“PART of the job of an intellectual community,” said Laurie Patton, president of Middlebury College in Vermont, “is to argue.” Introducing Charles Murray, a controversial author, on March 2nd, she emphasised the audience’s right to non-disruptive protest. Excitable students who thought Mr Murray unacceptably prejudiced—one of his books touches on the relationship of race to intelligence, though he has also written on the white working class—evidently considered that offer insufficient.

Their protests quickly escalated from jovial catcalling to prohibitive heckling and then—after Mr Murray was interviewed on camera by Allison Stanger, a Middlebury professor, in a separate room—into violence. Ms Stanger’s hair was yanked; the car in which the pair departed was mobbed. “I feared for my life,” she subsequently wrote.

In this latest tussle between campus advocates of free expression and those seeking to banish views they think lie beyond that concept’s ambit, there is some cause for optimism. Ms Patton turned up to the talk, organised by a student club, and afterwards apologised. The college ensured Mr Murray could be heard; it is investigating the scuffle. Still, like the trouble that erupted recently over an offensive speaker in Berkeley, California, the violence at Middlebury—real violence, not the imaginary sort some hotheads think Mr Murray’s beliefs inflict—is an ominous turn.

Ominous for the left, in particular. As with previous incidents, this was as much a clash between different generations of liberals as between left and right. Ms Stanger made clear that she sympathised with Mr Murray’s critics: “We have got to do better by those who feel and are marginalised,” she wrote, adding a sideswipe at Donald Trump. But, as elsewhere, the dust-up pitted her old sense of openness against students’ moral certitude and tightly circumscribed idea of proper discourse.

Meanwhile, Mr Murray was left to worry about academic freedom and to note that many of his assailants resembled figures from “a film of brownshirt rallies”. Middlebury’s agitators might ask themselves how a man whose work they decry as racist acquired the right to compare them to fascists. Students everywhere should wonder how free speech, a central liberal value, is instead becoming the banner of conservatives.