In the seven volumes of "Harry Potter," J.K. Rowling told millions of readers a story about the power and triumph of love. Earlier this month, accepting an award at the PEN American Center gala, she issued a reminder about dealing with hatred.

When Rowling mentioned the half-million online signatures a petition to ban Donald Trump from visiting Britain had gleaned in January, a few in the audience applauded. Rowling had referred to "tides of populism and nationalism currently sweeping many developed countries ... accompanied by demands that unwelcome and inconvenient voices be removed from public discourse," and perhaps these audience members expected her to side with those who signed the petition to Parliament.

That would hardly have been a shock: Rowling has compared Trump to Voldemort, the murderous bigot who is Harry Potter's nemesis.

But when the applause broke out, Rowling said, "Just a moment."

Only when those who offend are free to speak, she said, are their opponents free to speak against them. "If my offended feelings can justify a travel ban on Donald Trump, I have no moral grounds on which to argue that those offended by feminism or the fight for transgender rights or universal suffrage should not oppress campaigners for those causes."

There are, of course, those who would argue that there is a difference between silencing good causes and silencing bad ones.

But as Rowling concluded: "If you seek the removal of freedoms from an opponent simply on the ground that they have offended you, you have crossed a line to stand alongside tyrants who imprison, torture and kill on exactly the same justification." This means that everyone — and especially anyone who expects to be in the minority on any issue — has a vital interest in a culture of free speech.