Donald Trump is now back on the road, holding rallies in battleground states. These events, with people behind the president wearing masks but most others not, look awfully irresponsible to most of us — some polls show that as many as 92 percent of Americans typically wear masks when they go out.

Trumpworld sees these things differently. Mike Pence articulated the view in the vice-presidential debate. “We’re about freedom and respecting the freedom of the American people,” Mr. Pence said. The topic at hand was the Sept. 26 super-spreader event in the Rose Garden to introduce Amy Coney Barrett as the president’s nominee for the Supreme Court and how the administration can expect Americans to follow safety guidelines that it has often ignored.

Kamala Harris countered that lying to the American people about the severity of the virus hardly counts as “respect.”

It was a pretty good riposte, but she fixed on the wrong word. She could have delivered a far more devastating response if she’d focused on the right word, one that the Democrats have not employed over the past several months.

The word I mean is “freedom.” One of the key authors of the Western concept of freedom is John Stuart Mill. In “On Liberty,” he wrote that liberty (or freedom) means “doing as we like, subject to such consequences as may follow, without impediment from our fellow creatures, as long as what we do does not harm them even though they should think our conduct foolish, perverse or wrong.”

Note the clause “as long as what we do does not harm them.” He tossed that in there almost as a given — indeed, it is a given. This is a standard definition of freedom, more colloquially expressed in the adage “Your freedom to do as you please with your fist ends where my jaw begins.”

Now, conservatives revere Mill. But today, in the age of the pandemic, Mill and other conservative heroes like John Locke would be aghast at the way the American right wing bandies about the word “freedom.”

Freedom emphatically does not include the freedom to get someone else sick. It does not include the freedom to refuse to wear a mask in the grocery store, sneeze on someone in the produce section and give him the virus. That’s not freedom for the person who is sneezed upon. For that person, the first person’s “freedom” means chains — potential illness and even perhaps a death sentence. No society can function on that definition of freedom.

Joe Biden does a pretty good job of talking about this. At a recent town hall in Miami, he said: “I view wearing this mask not so much protecting me, but as a patriotic responsibility. All the tough guys say, ‘Oh, I’m not wearing a mask, I’m not afraid.’ Well, be afraid for your husband, your wife, your son, your daughter, your neighbor, your co-worker. That’s who you’re protecting having this mask on, and it should be viewed as a patriotic duty, to protect those around you.”

That’s good, but it could be much better if he directly rebutted this insane definition of freedom that today’s right wing employs.

There are certain words in our political lexicon that “belong” to this side or the other. “Fairness” is a liberal word. You rarely hear conservatives talking about fairness. “Growth” is mostly a conservative word, sometimes the functional opposite of fairness in popular economic discourse, although liberals use it too, but often with a qualifier (“balanced” or “equitable” growth, for example).

“Freedom” belongs almost wholly to the right. They talk about it incessantly and insist on a link between economic freedom and political freedom, positing that the latter is impossible without the former. This was an animating principle of conservative economists in the 20th century like Ludwig von Mises, Friedrich Hayek and Milton Friedman.

It’s manifest silliness. To be sure, when they were writing, it was true of a place like the Soviet Union. But it is not true of Western democracies. If they were correct, the Scandinavian nations, statist on economic questions, would have jails filled with political prisoners. If they were correct, advanced democratic countries that elected left-leaning governments would experience a simultaneous crushing of political freedom. History shows little to no incidence of this.

And yet, the broad left in America has let all this go unchallenged for decades, to the point that today’s right wing — and it is important to call it that and not conservative, which it is not — can defend spreading disease, potentially killing other people, as freedom. It is madness.

One thing Democrats in general aren’t very good at is defending their positions on the level of philosophical principle. This has happened because they’ve been on the philosophical defensive since Ronald Reagan came along. Well, it’s high time they played some philosophical offense, especially on an issue, wearing masks, on which every poll shows broad majorities supporting their view.

Say this: Freedom means the freedom not to get infected by the idiot who refuses to mask up. Even John Stuart Mill would have agreed.

The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.

Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram.