Here is the conventional wisdom about last night’s presidential debate, as I understand it. Hillary Clinton won in a rout, but that’s largely because Donald Trump flagged after an excellent first 30 minutes in which he hammered away at his strongest issue: trade.

“Donald Trump won the first 25 minutes of the first presidential debate,” writes Ross Douthat at the New York Times, in a representative piece. “He was too bullying and shout-y, too prone to interrupt, but he seized on an issue, trade, where Hillary Clinton was awkward and defensive, and he hammered away at his strongest campaign theme: linking his opponent to every establishment failure and disappointment, and trying to make her experience a liability rather than a strength.”

His colleague, Maggie Haberman, made much the same point. “Trump has a strong case to make on trade, when he makes it,” she tweeted. “He made it once and then chased shiny objects for most of the debate.”

This is how it felt to me, too. Stylistically, this section was Trump’s best portion of the debate. He kept slamming Clinton on NAFTA — "the worst trade deal maybe ever signed anywhere” — and spoke with the confidence of a man who knew what he was talking about.

But he didn’t know what he was talking about.

What was stylistically Trump’s best portion of the debate was substantively among his worst (I say among his worst because it is hard to beat the section where he said he both would and would not honor the NATO treaty, and then said he both would and would not adhere to the first-strike doctrine on nuclear weapons). Trump was arguing the central economic theory of his campaign — and he was just wrong. In a section that began with him demanding solutions for our economic woes, he showed himself completely confused as to the nature of not just our economic problems but the underlying labor market.

The tone of his voice and the confidence of his delivery shouldn’t distract us from the hollowness of his remarks.

Trump’s case on trade is wrong on the facts and wrong on the theory

"Hillary, I'd just ask you this,” Trump said early in the debate. "You've been doing this for 30 years. Why are you just thinking about these solutions right now? For 30 years, you've been doing it, and now you're just starting to think of solutions. I will bring back jobs. You can't bring back jobs.”

"Well, actually, I have thought about this quite a bit,” Clinton replied. “I think my husband did a pretty good job in the 1990s. I think a lot about what worked and how we can make it work again…”

"Well, he approved NAFTA,” Trump shot back. "He approved NAFTA, which is the single worst trade deal ever approved in this country.”

To Trump, NAFTA is some kind of original economic sin — a core cause of our current economic troubles. So let’s talk about NAFTA.

Economists disagree as to the exact effects of the treaty, but virtually every paper published on the subject finds the effect on the American economy was small. A review of 11 econometric studies of NAFTA finds effects ranging from a modest reduction in wage growth for blue-collar workers to a 0.17 percent increase in overall American wages.

A separate overview of the evidence from the Congressional Research Service concluded that NAFTA "slightly increased growth in output and productivity” and "had little or no impact on aggregate employment.”

On some level, all this is obvious. NAFTA was enacted in January 1994 — a few years before one of the strongest economies in American history. It would be foolish to credit NAFTA with the ’90s economic boom, but it is yet more foolish to pretend NAFTA devastated the American economy given the fact that unemployment fell from 6.6 percent in January 1994 to 4.0 percent in January 2000.

Trump’s efforts to connect his trade-first theory of the American economy to the present day failed perhaps more obviously. In the first question, he was asked how he would raise the wages of American workers. His response is worth quoting at some length:

Our jobs are fleeing the country. They're going to Mexico. They're going to many other countries. You look at what China is doing to our country in terms of making our product. They're devaluing their currency, and there's nobody in our government to fight them. And we have a very good fight. And we have a winning fight. Because they're using our country as a piggy bank to rebuild China, and many other countries are doing the same thing. So we're losing our good jobs, so many of them. When you look at what's happening in Mexico, a friend of mine who builds plants said it's the eighth wonder of the world. They're building some of the biggest plants anywhere in the world, some of the most sophisticated, some of the best plants. With the United States, as he said, not so much.

It’s a simple fact that none of this is true. Jobs aren’t fleeing the United States. August marked the 78th straight month in which the US economy added jobs — we’re in the single longest streak of private sector job growth in American history. China isn’t devaluing its currency — in fact, it’s propping it up to stop investors from fleeing the country. The biggest plant in the world is being built by Tesla in Fremont, California, and the existing biggest plant in the world is a Boeing factory in Washington state.

And these are just the narrow facts that Trump got wrong. He also seems confused about the basic structure of the US economy, and that’s led him to focus on the wrong issues entirely. “You would never know from Trump’s discourse that the vast majority of Americans work in jobs related to domestic service provision,” wrote Vox’s Matthew Yglesias. “They work in hospitals and restaurants and schools and stores working with nearby customers, not internationally traded manufacturing.”

If you believe the American economy is broken, you’re simply not going to fix it with trade deals. Trump’s promise to bring the jobs “back” is all the more hollow because he doesn’t seem to know where they are.

So here is the lesson of Trump’s performance in the first 30 minutes of the debate: He doesn’t know what he’s talking about. That seems relevant to whether he performed well or poorly in that section.

The media should inform voters, not try to reflect them

There’s a deep tension in the way the media judges presidential debates. On the one hand, we know that our coverage affects the public’s ultimate view of the event — in that way, we are key participants in the debate, not merely observers of it.

But that knowledge is uncomfortable. It’s not the role we are meant to play. The press wants to reflect reality, not shape it.

And so we attempt, peculiarly, to recast ourselves as observers of voter reactions we can’t observe. We judge the debate based not on what we think to be true about it but on what we think the public will think to be true about it. And so we end up asking not whether the candidates made good arguments given what we know to be true but whether they made good arguments given what we imagine voters know to be true. And once you’re in that mindset, a section where Trump sounded good can be a win even if nothing he said made sense — after all, fairly few voters are trade policy or labor market experts.

But the public isn’t relying on us to tell them what we thought they thought watching the debate. They’re relying on us to tell them what we found when we compared the candidates’ answers to reality, and to the best analysis on offer from experts, so they can make a better-informed judgment on what actually happened in the debate. And sometimes there’s a very big gap between how good a candidate’s answers sounded and how good his or her answers actually were.

That’s the case for Trump’s opening section last night. He was speaking on the issues where he’s supposed to be strongest — his whole pitch is he’s a businessman who knows how the economy really works and what is really needed to fix it — and he showed he didn’t have any real idea what he was talking about. Voters deserve to know that.

How presidential debates are won and lost