Over the weekend, just hours before Donald Trump made an underwhelming presentation on a doomed plan to end his government shutdown, the president spoke briefly with reporters. Most of the rhetoric was familiar, though he did add a new twist to his usual pitch.

“Everybody knows that walls work. If you look at different places they put up a wall – no problem. If you look at San Antonio, if you look at so many different places, they go from one of the most unsafe cities in the country to one of the safest cities immediately. Immediately. It works. We have to put them up and we will put them up. We got to.”

Ordinarily, this is about the time I start explaining that we don’t “have to” put up a giant wall, and by all appearances, even Trump doesn’t really believe that. If he did, the president wouldn’t have waited two years to push the issue. He also wouldn’t have turned down Democratic offers that provided the administration with wall funding. He also wouldn’t have endorsed a clean spending bill, without wall money, just last month, before dramatically changing his mind after conservative media told him to.

But in this case, there’s another problem: Trump wants us to “look at San Antonio.”

OK, let’s do that. San Antonio is over 100 miles from the Mexican border, and it does not have – nor has it ever had – a border wall. (The Alamo may have been surrounded by a wall, though if memory serves, it was of limited utility.)

I’ve seen some speculation that the president may have been trying to refer to El Paso, but that would also be factually wrong, since it did not “go from one of the most unsafe cities in the country to one of the safest cities immediately.”

I realize that highlighting the latest in a series of Trump mistakes about immigration only gets us so far. But I think there’s a larger arc to this: how are policymakers supposed to negotiate with a president who is so routinely confused about the basics of his own favorite subject?

This came up last week, when Trump sat down with the bipartisan membership of the House’s “Problem Solvers Caucus,” and proceeded to peddle transparently nonsensical claims about the border. One participant marveled at the president’s “very serious misconceptions,” adding, “I was listening to him today. He makes a lot of comments that are so untrue. But I believe that he actually believes them.”

A few days earlier, Trump made observations about border apprehensions that were the opposite of reality.

A few days before that, the president was peddling claims about terrorists entering the United States through the southern border, and soon after, even the White House wasn’t willing to defend the rhetoric.

There’s probably no point in documenting literally every example of Trump repeating lies, falsehoods, and misconceptions about immigration, but the bottom line remains the same: the president’s ignorance on one of his signature issues makes negotiating with him that much more difficult.

Even if his rivals could trust him (they can’t), and even if Trump were willing to be consistent (he isn’t), working on a deal with the president would require him to have some basic understanding of the relevant policy details.

His call for observers to “look at San Antonio” served as a reminder of just how lost Trump is on the issue he claims to care so deeply about.