of Down: Why Failing Well Is the Key to Success."