Three weeks ago, a California professor was ambling across her campus when she saw a teenager holding up a political sign she didn't like. What happened next reveals a larger problem: the steady assault on free speech at universities.

Associate professor Mireille Miller-Young and a group of her political allies marched over, confronted the kid and demanded that she remove her sign. When the youngster refused, Miller-Young ripped it out of her hands, took it to her office and cut it up. When the professor was asked if there had been a struggle, she said, no, "I'm stronger so I was able to take the poster.'"

A professor stopping speech because she's "stronger"? What's going on here, and why does it matter?

American democracy is built upon free speech and free assembly, including the right to express views others might abhor. America's universities should vigorously defend that essential freedom and pass it on to the next generation. If you disagree with someone, put up your own signs. Explain your own views. Say why others are wrong. What you cannot do — what you must not do — is shout down anyone or overpower them, as Miller-Young did. The only exception is when speech poses an immediate danger to specific people, such as a Klansman burning a cross at a civil rights worker's home. That conveys a deadly threat. Except for these rare cases, the answer to "bad speech" is always "more speech." To see that value affirmed here in Chicago, just walk into the Tribune's inspiring entryway. Col. Robert McCormick covered its walls with quotations defending free speech and its corollary, a free press. Those are bedrock values in a liberal democracy like ours.

Alas, Miller-Young never learned those values. She has a lot of company in the faculty lounge. Violations of free speech happen all the time on campus, usually without much attention or any push-back from administrators. When former Israeli Prime Minister Ehud Olmert spoke at the University of Chicago a few years ago, his opponents objected vehemently. Some demonstrated across the street, which is perfectly fine. That's what "more speech" means. But some Palestinian activists infiltrated the lecture hall and stood up repeatedly to disrupt the talk. That's shameful, and the university did nothing.

The same thing happened this year at Brown University when a former New York City police commissioner tried to speak about stop-and-frisk laws. Some students called him a racist and stomped and shouted to prevent him from speaking. They succeeded. Last year, at one of the country's leading colleges, Swarthmore, some radical environmentalists took over a university investment meeting and prevented others from speaking. They wanted the school to sell its stock in an energy company. Swarthmore's president and dean of students were actually present — and did nothing. That shouldn't be a surprise since the college's own speech code is a direct assault on free speech. The school proudly announces that it will remove any signs with "uncivil expression." Who decides what's "uncivil"? Why high-ranking school officials do, of course. They also define "harassment" as any unwelcome conduct. So, if you happen to think something is unwelcome — anything at all — then you've been harassed. There is more free speech on a Saturday morning cartoon show.

The same intolerance seeps into classrooms, usually from activist professors who know exactly what's right and insist you parrot their views. Students are wary, naturally, of disagreeing with the authorities who grade them, especially if those professors are rigid ideologues. Smart students simply avoid those professors or feign agreement. Just imagine you are lucky enough to get into one of Miller-Young's classes in her specialty, "feminism, queer theory and pornography." What would happen if you spoke up in class and, say, opposed her views on abortion or gay marriage? That's only a hypothetical, of course. No one in his or her right mind would do it.

Yet good seminars thrive on thoughtful differences. If you disagree with a position, whether it's the professor's or another student's, explain why and ask for a reasoned response. Universities should encourage this kind of discourse — and demand accountability from teachers who quash dissent.

Fortunately, Miller-Young's school does provide a place for dissent. The University of California at Santa Barbara has set aside a small area as a "free speech zone." That's where the teenager was holding the sign. Yep. They have a little leper's colony for free speech. Why isn't the whole university a place for free speech? It's California, not Crimea.

Universities should be citadels of open debate, even when the views expressed make people uncomfortable. Get over it. Respond with a better argument. Make your own sign. Hold your own meeting. Great universities are built on free speech, free inquiry and spirited debate. So are great countries.

Charles Lipson is a political science professor at the University of Chicago.