I used to dread teaching John Stuart Mill. A staple of introductory courses on political theory, On Liberty seemed more like a relic of Victorian conventionality than something relevant to the 21st century. Aristotle challenges students' assumptions about nature; Hobbes forces them to consider the connection between violence and order. But Mill? Who still needs his defense of a freedom to read and say what you like? Better to provoke students with his censorious antagonist, James Fitzjames Stephen.

I don't feel that way any more. The ubiquity of social media, coalescence of new taboos based on progressive theories of race and gender, and moralization of major institutions have rescued Mill from the syllabus of forgotten classics.

When I began teaching about 15 years ago, Mill's claim that the expression of bitterly contested and even deeply unpopular opinions should not only be permitted but encouraged seemed like a truism. Today, it is subject to debate, just as it was in Mill's day, when universities — at that time, often run by churches — were less hospitable to contested inquiry than were the press or political forums.  

But Mill's defense of liberty in "thought and discussion" isn't exactly what students expect — or what journalists and politicians often mean when discussing "free speech." In the first place, it's not primarily about the law. Even in the 19th century, Mill took for granted that legal constraints on what could be said or published were loose and ineffective, at least in "constitutional countries."

That assumption was wrong in the short term, as prosecutions of political, moral, and religious arguments continued to occur in Britain, the United States, and elsewhere during Mill's lifetime. In the long run, though, he accurately perceived the difficulty of justifying such restrictions in a nominally free society. Instead of targeting repressive laws, then, Mill focuses on social and cultural obstacles to dissent. "Our merely social intolerance kills no one, roots out no opinions," he wrote, "but induces men to disguise [their views], or to abstain from any active effort for their diffusion." Mill didn't use the phrase "cancel culture," but that's what he has in mind.