'In the US, invoking offended feelings is morally equivalent to saying you feel raped, starved or strangled'

Disagreement is one the greatest sources of pleasure available to intellectuals. If I find someone agrees with me, I think it's time to change my mind. Academics' lust to ban speakers because they are controversial, off piste, unfashionable, pernicious or even just plain wrong seems inexplicable to me. Without controversy, we stagnate in like-mindedness - and where's the fun, stimulus or satisfaction in that? Most readers of The Times Higher , therefore, are unlikely to jib at advocacy of academic freedom. But suppose that for "academic freedom" we substitute an increasingly popular phrase: "intellectual diversity"? How does that change the game?

The Missouri State legislature has passed a Bill - not yet enacted in law - mandating "intellectual diversity" in universities. College authorities would be obliged to meet "concerns (that) shall include but not be limited to the protection of religious freedom, including the viewpoint that the Bible is inerrant". The agenda underlying the legislation is therefore pretty clear. And it is right and helpful to air and contest religious views in academic arenas. The question whether there is a useful sense of "inerrant" that can properly describe the Bible seems to me impassioningly interesting.

So far, so good. But there are a couple of weird features that make "intellectual diversity" very different from that of academic freedom, and much less attractive.

First, the Missouri legislation would "protect" students who feel they have been "penalised for the expression of an opinion". So if a teacher marks a student down for insisting, in defiance of correction, on an obviously false proposition, such as Henry VIII was a rhinoceros, or that the Nazis killed no Jews, the student could file grievances; the professor would be investigated; much time, emotion, money and talent would be wasted; and, likely as not, the relevant department's funding would be cut.

Second, the effect of "intellectual diversity" would not be to encourage debate, but to arrest it. If all opinions command equal and mandatory reverence, no one can safely venture to disagree; for to tell people they are wrong is, in a land of over-developed sensitivities such as the US, to "penalise" them.

The latest academic-freedom controversy in the US - the decision of the regents of the University of California to withdraw an invitation to Larry Summers - is shocking for three reasons: first, the petitioners against Summers included university teachers. We are used to attacks on freedom from outsiders - usually paranoid bureaucrats and self-interested lobbyists - but academics generally have enough sense to realise that academic freedom has, at least, the merit of being in their own interests. The petitioners justified victimising Summers by saying they felt "offended" by his views on the obstacles to scientific attainment by women. In the US, invoking "offended" feelings is now morally equivalent to saying you feel raped, starved or strangled. But it is good to be offended: it shakes you out of complacency. If you have a true academic vocation, you tackle contrary opinions; you don't bleat about side-effects on your emotions.

Second, the regents' professed excuse for "disinviting" Summers was "avoiding controversy". That is not a good reason for doing anything in the academic world.

Finally, the petitioners' charges were worryingly vicious: Summers, they claimed, "symbolised sexist and racist views". There is no record of his ever uttering a view that could reasonably be construed as racist, though he once questioned the bona fides of an Afro-American scholar. The nearest he has got to sexism is musing whether women on average have less aptitude for science than men: there are similar grounds for the hypothesis that men have less aptitude in professions where women predominate. It would be sexist to say that women are generally inferior to men, but Summers did not say that. It would also be sexist to use the claim of different aptitudes to deny women a chance in science. But the context of his remarks showed that Summers was actively - though not necessarily very effectively - looking for ways to increase and encourage female scientists.

On present showing, it looks as if we are approaching a future in which irate professors can ban Larry Summers from challenging conventional wisdom while having to endure drivelling lip from a kid who thinks the moon is made of green cheese. We need to be clear about this: academic freedom exists to enhance education. It includes teachers' right - and inviolable duty - to disagree frankly with each other, to tell pupils when they are talking rot, and to privilege truth only through debate that neither demonises candour nor divinises consensus.

Felipe Fernández-Armesto is Prince of Asturias professor of history at Tufts University in the US.