March 16, 2007 -- "The Secret," as described in the thunderously successful DVD and book, essentially says that you can get anything you want -- health, wealth, love -- simply by thinking it.
The book calls it the law of attraction: "When you think of the things that you want, and you focus on them with all of your intention, then the law of attraction will give you exactly what you want … every time."
Proponents of the secret say it's scientific: Positive thoughts attract positive things; negative thoughts attract negative things.
You don't get fat because of food. You get fat because you think food will make you fat, according to the book's author, Rhonda Byrne, who would not grant ABC News an interview for this story.
Her advice: Don't look at overweight people.
"After I saw it, I went and bought 30 copies," Oprah Winfrey said, talking about the book on her show.
She's featured "The Secret" twice on her show in recent weeks. It's being taught in packed seminars across the country.
"If you're constantly thinking, feeling and acting broke, then you're never gonna attract prosperity in your life," said motivational speaker James Ray.
Scientists we've spoken with say that there is evidence that positive thinking has genuine power -- for example in improving your health -- but that "The Secret" emphasizes this power to an extent that is dangerous.
The DVD features a woman who says she cured her breast cancer by watching funny movies and visualizing herself healthy. No radiation or chemo.
"If you think that you're not going to get cancer, you don't want to get cancer, you believe you're not going to get cancer, are you not going to mammography? Are you not going to have a colonoscopy? Are you not going to quit smoking?" asked Richard Sloan, professor of behavioral medicine at Columbia University Medical Center.
And what if you practice "The Secret" and you still get sick?
"I think we get into the area here where you can start feeling guilty if you get sick because I must have been thinking wrongly about this," said Dr. Redford Williams at Duke University.
Proponents of "The Secret" insist they're not telling people to avoid doctors and that "The Secret" is helping people the world over.
With millions of people buying it, "The Secret" is most definitely helping the people who are selling it.
Dan Harris filed this report for World News with Charles Gibson.