Donald Trump’s announcement yesterday declaring the opioid crisis a public-health emergency was disappointing for a variety of reasons. Not only did it fall short of what the president vowed in August, but the White House’s plan, at least at this point, doesn’t include additional resources to combat the epidemic.

Making matters slightly worse, Trump seems to think he can address the problem with a bogus solution. From his speech yesterday:

“We must stop the flow of all types of illegal drugs into our communities. For too long, dangerous criminal cartels have been allowed to infiltrate and spread throughout our nation. An astonishing 90 percent of the heroin in America comes from south of the border, where we will be building a wall which will greatly help in this problem. It will have a great impact.”

No, actually, it won’t.

The president has dabbled in this area before, arguing in July, for example, that he wants his wall to have literal “transparency” so that people can protect themselves from 60-pound bags of drugs that Mexicans will somehow catapult over Trump’s beloved wall.

But yesterday’s argument was more direct: he believes the wall will serve as a physical barrier, helping keep heroin out of the United States. We already know – as Trump should already know – that this isn’t true.

“A wall alone cannot stop the flow of drugs into the United States,” Christopher Wilson, the deputy director of the Mexico Institute at the Wilson Center, recently explained. A Washington Post report added, “[E]xperts on the drug trade say a border wall, even one as big and beautiful as Trump promised, would be near-impotent in stemming the supply of illegal drugs.”

That’s because most illegal drugs arrive by way of ports of entry, not illegal border-crossings. One of the central pillars of Trump’s argument for wall is based largely on his confusion about what a wall would do.