In Herman Melville’s Moby-Dick, Captain Ahab lusts for vengeance against the titular white whale for biting off his leg in an earlier voyage. His hatred overwhelms his senses so completely that he comes to regard the whale as the embodiment of humanity’s fears and anxieties. Ahab, wrote Melville, “cherished a wild vindictiveness against the whale, all the more fell for that in his frantic morbidness he at last came to identify with him, not only all his bodily woes, but all his intellectual and spiritual exasperations.”

President Donald Trump is similarly and strangely haunted by the U.S. border with Mexico. In his public remarks and private complaints, he treats it as the source of nearly everything he finds wrong with modern American society: human trafficking, murder and rape, unemployment, the opioid crisis, Democratic electoral victories, and more. To him, almost all that ails the United States is caused by the supposed porousness of that thousand-mile strip of land.

So, last week, he proposed a drastic solution: closing the border entirely until the Mexican government takes more dramatic steps. “If Mexico doesn’t immediately stop ALL illegal immigration coming into the United States through [sic] our Southern Border, I will be CLOSING the Border, or large sections of the Border, next week,” he wrote on Twitter.

This wouldn’t be as straightforward as he seems to think. The border is not an open door that can simply be shut. While there are legal border crossings that Trump can try to close, his legal authority to do so is dubious and untested. And whether the courts sign off on that or not, any attempt would carry a colossal economic and political toll.

Trump’s latest clash with reality demonstrates the president’s failed approach to governance. He makes grandiose promises to rile up his most fervent supporters, apparently without thinking through the implications. Most politicians would back down when those implications became clear, but Trump is undeterred. He grasps for ad hoc ways to fulfill those promises that often do more harm than the status quo. “Securing the border” is an ill-defined and ultimately unattainable goal. But the president is still determined to chase it around the Norway Maelstrom and perdition’s flames before he gives it up.