It’s not exactly a secret that Donald Trump tells a staggering number of lies on a nearly daily basis, but I’m especially interested in the ones he returns to, over and over again, even after being told he’s wrong.

In this week’s interview with Forbes, for example, the president boasted, “I’ve had just about the most legislation passed of any president, in a nine-month period, that’s ever served. We had over 50 bills passed. I’m not talking about executive orders only, which are very important. I’m talking about bills.”

This is, of course, demonstrably ridiculous, as Trump surely knows. But there’s a rationale behind the lie: the president is embarrassed by his failures, and he can’t explain his lack of accomplishments, so he’s made up a legislative record that exists only in his imagination.

Similarly, Trump needs a rationale to sell his plan for massive tax cuts. The truth won’t do, so as Politico noted, the president is clinging to a specific lie.

“We are the highest taxed nation in the world,” President Donald Trump has repeated over and over again. He said it Tuesday during a meeting with former Secretary of State Henry Kissinger. He said it at a White House event last Friday. He’s tweeted it, repeated it in television interviews and declared it at countless rallies. It is his go-to talking point, his favorite line as he tries to lead the Republican Party to a once-in-a-generation overhaul of the federal tax code.

Despite the repetition, the claim is plainly untrue. Indeed, as the president almost certainly realizes, it’s not even close to being accurate. Asked for an explanation yesterday, White House Press Secretary Sarah Huckabee Sanders said some other claim that Trump didn’t say is true, so there’s no need to dwell on the president’s bogus argument.

When a reporter noted that this doesn’t explain why Trump keeps lying, Sanders responded, “Sorry, we’re just going to have to agree to disagree.” She then moved on.

The significance of this extends beyond the president’s truth allergy. There’s a more substantive angle to keep in mind.

The reason Trump keeps lying about U.S. tax rates is that he needs it to be true. In Trump’s model, the U.S. is “the highest taxed nation in the world,” which serves as a foundation for his pitch to overhaul the federal tax code in a regressive direction.

If we’re not “the highest taxed nation in the world” – in other words, if reality matters – then the White House no longer has a reason to push the president’s tax plan.

And so, Trump’s lie persists.