The first question asked by a "normal American" during the CNN Democratic presidential debate — and the only question asked by a person of color — was this: "Do black lives matter, or do all lives matter?"

There's a reason the question was asked this way: Martin O'Malley, in particular, got in trouble this summer for responding to activists affiliated with the Black Lives Matter movement by saying "all lives matter." But the reason that O'Malley got in trouble for that is that the two aren't supposed to be mutually exclusive.

"All lives matter" has been used, as a slogan, to oppose the Black Lives Matter movement. As the phrase "black lives matter" has moved into the mainstream of progressive politics over the past year, the response that "all lives matter" has been used to try to shut it down — or to obscure the real racial disparities in police/community relations. But while Black Lives Matter activists take issue with the phrase "all lives matter," they certainly don't take issue with the belief that they do. The entire point of the Black Lives Matter movement is that all lives matter — even, and including, black ones.

O'Malley had to apologize for saying "Black lives matter. White lives matter. All lives matter" this summer

The "issues on the campaign trail" that Cooper was referring to stem from a confrontation between black activists and Martin O'Malley and Bernie Sanders at the major progressive conference Netroots Nation this summer. The activists were affiliated with the Black Lives Matter movement, which has sprung up in the past year to protest and respond to the deaths of young black men and women at the hands of police and while in police custody. During his attempt to respond, O'Malley said, "Black lives matter. White lives matter. All lives matter." The protesters, as well as other attendees and progressives watching the events unfold, were deeply offended — and O'Malley was forced to apologize.

Do activists really believe that only black lives matter? No, of course they don't. But while O'Malley almost certainly wasn't trying to diminish the movement, he touched a nerve — and set off another round of argument about what the slogan really means.

Activists feel police often act like black lives don't matter

The hashtag #BlackLivesMatter, and the campaign that's organized around it for the past year, isn't about the general improvement of the well-being of African Americans. It's a protest against police killings of young black men and women and the deaths of young black men and women in police custody. To the activists in the Black Lives Matter movement, the frequency with which black Americans die at the hands of police — and the circumstances of those deaths — are powerful evidence that law enforcement doesn't care as much about black lives as white lives. And they point to the media reaction to those deaths, which often focuses on the criminal records of the victims, as evidence that American society doesn't care as much about black lives either.

In April, after the shooting of Walter Scott by a police officer in South Carolina, Tavis Smiley put it this way:

Now how do you know [the officer who killed Scott] didn't care about his humanity? Because he shot him, like a coward, 8 times in the back as he's running away. How do you know he doesn't care about his life? Because as he's dead on the ground, you're so afraid of a black man that as you shoot him 8 times in the back and he's face down he's dead and you still handcuff him. What does that say about how you regard or disregard the humanity of that particular human being?

All lives matter, but not all lives are equally under threat

Black Lives Matter is a specific cause. And it's pretty well accepted that if someone supports one cause, it doesn't mean they don't care about other ones.

#BlackLivesMatter doesn't mean other lives don't. Like people who say "Save The Rainforests" aren't saying "Fuck All Other Types of Forests" — Matt McGorry (@MattMcGorry) July 18, 2015

And because "black lives matter" does refer to the specific threats that activists feel law enforcement poses to black Americans, countering with "all lives matter" can sound like an attempt to deny that those racial disparities exist. No one disagrees that all lives ought to matter — but one of the difficulties with talking about race in America is that even when race ought not to matter, for many people, it does.

Here's an analogy from Reddit user GeekAesthete, in a thread in which another user asked redditors to explain why #AllLivesMatter was offensive, that gets at this:

Imagine that you're sitting down to dinner with your family, and while everyone else gets a serving of the meal, you don't get any. So you say "I should get my fair share." And as a direct response to this, your dad corrects you, saying, "everyone should get their fair share." Now, that's a wonderful sentiment -- indeed, everyone should, and that was kinda your point in the first place: that you should be a part of everyone, and you should get your fair share also. However, dad's smart-ass comment just dismissed you and didn't solve the problem that you still haven't gotten any!

#AllLivesMatter has often been used to attack #BlackLivesMatter

After O'Malley said "all lives matter" at Netroots Nation, he told MSNBC that he hadn't been aware of the connotations of the phrase. To activists, though, that's hardly an excuse — in fact, it almost makes the problem worse. Because for nearly as long as "black lives matter" has been a slogan of activism against police violence, the phrase "all lives matter" has been used by its opponents.

The Facebook page ALL Lives Matter features complaints about how there's no such thing as "police brutality" and photos of white people allegedly killed by African Americans. And the hashtag #AllLivesMatter on Twitter is a decent mix of earnest entreaties to stop being divided by race, blaming Obama for racial divisions, and articles about black-on-black or black-on-white crime.

Just as he told MSNBC, O'Malley probably wasn't aware before the Netroots conference that "all lives matter" is often used to attack "black lives matter." He may have reflexively answered thinking he was accepting their point and reiterating it. But the honest mistake itself illustrated that he hadn't been paying very close attention to how the #BlackLivesMatter movement has developed over the past year, and how it's been attacked — both by people simply trying to deny the continued relevance of race, and by people trying to paint African Americans as the real thugs.

And it means O'Malley hadn't been paying attention to the conversation some Black Lives Matter activists have been trying to start within the progressive movement — to ensure that their issues are seen as progressive issues. O'Malley was at Netroots Nation to appeal to progressives looking for an alternative to Hillary Clinton. The protesters who challenged him took the stance that for a candidate to label himself progressive in the year 2015, he should have a well-considered position on the biggest progressive movement of the past year.

Maybe not everyone would be forced to apologize for saying that "all lives matter" — although, like many other things, the real question is whether you keep saying it after it's explained why people could take offense. But Martin O'Malley isn't all people. And the broader lesson of what happened at that conference is that black progressives don't feel they're currently part of the progressive agenda — and they're fighting to change that.

Correction: This article originally said that O'Malley apologized Sunday; he apologized Saturday.