eadphones was another world. Everyone in this world thought the same way, except for liberals, and they were only cartoon characters, to be defeated as though in a video game. In the real world, my friend was unemployed and had been staying with me, rent-free, for two months. But inside the headphones, he could laugh about welfare bums instead of pounding the pavement.

I thought about that last week when Limbaugh went after his latest target: Michael J. Fox. The actor, who has Parkinson's disease, has been appearing in ads for candidates who support government-funded embryonic stem cell research. The ads promote such research as a potential cure for Parkinson's and other diseases.

On Monday, Limbaugh played one of the ads for his audience. "In this commercial, he is exaggerating the effects of the disease," he said of Fox. "He is moving all around and shaking. And it's purely an act. This is the only time I have ever seen Michael J. Fox portray any of the symptoms of the disease he has. . . . This is really shameless of Michael J. Fox. Either he didn't take his medication or he's acting, one of the two."

Where had Limbaugh seen Fox? "I've seen him on 'Boston Legal,' I've seen him on a number of stand-up appearances," he said. He pointed to Fox's autobiography. Fox "admits in the book that before a Senate subcommittee . . . he did not take his medication, for the purposes of having the ravages and the horrors of Parkinson's disease illustrated, which was what he has done in the commercials," Limbaugh charged.

In the book, Fox explains his life in the real world -- the world his body inhabits, as opposed to the make-believe world Limbaugh saw on television. Fox describes how, during "the years I spent promoting the fiction that none of this was actually happening to me," he learned "to titrate medication so that it kicked in before an appearance or performance. . . . I did everything I could to make sure the audience didn't know I was sick. This, as much as anything, had, by 1998, become my 'acting.' " When he came out of the Parkinson's closet, Fox recalls, he chose "to appear before the subcommittee without medication. It seemed to me that this occasion demanded that my testimony about the effects of the disease . . . be seen as well as heard."

Here we have two completely different notions of reality. Fox's job is to portray characters in movies and on television. For him, Parkinson's was an invasion of the fake world by the real one. The medication, designed to hide this from the audience, became part of the fiction. In going off his meds, he was dropping the act.

Limbaugh's life story has gone the other way. His job is to explain politics, a branch of nonfiction. But for him, the fake world has overtaken the real one. He thinks "Boston Legal" is reality. Anything that doesn't match this must be "acting." If you go off your meds, you're not revealing your symptoms. You're "portraying" them.

Radio, television and the Internet greased Limbaugh's descent into fantasy. Years ago, a profile described him "holed up in his New York apartment with Chinese takeout and a stack of rented movies." In another profile, he "complained that he has virtually no social life." Click the video links on his Web site, and you can peer into his world. He sits in a soundproof studio. He never has to go outside.

In Limbaugh's world, "there never was a surplus" under President Bill Clinton. AIDS "hasn't made that jump to the heterosexual community," and cutting food stamps is fine because recipients "aren't using them." Two years ago, he said the minimum wage was $6 or $7 an hour. Last year, he said gas was $1.29 a gallon.

Limbaugh has particular trouble distinguishing reality from entertainment. The abuse at Abu Ghraib "looks just like anything you'd see Madonna or Britney Spears do on stage," he told his listeners. Last month, he defended ABC's Sept. 11 movie against the document on which it purportedly relied: "The 9/11 commission report, for example, says, well, some of these things didn't happen the way they were portrayed in the movie. How do they know that?"

Last year, Limbaugh, who used a tailbone defect to get out of the Vietnam War draft, accused a Democratic candidate of having served in Iraq "to pad the resume." He charged veterans -- including former senator Max Cleland (D-Ga.), who lost his legs and an arm in Vietnam -- with trying "to hide their liberalism behind a military uniform . . . pretending to be something that they are not." When war is just a television show, a uniform is just a costume. Liberalism is real; losing your limbs is a pretense.

Which brings us back to stem cells. Limbaugh says Fox's ads dangle a prospect of imminent cures "that is not reality." He's right. But the ads convey another reality: a man dying of a disease that might be cured more quickly if the government dropped its restrictions on research funding. Limbaugh dismisses this as a "script" being followed by Fox's "PR people" and "the entertainment media." Script? Entertainment? This is life and death.

I have another friend. He has Parkinson's. I've seen him on good days and bad days. That's how I know Fox isn't faking. My friend doesn't see the destruction of embryos as a dangerous price to pay for stem cell research. I do. But if you worry about the embryos, you had bloody well better look into the eyes of the people dying of these diseases. You had better ask yourself whether slowing research that might save them is an acceptable price for your principles.

If you can't -- if all you can see is "acting" -- then you need more help than they do. Fox's disease can only take your body. Limbaugh's can take your soul.

William Saletan covers science and technology for Slate, the online magazine at www.slate.com.