Just how powerful IS positive thinking?

Experts debate how much effect, if any, a positive attitude may have on medical patients battling potentially fatal disease.

When people recover from illnesses against all odds ... should their claims that "positive thinking" made the difference constitute PROOF POSITIVE? Turns out, not even the "experts" entirely agree on that one. Our Cover Story this morning is from Rita Braver:

Rookie New York Giants Mark Herzlich was on top of the world last weekend - starting for the very first time.

But Two years ago, Herzlich was facing a much bigger challenge than winning a football game - he was diagnosed with bone cancer.

"My doctor at the time had told me not only would I never play football again, I'd probably never be able to run again," Herzlich said.

Herzlich had been an All American linebacker at Boston College, a fierce competitor on the field and off.

"I made a goal in my mind," he said. "It was that I was going to beat the cancer and come back and play football."

Like Herlizich, Lance Armstrong credits not only top quality medical care but also positive thinking with beating his cancer: "You can't deny the fact that a person with a positive and optimistic attitude does a lot better," he said.

In 2006 Leigh Fortson, a freelance writer from Colorado, was diagnosed with anal cancer. She was later diagnosed with two more serious cancers. But she has survived and is still fighting the disease.

"I have absolutely no doubt" that her mindset made a difference, Fortson said.

Positive thinking - a can-do attitude. That's the key, according to all these people and others who faced life-threatening illnesses. But here is a surprising and disheartening fact: What you think, how you think, may not make a difference.

"That's dangerous nonsense, to think that you can think your way out of cancer, or think your way out of heart disease," said psychologist Richard Sloan.

A professor of behavioral medicine at Columbia University Medical Center, he has done extensive examinations of survival studies.

"What do you say to somebody who thinks that the reason that they're still alive now [is] because of their attitude?" asked Braver.

"I'd say, I'm very happy for you, I'm glad you survived. But for every one of you who said you were going to fight your way out of it, there are probably dozens of people who said precisely the same thing and didn't survive," Sloan said. "One person's anecdote doesn't make evidence."

One of the major studies on whether mindset affects recovery was conducted by University of Pennsylvania psychologist James Coyne.

"First, we asked simply is there any relationship overall? Putting biology aside, does emotion and well-being predict survival?" said Dr. Coyne. The answer? "It was as close to 'not at all' as you could get."

He said the few studies that conclude otherwise are hype, all based on bad science.

"It's disturbing at some level. Attitude doesn't matter for survival," said Dr. Coyne. "There are some things you can modify in life, but cancer is not one of them."