Substantively, perhaps no single theme came across more clearly in Monday night's presidential debate than the argument that America has lost countless jobs overseas. The candidates spent the first half hour or so of their debate on this subject, trading jabs on whether trade deals help Americans and Hillary Clinton's support for the Trans Pacific Partnership.

As much as Donald Trump's campaign can be said to have a coherent policy theme, trade would be it. The Republican nominee has repeatedly claimed that America loses millions of jobs overseas to bad trade deals such as NAFTA, which he argues is "the single worst trade deal ever approved in this country."

Clinton neglected to test the truth of this claim during the debate, offering little more than a half-hearted "that's your opinion" in response. So let's pick up where she left off. How many jobs does America actually lose to foreign trade?

We have no idea, but we can be very confident that the answer is not "millions." Here's why.

No one actually knows how many jobs go overseas

Factory work in America is often, and not unfairly, portrayed as decimated.

Despite a recent uptick, overall skilled manufacture has lost approximately 5.5 million jobs over the past 20 years. Extended back to 1979 that number rises to nearly 7 million. The days where a high school-educated factory worker could buy his lake house in Michigan and retire comfortably are, increasingly, over.

What's less certain is specifically how many of these jobs have actually been outsourced.

When politicians talk about job losses they're referring to a very specific situation: companies that rely on foreign labor for work that would otherwise be done in the United States. There's no consensus among economists on how big that category is.

"I don't know that anybody has a clear measure of whatever jobs have really been lost overseas," said Kyle Handley, a professor at the University of Michigan's Ross School of Business, "and certainly not if those numbers are in the millions."

There are many reasons for this, including the difficulty of finding out whether a given job would have survived in America at all.

Trade isn't always the culprit

While the last 30 years have been marked by major trans-national trade deals, these decades have also seen major upheavals to the economy. From the growth of skilled and service-sector jobs to the productivity impact of computers and robotics, Americans are competing in a very different domestic landscape than ever before. Many formerly successful firms and professions have struggled and died since the 1980s while others have sprung up.

J. Bradford Jensen, the McCrane/Shaker Chair in International Business at Georgetown's McDonough School of Business, argued that manufacturing job losses have a lot more to do with this new economic climate than global trade.

"I don't know that trade is the predominant reason for that shift," he said. "There are other things going on that are changing the composition of the U.S. economy, and the most important is changes in productivity."

Take farming, he suggested. "If you think back 100 years ago there were very rapid growths in productivity in agriculture," he said. "It used to be that the majority of employees in the United States were working in agriculture, but gains in productivity reduced that share of labor to two percent… The same thing is happening in manufacturing. Fewer workers are working in manufacturing, even though the dollar value of output continues to increase."

To assume that any given factory job is "lost" is to assume it wouldn't have been automated or deemed too expensive, which is not always a safe conclusion.

Job losses are only half the picture

Although the conversation around trade focuses on jobs lost, millions of Americans are also employed as a direct result of globalization. More than 14% of the GDP is based on exports, which directly support more than 11 million jobs nationwide.

The U.S. exported more than $186.3 billion worth of goods and services in July of this year alone, finding its chief markets in Canada, Mexico and China. (Mexico alone buys nearly 16 percent of everything this country ships abroad.)

This is why economists and industry experts don't like referring to job losses. According to them, that reflects a very one-sided picture.

"When people talk about losing jobs, it's kind of a slippery thing," Jensen said.

"The way economists think about it is that trade does not change the number of jobs, it changes the composition of jobs," Jensen added. "By that I mean, if we didn't have any trade we would undertake a different set of activities in the United States than we do now that we're relatively open to trade."

The reason, Jensen and others argue, that this is a good thing is because it allows countries to specialize in what they're good at. A high skill country like the United States is better at producing goods and services that line up with those skills. Businesses can open more easily based on access to a cheap manufacturing base, while more Americans can get jobs in high value exported services like programming, law and intellectual property. At the same time, Americans get the benefit from cheaper goods sold by countries that are better at producing inexpensive labor. That creates export-related jobs and frees extra spending power which, economists argue, generates still more jobs right here at home.

"Because people can now buy a smart phone at a lower price they're going to have more money in their pocket," Handley said. "They're going to spend that money on something else, even on something as simple as getting their hair cut more often. So there are more opportunities to be a barber or go into cosmetology."

And made in America wouldn't necessarily make Americans richer

Trump's solution to America's very real trade gap is to propose high tariffs that would make it prohibitively expensive to manufacture goods overseas. However this would act as a tax on virtually every product sold nationwide.

A study from the National Foundation for American Policy found that Trump's plan to raise tariffs against China, Japan and Mexico would act as an effective tax of $2,200 per year on the average household and "would be ineffective in shielding American workers from foreign imports, since producers from other countries would export the same products to the U.S."

Waging a trade war with the entire world would increase that tax to $6,112.

The reason is because of how thoroughly globalized the U.S. economy has become. Overseas producers, leveraging advantages that don't exist domestically, can manufacture many goods far more cheaply than can be done here. American consumers are the net beneficiaries.

"That's one thing that doesn't get talked about a lot with regard to those jobs that have been lost," Handley said. "I'm not sure those jobs are going to come back just because we increase trade barriers, but the products that will be produced again in the United States are not going to be nearly as cheap as they are now."

Just think of the cost of a car back in the 1970s.

"As a share of the typical household's disposable income it was much more expensive in the 1970s for a family of four say to put aside some of their income to buy an automobile than it is today," Handley said.

You can only get so far with trying to protect anything.

Forcing jobs back into America through trade barriers would probably move some outsourced jobs back to the U.S. while at the same time reducing export jobs, but the math is complicated and barely known, when it's math at all.

Neither economists nor industry experts know how many jobs have been shipped overseas, and of those no one is sure how many would simply have ceased to exist here at home.

No one has any more confidence on how many jobs could be clawed back by erecting tariffs either, although doing so would impose thousands of dollars in expense on the average consumer. And for the jobs that have been outsourced to China or India, millions of others have been create here at home selling to those very same markets.

That's not an attempt to deny the reality of factory closings or the suffering of Americans who are out of work, but to say that this is far more complicated than a black and white slogan would make that seem.