The right to free speech is one of the most important democratic freedoms. It enables the flow of information and encourages diversity of opinion in the public sphere, as well as criticism of political leadership, all of which are in the public interest. But like most freedoms, it is not absolute, nor should it be.

The Supreme Court of Canada is currently pondering whether to jettison provisions in the Saskatchewan Human Rights Code that restrict free speech in the interests of protecting a vulnerable minority from publicly proclaimed hatred. The case in question concerns the rights of homosexuals, but the issue is broader: The court's judgment will have a ripple effect on anti-hate laws and the rights of minorities everywhere.

Given its multicultural fabric, contemporary Canada is vulnerable to the potential rise of ethnic hatreds, and it is naive, not to mention ahistorical, to assume that our mythologized consensus over tolerance cannot easily be eroded.

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Back when Canada was white and Protestant, respected academics proudly announced that Catholics, Jews and immigrants from southern Europe were "unassimilable" (their favourite expression). Their words carried weight. Signs in the Beaches neighbourhood of Toronto read: "No Jews or dogs." There was a riot against Jews in the 1930s. In Quebec, a virulent fascist movement held sway with ideas that continued into the postwar decades. The leader and mainstay, Adrien Arcand, was supporting neo-Nazi Ernst Zundel as late as 1967.

Happily, those days are gone; in fact, one reason Canada has successfully incorporated millions of immigrants from around the world is that Canadians came to understand that, in an emerging milieu of ethnic difference, hate speech was a danger to the maintenance of social peace. Equality of citizenship, including freedom from harassment, required compromise and moderation, values that were incorporated into the Canadian zeitgeist.

European countries without our tradition of upholding anti-hate laws, and without a history of ethnic pluralism, have had a much harder time coping with growing diversity. Only the United States allows almost unmitigated speech, but some legal scholars, such as Jeremy Waldron of New York University, are beginning to believe that America should align itself with the rest of the world's liberal democracies – countries that, in his words, "take affirmative responsibility for protecting the atmosphere of mutual respect against certain forms of vicious attack."

This protection has become urgent in the years since 9/11 with the concomitant increase in verbal attacks on Muslims. In Canada, the most prominent free-speech extremists are Ezra Levant, who provocatively republished the insulting Danish cartoons denigrating the Prophet Mohammed in 2006, and Mark Steyn, who has been generalizing about Muslims for years. Both these authors have tried to shift the Canadian consensus by normalizing previously unacceptable levels of speech.

And normalizing is exactly how it happens. Shifts in the general consensus regarding minorities are progressive and incremental. We don't notice. Then we wake up to find ourselves in a changed environment, as did the Norwegians after Anders Breivik went on a murderous rampage last summer. Mr. Breivik was part of a growing continuum of radicals fixated on the idea that Muslims, as an undifferentiated group, are conspiring against the West. In his eyes, he was a patriot. Unfortunately, history is rife with such people. They are nourished on incessant, unbridled hate speech.

It has been more than 20 years since our Supreme Court last wrestled with this issue, then upheld Canada's hate-speech provisions by ruling (narrowly) that James Keegstra, an Alberta teacher who quizzed his students on their "knowledge" of his anti-Semitic views, was willfully promoting hatred of a Canadian minority. The Canada we live in today is considerably more fragile than it was in 1990. Freedom of speech must be balanced with freedom from the destabilizing effects of public hatred in this, the world's most heterogeneous society.

Erna Paris is the author of Long Shadows: Truth, Lies and History .