Alex Jones, the conspiracy-theory-pedding radio host who is a favorite source of Donald Trump, speaks to a crowd near the Republican National Convention in Cleveland in August. (Brian Blanco/European Pressphoto Agency)

“Consider the source” is a good rule in life, and even more so in the realm of news.

In recent days, Donald Trump stood in front of riled-up crowds and argued that both candidates should undergo drug tests before the final presidential debate Wednesday. Why? Because Hillary Clinton, he claimed, is taking performance-enhancing substances.

“I don’t know what’s going on with her, but at the beginning of her last debate, she was all pumped up at the beginning, and at the end it was like take me down,” he said. “She could barely reach her car.”

He provided no evidence for any of this. In fact, he seemed to be purposely mixing up Clinton’s debate performance with her recent bout with pneumonia. (In a much-viewed video, her knees buckled as she departed early from a 9/11 commemoration in New York.)

But here’s how Roger Stone, Trump’s ally and longtime dirty-trickster, described Clinton’s second debate behavior, in a recent interview with Alex Jones, the syndicated radio host and proprietor of InfoWars, a website that thrives on far-right conspiracy theories.

Republican presidential nominee Donald Trump believes there’s a global conspiracy to stop him from becoming president – but it’s not the first time he’s pushed unfounded theories. (Peter Stevenson/The Washington Post)

“Look, of course she was jacked up on something. I assume some kind of methamphetamine.”

[It’s time for TV news to stop playing the stooge for Donald Trump]

It sounds like a perfect circle of disinformation: Stone provides unfounded allegations to InfoWars, and lately, Trump has been using InfoWars like a news source.

Let’s be clear: If InfoWars is news, the yowling of feral cats is classical music, and Trump University the best place to invest your hard-saved tuition dollars.

InfoWars was founded by Jones, a purveyor of various crackpot notions, including that the Sandy Hook massacre of tiny children in 2012 was a government hoax intended to promote gun control. (It was all done with actors, Jones claims.)

And the California drought? Made up. InfoWars is also a great place to go for 9/11 “truther” rumors; Jones proudly calls himself a founder of those.

But Trump seems to be a fan: He did an interview with Jones last year, telling the host his “reputation is amazing.” Which is indeed true, but not in the complimentary way Trump intended it to be taken.

[What could a future Trump TV venture look like? Tune in here.]

“InfoWars is poisonous, and its journalistic value is negative,” said Rick Perlstein, the historian who has chronicled the modern conservative movement in books about Barry Goldwater and Richard Nixon. He called the circularity of Trump referring to Roger Stone’s interview in InfoWars as “a burlesque version” of Dick Cheney’s planting a story in the New York Times in the run-up to the Iraq War and then citing that story on NBC’s “Meet the Press.”

Perlstein wrote about the Jones/Trump connection in Salon in the spring, calling Jones “a lunatic,” observing that Trump was citing Jones’s denials of a California drought.

It’s well known, of course, that Trump increasingly is campaigning against what he calls “the corrupt media,” slamming news organizations for “false stories, all made up . . . lies, lies.” He calls reporters “scum” and insists that they are all tools of the Clinton campaign.

Still, he makes a few exceptions. He borrowed his top campaign executive, Stephen Bannon, from Breitbart News, the far-right website which is practically a wing of his campaign, often referred to as Trump Pravda.

[When Trump went low, Steve Bannon found a new direction: Lower.]

He has repeatedly suggested that the National Enquirer, the supermarket tabloid that sometimes pays for news stories, should have won a Pulitzer Prize for exposing the existence of former North Carolina senator John Edwards’s love child. Trump also used the Enquirer as the basis of his discredited claim that Ted Cruz’s father was linked to the assassin of President John F. Kennedy.

The Enquirer returns the love — it’s one of the very few newspapers in the nation that have endorsed Trump for president, while many papers that have never backed a Democrat are endorsing Clinton or at least warning their readers off Trump. (The Enquirer’s chief executive, David Pecker, and Trump reportedly have a personal friendship.)

And in recent weeks, Trump has retreated almost entirely to Fox News, largely to the open arms of Sean Hannity, who has taken to professing that he’s not a journalist.

Given all this, where does the Republican nominee for president get his news? I asked Jason Miller, a senior communications official for Trump, that question by phone and email Tuesday.

And, I asked, does the candidate think as highly of Alex Jones and InfoWars as it seems? There’s been no response, but the evidence is there for all to see.

Trump’s embrace of the worst in American journalism, and his scorn for the best, is absurd. But it’s not actually funny.

If he loses on Nov. 8 and his next act truly does turn out to be Trump TV — filled with wild-eyed conspiracy theories, checkbook journalism and conjecture presented as fact — no one can say he didn’t warn us.

For more by Margaret Sullivan, visit wapo.st/sullivan