The free exchange of ideas is an idea itself. Can it be defended against attack without collapsing into paradox?

Can tolerance abide intolerance?

Though these questions seem to present a hall of mirrors, the answer to each is yes. It has to be, if a society wishes to strike a balance between progress and pragmatism. Between tough and tender. Or between any other alliterative inverses you can think of — if you indeed perceive them as being only at odds.

Yet the defense of discourse holds precariously, era to era, moment to moment, including now.

Consider a recent open letter signed by more than 150 luminaries in defense of free speech — and the deluminating reaction to it.

An “intolerance of opposing views, a vogue for public shaming and ostracism, and the tendency to dissolve complex policy issues in a blinding moral certainty.” That was the concern of the letter, which was published by the 170-year-old Harper’s Magazine — or, more specifically, on its website. It will be printed in October’s issue. Harper’s, with its typically contemplative articles, has a long lead time.

Signatories ranged from authors like J.K. Rowling and Salman Rushdie to intellectuals like Garry Kasparov and Noam Chomsky. Rowling, a particular lightning rod, has fallen out of favor among a number of her “Harry Potter” fans for making comments perceived as transphobic. Small consequence. Rushdie has lived for 30 years under Islamic death threats for a book he wrote.

Silence! came the response to the Harper’s letter.

“Fatuous, self-important drivel,” wrote one critic on Twitter, a medium not known as pensive. Better-nuanced arguments have followed in other media, though the focus is largely the same:

• That those signing the letter are cultural elites who already have a forum, among other blessings, and who now should stand aside.

• That the letter interprets mere strong criticism as a threat to free speech.

• That the culture must be scoured, at long last, of forces that marginalize others.

It’s true that the signatories of the Harper’s letter are generally successful people, including in the marketplace of ideas. But left unexplained is how that reduces their concerns or warrants a forfeiture of their contributions.

It’s also true that Western democracies are not suffering anything as stifling as, say, the Chinese government’s new “national security” law in Hong Kong, where freedom of expression had previously set the territory apart from the mainland. An episode of “The Daily,” a New York Times podcast, described the spectacle of the law’s introduction, including enormous red banners on a barge passing through Victoria Harbor. The clear goal was not just punishment of perceived offenses but intimidation of all.

Yet it isn’t always governments that blow in the chill. Ask authors of Young Adult fiction, who learned in recent years that their skilled imagination isn’t welcomed if applied to a culture other than their own. Consider the fate of the New York Times opinion editor James Bennet, who resigned under criticism from within his newspaper after publishing a commentary from a conservative U.S. senator with an unpopular idea. Or the story of Times opinion editor and writer Bari Weiss, who threw in the towel last week. “My own forays into Wrongthink have made me the subject of constant bullying by colleagues who disagree with my views,” she wrote. “… Even now, I am confident that most people at The Times do not hold [to this new orthodoxy]. Yet they are cowed by those who do.”

Finally, it’s true that the human hive is messy. Some expression is distasteful or outdated. Some thinking is defective. Some is dangerous. Even the pursuit of ideals can go awry.

The Harper’s letter has an obvious response to all this: “The way to defeat bad ideas is by exposure, argument, and persuasion, not by trying to silence or wish them away.”

That’s hard work, and unremitting. The alternative is … what’s that term again? …

Deluminating. Which is not a dictionary word but rather one we’ve derived from the name of a fictional gadget in Rowling’s books. It puts out the lights, so that those who possess it can go on about their business under the cover of darkness. For better or for worse.