On Tuesday afternoon an Israeli academic was shouted down by two dozen protesters as he tried to begin a lecture before about 100 students and faculty at the University of Minnesota. The speaker was Moshe Halbertal, a professor at NYU Law School and a professor of Jewish thought and philosophy at Hebrew University. He was invited to deliver the Dewey Lecture in the Philosophy of Law, which is organized annually by the law school. That the freedom to present a lecture is threatened in this way at a public university is appalling, calling not only for punishment of violations but for a clear statement by university officials defending the free exchange of ideas.

The lecture, which I attended, was delayed half an hour as one by one the protesters stood up to shout denunciations of Israel and were escorted from the hall by university police. One young woman came screaming back into the lecture after having been ejected. Outside the hall, the protesters chanted so loudly that it was difficult to hear Halbertal, much less to concentrate on what he was saying, until 45 minutes after the lecture was to have begun.

The protests were apparently organized by a group calling itself the “Anti-War Committee,” which bragged on its Twitter feed about having disrupted the lecture and complained that the protesters’ “free speech” rights were violated when a few were arrested. It appears that no law students were involved, but many of the demonstrators were college-aged and the protest was endorsed by a group called Students for Justice in Palestine (SJP), a university group. According to its Facebook page, SJP “promotes justice, human rights, liberation, and self-determination for the Palestinian people.”

The lecture was entitled, “Protecting Civilians: Moral Challenges of Asymmetric Warfare.” The talk did not directly address the Israeli-Palestinian conflict, though Halbertal drew in part on his experience helping to draft the Israeli army’s code of ethics. When he was finally able to speak, Halbertal argued that in fighting “asymmetric wars” (typically, wars between professional militaries and insurgencies or resistance movements) professional combatants should err on the side of protecting noncombatants from casualties, even when they thereby increase risks to themselves or to their cause.

It was a careful and nuanced presentation, one that was far more dovish and human-rights oriented than caricatures of Halbertal as a “war crimes apologist” by protesters suggested. But the protesters had no interest in hearing the lecture or in allowing the audience to hear it. Halbertal told me that in all of his lectures on the subject of warfare, including at Columbia University, this was the first time he had been subjected to a disruptive demonstration.

There are two core principles at stake for a university in what happened Tuesday. One is that there is no right to shout down a speaker at an academic lecture on the grounds of a public university. The University of Minnesota prohibits such disruptions, as do other universities. Such policies raise no First Amendment problems on their terms. University police acted appropriately to remove the demonstrators. Now the university has an obligation to investigate and punish any violations of these policies.

The second principle is that a university community–including its faculty, staff, administrators, and students–must cultivate a norm of respect for free speech that goes beyond ensuring mere First Amendment compliance. Members of a university community have an obligation to consider opposing viewpoints and, if not always a duty to listen to them, then at least a duty to allow others to listen to them.

The problem is that, increasingly it seems, some members of university communities believe they are entitled to shut down speakers because they deem the expression offensive. This can occur before the speech by having the speaker “disinvited,” during the speech by heckling the speaker, or after the speech by filing formal complaints claiming that the speaker created a hostile environment. These are all forms of a heckler’s veto. But mere offense cannot justify the heckler’s veto; otherwise no speech on any interesting topic would be heard on campuses.

Others say that whatever the constitutional obligation of government (and that includes public university administrators) to allow offensive speech, they are not personally morally obligated to sit silently while, say, a Nazi advocates genocide. But the Nazi-exception-to-all-rules-of-conduct-and-civility should be used very sparingly. I’d concede the moral force of the point as applied to actual Nazis, but not much beyond that. There are legitimate criticisms of Israeli policies and innumerable legitimate ways those criticisms can be aired. But preventing others from hearing Israeli speakers who aren’t even defending Israeli policy cannot be one of them.

So beyond punishing the disruption of a public lecture, a university cannot accept any moral legitimacy in such an act or equivocate in denouncing it. It is fundamentally illiberal and destructive to the core values of an educational institution. And ironically, those who espouse unpopular and minority causes would be most vulnerable in a world where mobs feel authorized on principle to decide who may speak.