President Trump lies so reflexively on trivial matters that world leaders do not know whether to believe him on important ones. This conduct has become so routine it barely merits notice. He denounces the press as “the enemy of the people,” derides his critics as treasonous and openly fawns over an autocrat whose modern-day gulags practice extermination, torture and sexual violence. The president’s most strenuous apologists have long swept all this away with the breezy assurance that he should be taken “seriously, not literally.” Instead of his bombast, they say, look to policies of which conservatives approve.

This image of Mr. Trump as a political Robin Hood whose illicit behavior is justified because it serves a greater good is doubly flawed. First, the lying and vulgarity are unrelated to the policies Mr. Trump’s base wants implemented. Second, his supporters purport to seek a restoration of American founding principles. This increasingly strains credulity. But if they profess constitutionalism, they should at least understand that it is more about process than policy. Constitutions depend on habits and traditions, not the momentary outcomes they produce. Mr. Trump’s upending of these customs, not his transient policies, will form the legacy that endures.

The first flaw arises from what might be called the “post Trump, ergo propter Trump” fallacy. It is a form of the “post hoc ergo propter hoc” error in logic: “after, therefore because of.” The classic illustration is the supposition that the rooster’s crow causes the sunrise because the second event follows the first.

In the version of the fallacy his defenders espouse, Mr. Trump violates customary standards of presidential behavior and then delivers desired policies, so the assumption is that the violations produced the policies. No one believes this more vehemently than Mr. Trump himself, a man who crows before the stock market rises and believes he caused it to occur. The challenge in his case is that the boorishness that supposedly yields conservative outcomes is so unrelenting it is impossible to correlate with anything and plausible to associate with everything.