Just a month ago, his message was less certain. Trump threatened to close the U.S. border to Mexico, citing the threat of undocumented immigrants. He scaled that back the following week but issued a strong proclamation, “We can’t take you anymore. Our country is full.” The following Monday, the Departments of Homeland Security and Labor gave an opposing view when they issued a temporary increase in visas for nonagricultural, seasonal workers.

These disparate actions embody the messiness of immigration policy. There are a lot of stakeholders with different needs. While everyone seems to agree the country’s current immigration policy is outdated, there isn’t a consensus on how to fix it—or what parts to fix.

“There’s not really a set ‘goal’ we’re working toward; rather, [visa limits] just keep increasing, because a huge number of special interest groups all want it to,” said Matthew Sussis, a spokesman for Center for Immigration Studies, a group that argues for much lower levels of immigration.

The framework for modern day immigration was set in 1965 and reworked in 1990. The result is a complicated matrix of green cards and visas, some limited by caps, others not—some providing a pathway to citizenship, others not. The approach to undocumented immigration is just as muddled. Some administrations prosecuted heavily, others less so.

There are many competing interests in these conversations, all armed with white papers and competing data. Some stakeholders, like businesses, want to increase the workforce. Others, like unions, may argue that increasing immigration decreases work for Americans.

“The [temporary worker] caps were actually arbitrarily set in 1990 but based on politically agreeable numbers,” said Rebecca Peters of the Society for Human Resource Management, which advocates for employers to have a say in workforce needs. “To date, there have been a number of proposals looking out how best to determine cap levels, but it had always been arbitrary.”

If you look for the rationale behind visa limits within the Department of Homeland Security, they will refer you to the Department of Labor. And if you ask at the Department of Labor, you’ll wind up back at Homeland Security.

And if you try to get the White House to talk about the numbers in their proposal, well, no one wants to talk on record. The legislative text hasn’t been drafted and it’s already been declared dead on arrival. Among other things, Trump’s proposal would employ a merit system that favors higher skilled workers.

As yet, there is no indication in Trump’s plan that they are developing a mechanism that moves with the economy and no indication that there are provisions for DREAMERS, those young undocumented immigrants who have lived in the U.S. for most of their lives. Instead, the administration is proposing a reallocation within the current framework of visas and green cards.

Even economists say that a mathematical formula would be impossible. There are too many variables to neatly determine a sweet spot. Do we want high skilled workers or low skilled workers? More citizens, more temporary workers? Do we want to grow certain industries, or all industries equally?

“An ideal system doesn’t exist unless [a government] can tell what the goal is,” said Giovanni Peri, professor and chair of the Department of Economics at the University of California, whose work focuses on migration and labor markets.

That’s not to say there haven’t been attempts to root the numbers in something tangible. There have been mechanisms proposed to create a system that can respond to some elements of the economy. Peters’ group, for example, proposed a market-based approach. The demand for visas would be an indicator—a sudden increase in applications would trigger more visas to be issued, with a cap that could carry into the next year. A decrease would begin a similar contraction process.

“Some of the needs to modernize the work-based immigration system haven’t come forward as quickly as we’d like,” Peters said, adding that border security and DACA have taken Congressional focus.

There have been proposals in the past to take the politics out of immigration discussions.

“It is a mistake to think that somehow there is a formula, a rational process that leads to these numbers,” said Demetrios Papademetriou, co-founder of the think tank Migration Policy Institute. In 2006, the group put together a panel that ultimately concluded that the best way to comprehensively tend to immigration would be through an independent commission that would analyze data to determine categories and levels of visas. “It was not a recommendation that was liked.”