Hostility toward freedom of speech in American colleges and universities has been growing quietly for decades, but lately it has become impossible to ignore.

The toleration of unpopular opinion was once considered central to the purpose of a liberal education, which was not to indoctrinate students dogmatically but to teach them how to form beliefs.

But these classically liberal norms of toleration and open inquiry have given way to an activist conception of the mission of higher education. In practice, this amounts to the establishment of a specific ideology that its advocates refer to only in such generic terms as “social justice.”

Dissent from this new orthodoxy is increasingly treated as heresy: beyond the pale of argument. The irony of this retreat from the classically liberal mission of the university is that it vindicates the very arguments for freedom of speech and intellectual diversity that it rejects.

Consider the obsession with diversity, understood almost exclusively in racial and sexual terms, which ignores the decrease in ideological and political diversity — especially in the humanities departments of our elite institutions.

The University of Michigan, where I am a professor of philosophy, consistently ranks as one of the top public universities. Recently our new president announced that he wanted to broaden our definition of diversity a bit: “All too often it becomes focused on race and ethnicity, and those are incredibly important. But to me, equally important is the diversity of experience, diversity of culture, socioeconomic diversity, or geographical diversity.”

Notably, intellectual diversity is left off the list. To be fair, though, Michigan does sponsor an endowed Lecture on Intellectual Diversity. This year’s lecture was titled, “Beyond Double Jeopardy: Exploring the Intersection of Race and Gender.”

In order for our beliefs to be justified, we must be able to answer the best arguments against them. Yet John Stuart Mill observed that people naturally dislike confrontation with opposing arguments and tend to avoid it. These widespread human tendencies undermine the justification of our beliefs, making the toleration of unpopular opinions a prerequisite for knowledge.

For these reasons and others, Mill defended freedom of speech in uncompromising terms: “there ought to exist the fullest liberty of professing and discussing, as a matter of ethical conviction, any doctrine,” regardless of its falsity, immorality, or even its harmfulness.

Ideals of open inquiry and freedom of speech have been so widely rejected in academia because power in academia has shifted from classical liberals who value toleration to progressives willing to use intolerance to advance their political ends. Perhaps the most destructive development is the dogma that dissent from social justice orthodoxy constitutes literal violence against vulnerable groups which must not be tolerated.

This conflation of speech and violence is the inevitable consequence of the popular dogma that so-called “hate speech” falls beyond the pale of free speech immunity. The idea that opinions can trigger traumatic emotional episodes and that people should be safe from offensive views is a menace to the very idea of free expression.

In order to argue that some opinion is beyond the pale of toleration, one merely needs to claim that it constitutes “hate speech.” If putatively harmful speech is banned, then those who wish to suppress unorthodox opinion will attempt to frame it as hateful and violent. This is just what we now see playing out on campus, as when the University of California warns professors against committing such “microaggressions” as saying that America is the land of opportunity, or that the most qualified person should get the job.

This self-righteous intolerance generates pressure to deny conservatives a forum on campus, and to shout them down or threaten them when they are allowed to speak. When DePaul University in Chicago recently banned conservative speaker Ben Shapiro due to what it called “security concerns,” it effectively gave a rioter’s veto to those who threaten violence.

Since it is impossible for everyone to be protected from ideas and emotions they find abhorrent, this right can only be granted unequally: to some, not to all. No one considers making campus conservatives safe from radical ideas, nor should they.

But there is an increasing need to protect them from actual violence in the name of social justice. The right not to be assaulted can be guaranteed to all, but the demand not to be offended by the opinions of others undermines not just freedom of speech but equality of rights.

By officially discouraging the profession of these ideas, the university undermines the mission of teaching its students how to form their beliefs in a manner worthy of intelligent beings.