On September 11, 2001, Ron DiFrancesco went to work at Euro Brokers, a financial trading firm on the 84th floor of the South Tower of the World Trade Center, like it was any other day. He was at his desk when the first plane hit the North Tower at 8:46 a.m. A friend called and told him to get out. He headed toward the elevators when the second plane hit his tower at 9:03 a.m. That left the stairs as the only way out. In all the commotion, he quickly got stuck on a smoke-filled landing at the 80th floor, blocked by a collapsed wall. People around him were panicking, gasping for air and crouching low to breathe. DiFrancesco lay down on the concrete floor for a minute, feeling hopeless. Then he heard a voice telling him to get up.

The voice didn't belong to any of the people around him, but it was as real as any human voice to DiFrancesco. A strong presence of someone unseen was guiding him, directing him to another stairwell, telling him to break through the flames. He essentially ran into the fire to escape it, and though flames surrounded him as he made his way down three stories, he did find a clear path on the 76th floor and was able to escape just as the building was coming down. He was the last person out of the South Tower before it collapsed at 9:59 a.m.

This is one of hundreds of accounts (both verbal and written) of being aided by an invisible guardian, coined in a T.S. Eliot poem as the third man, explored in John Geiger's new book, "The Third Man Factor: Surviving the Impossible." Geiger researched this phenomenon -- when someone seems trapped in a life-or-death situation, only to be led to safety by a presence that feels as real as the danger -- for six years, combing through books, newspaper accounts, personal journals and psychological research, as well as interviewing people who have been aided by an unseen helper.

In an interview with AOL Health, Geiger explores the "third man" phenomenon that many adventurers, as well as people in the wrong place at the wrong time, have described. He explains that, while these occurrences may sound like a good old-fashioned "invisible friend," the "third man" is anything but child's play or make-believe.

AOL Health: Have you ever experienced this phenomenon?

John Geiger: When I was 7, I had an altercation with a rattlesnake [while hiking] in southern Alberta [Canada]. My father was ahead of me as I was coming up a steep embankment. I came face to face with this rattlesnake, and it was very terrifying, as you can imagine. I actually had a sense of duality, as if I could see the scene unfold. There seemed to be two of me -- one facing the rattlesnake and one kind of watching from a distance. In an instant, my father grabbed me and lifted me out of danger, and it's as if I saw it happen. To this day, I don't know whether or not it was just an overactive child's imagination or whether it was a real mental experience that I had, but it certainly made me interested in the ways we mentally cope with extremely stressful situations. Then, when I came across Sir Ernest Shackleton's experiences in Antarctica with a "third man," [when on the Imperial Trans-Antarctic Expedition of 1914 to 1916, the crew became stranded and was reportedly guided by an unseen force] I was very interested and began looking for other examples. I very quickly found scores of them.

AOL Health: How does science explain this seeming "guardian angel" experience?

Geiger: What's interesting to me is that there has been so little study of this phenomenon. When you think about the significance of this, that people in their greatest hour of need can turn and find help and escape situations that are extremely difficult, it's a beautiful idea. It strikes me as astonishing that there hasn't been more scholarly work done in this area. Having said that, there have been some notable studies, which started in the 1950s with Macdonald Critchley, a British neurologist who first seriously looked at the phenomenon of the third man. He looked at wartime shipwreck survivors, and he felt that the origins of the third man lay not outside the body, but within -- a neurological response brought on by extreme stress. And more recently, there have been other neurological studies. So it is an area that is gradually being recognized by the scientific community as a subject of interest.

There are two major explanations as to what is happening. One is that it's a psychological response to extreme stress and that people have an inner resource they can call upon, and that is a sense that there is another being. They have a companion when they need one, which acts as a coping mechanism and allows them to overcome these apparently insurmountable obstacles.

The other is from very recent work in Switzerland that suggests that by using an electrical stimulus in the brain, some neurologists were able to evoke a sense of a presence in a clinical setting. So by simply stimulating part of the brain, they were able to create a sensation for a patient that there was another being beside her, when objectively there was no one there. Now, what was interesting about that is it did not have the same overwhelmingly benevolent and beneficial powers for her. It was simply an odd, slightly unusual kind of sensation.

AOL Health: Do you find many people who write these experiences off as anecdotal or made up?

Geiger: There are so many examples in the book because I didn't want to restrict it to a handful of cases because then a critic would be able to dismiss it. But here, we have scores of examples over time and in all sorts of different environments. Nobody is questioning whether the experience is real. It's accepted and I think the book makes an overwhelming case that this a common experience for human beings.

AOL Health: Why do some people experience this, but not others? Did you struggle with understanding the seemingly random occurrence?

Geiger: Yes, absolutely, and there are a number of considerations. Some people are just more open to this experience. Some people are just more willing to admit they are having this kind of experience, and are prepared to pay attention to it. Other people might not take the advice: They block it out and think, "This is crazy" and that they are obviously coming apart at the seams and dismiss it as a hallucination. Essentially, the third man requires a willing partner. You can't benefit if you are unwilling to take the directions and guidance and the suggestions that are provided and admit that you are having this experience.

AOL Health: Can you explain the five principles you mention in your book that are usually in place for the "third man" to appear?

Geiger: I had a lot of material to work with, in the low hundreds of examples, and I began to see patterns emerging. I was able to draw some theories as to likely triggers for the experience. This experience has happened to people in all sorts of situations: at sea level, on mountaintops, in space, in airplanes, on the Antarctic continent, and other places. It can happen anywhere, but what evokes it?

One is the principle of multiple triggers. So it's not enough that you are under one particular stress, like you haven't eaten for a long time or you are at high elevation. Those are all stresses, but it requires more than a single stress: It seems to be a group of stresses combined that bring on the phenomenon.

Then, there is the widow effect. For example, Nancy Reagan recently gave an interview in "Vanity Fair," where she says that she still feels the presence of the late Ronald Reagan. If you ask them, a surprisingly high number of widows and widowers will tell you that they still feel the presence of their loved one. So a pattern I found in the book is that most often the third man seems to come when someone loses their companion. For example, if you have two climbers and one falls back and the other continues, it's the absence of that second or third person that seems to [produce this effect]. When you lose someone on an adventure -- they die or turn back -- and you suddenly find yourself alone, it's the absence of a real-life companion that seems to encourage the presence of the third man companion.

I also talk about the pathology of boredom. Monotony plays an important role in this phenomenon. So maybe you're high on a mountain in a white-out [a blizzard-like situation of zero visibility] and there's not a lot of sensory input. Or, if you're on the Mir space station, where you have a very monotonous routine that persists for many weeks -- this all contributes to the third man.

Another principle is the muse factor. Some people seem to have an openness to this kind of experience, and others do not. So if you are not open to the experience, then it's less likely that you would be inclined to believe what you were encountering and accept it.

Finally, we have the power of the savior. The power of the savior is the most simple of the underlying conditions, namely that there must be faith in one's ultimate survival. The third man cannot help those who will not accept help, and cannot save those who are resolute in accepting death. He needs a willing partner.

AOL Health: How has religion explained this phenomenon?

Geiger: Essentially, for religious believers, the third man phenomenon is the same as a "guardian angel" experience. You could look at my book as either proof of existence of guardian angels, or it is evidence that guardian angels are a function of brain processes, and this is something that human beings have a capacity to evoke when we're most in need. So, if you are in a life-and-death situation, it's often the sense of human companionship that is the key to survival. In the case of the third man, these presences will actually provide helpful survival tips and point the way to survival. The idea of having a guardian angel fits in well with Judeo-Christian traditions. But other major faiths all have similar accounts of angels or deities of some sort who are there in rugged areas in nature to guide people to safety.

AOL Health: Are there people who have experienced the "third man" who are religious skeptics? Can it occur completely outside the realm of religious beliefs?

Geiger: Yes. What's interesting is that people view it through their own particular lens. Ron DiFrancesco is a practicing Roman Catholic. So for him, it was an angel that helped him out of that situation. But many other people I spoke to are religious skeptics -- agnostics or atheists, or people who just say they're not very religious. They will uniformly tell me that to them, it was a power that they were able to call upon from within themselves, even though it seemed like another being was there. A non-believer would see it as a product of brain processes, whereas a religious or spiritual person would see it as a guardian angel.

There's no difference between the experiences themselves; it's just the interpretation. So people might attribute an identity. Sometimes if they are religious people, the identity will be related to faith, and if not, then it may well be someone they know, like a relative or a friend who recently passed away.

AOL Health: Is the "third man" always a positive, benevolent presence?

Geiger: I didn't find a single case of a person who survived some horrific ordeal who came forward and said that they had a malevolent being, an evil companion who was trying to lead them astray. The people who come through always tell you exactly the same story, that their lives were saved by an unseen presence who encouraged them, provided comfort and support, and also gave them specific directions as to how they might be able to get out of their situation. You would think that in six years of research, I would have found examples otherwise if it were anything but a benevolent survival force. I also think more people have had this experience than are willing to admit and that it's much more common that we might think. I think it's likely underreported, because people are disinclined to admit to unusual experiences. To them, it may be evidence that they were having some sort of breakdown.

AOL Health: Do you feel like we have any control over the "third man" presence?

Geiger: I think we do have control, but because of the lack of recognition of the phenomenon, people are unaware and unable to access it and make use of it. But if you talk to someone like Reinhold Messner, the great climber who's had the third man experience a number of times, it's almost now expected in certain situations. He knows when it's likely to happen. So for people who have found themselves in these situations more than once, the third man is sort of always there and can be called upon when needed. Vincent Lam, a medical doctor and a guy who practices both emergency and expedition medicine, who wrote the forward to my book, had this experience, and he feels like he can call upon it when he needs it. It's now fundamentally part of who he is.

One of the things I've been struck by is the large number of e-mails I'm already receiving from people who have had third man experiences in their everyday lives -- people in terrible car accidents, or who got lost in a wilderness, or who were in abusive relationships or battles with cancer. Human beings are hard-wired for companionship, and this is a way that we can cope with terrible stresses, even in our everyday lives.

AOL Health: Can this be harnessed in everyday life?

Geiger: Fundamentally, the most important thing is that people are aware of the phenomenon -- that they realize that it is real and has been quantified and that there are scores of high-achieving people who have come forward and admitted to having had the experience. Once people recognize the phenomenon and realize that it's an experience many have had, then the ability to utilize and harness it to good effect is tremendous. As much as anything in our ability to cope with severe stress, the third man factor is a fundamental tool in our arsenal for self- preservation. I'm talking not just physical situations like being trapped under a boulder in the wilderness, but in our everyday experiences. I think this phenomenon will touch -- and has touched -- many people. But there's just been a failure to recognize the experience for what it is, which is an astonishing survival capacity.

AOL Health: In your research, was there one story that was more compelling than any other?

Geiger: I open the book with Ron DiFrancesco's story of escaping the World Trade Center. September 11th is an experience that people in America, and people all over the world, have so vividly imprinted on them. It was such a shared trauma. To think that someone in that situation was able to get out because of an intervention from a third man was a remarkable discovery. DiFrancesco's story never ceases to move me. It's an astonishing example of the phenomenon.