Image credit: British Library Trustees; book cover designer: Chojnowski

Did an angel give encouragement to Sir Ernest Shackleton and his men? Image credit: British Library Trustees; book cover designer: Chojnowski

I heard this anecdote somewhere: Though there were 10 people in his group (including himself and guides), a mountain climber counted 11 people as the climbing party made its ascent. At base camp, he counted again and found there were 10. Who was that 11th person?

What the climber experienced is familiar terrain in the Bible (where there are plenty of unexpected encounters with strangers who mysteriously vanish), and it's probably an experience familiar to anyone who has felt an unexplained, consoling "presence" -- there's no better word for it -- during a time of great need: at a dying family member's bedside or during a crisis.

What is that "presence"? Is it a guardian angel?

Or is it an example of a psychological phenomenon that John Geiger writes about in "The Third Man Factor" (Weinstein Books: 300 pp., $24.95)?

Geiger begins with the stories of three people who had near-fatal experiences -- Ron DiFrancesco, one of the last people out of the South Tower of the World Trade Center before it collapsed on 9/11; James Sevigny, a mountain climber severely injured in an avalanche while climbing in Canada; and Stephanie Schwabe, an underwater explorer who lost her guideline in an underwater cave in the Bahamas.

As different as each experience was, all three were visited by a ghostly visitor that calmed them, instilled hope and enabled them to rescue themselves. Injured climber Sevigny, for instance, "heard" -- not physically, but in his mind -- a voice tell him to get up and start walking: "The presence, which stood behind his right shoulder, implored him to continue even when the struggle to survive seemed untenable," Geiger writes.

Geiger gathers many such stories and reports on various theories offered to explain them. His book is engrossing, balanced (no barbs are directed at skeptics or believers) -- and, in the end, this book may be more challenging to some people's religious beliefs than either Christopher Hitchens' "God Is Not Great" or Richard Dawkins' "The God Delusion."

Why? Most of us, when the question of God's existence comes up with friends in a restaurant, don't debate it in terms of what Hitchens or Dawkins has to say (though I'm sure there are some who do). We speak more personally -- about what we feel, about incidents that seem providential and not coincidental, about what our grandmas taught us or about attitudes to the afterlife -- drawing not on Aquinas and Dante but on CBS' "Ghost Whisperer."

And this is why Geiger's book is so engrossing -- and formidable. Though he doesn't push an agenda, his survey of stories about "the third man" -- a term arising out of experiences involving pairs of individuals in dangerous situations -- invariably attacks a common belief in angels.

Geiger notes how, in the 20th century, the outlook on what causes such visitations has been a "gradual reduction from the outside in, from God, to the mind, to the brain." This reduction has been given many names: presence hallucination, pathology of boredom, stimulus hunger, hypoxia (oxygen deficiency), bicameralism (how the brain's perceptions can become detached so that they are viewed as belonging to another person).

On the side of the angels, there are the stories of the disastrous Antarctic expedition of Sir Ernest Shackleton in 1914-1916 and the World War I battlefield experience of the British Army at Mons. Angels were said to have protected the retreating soldiers there, while Shackleton and two other survivors had the comforting feeling that "we were four, not three."

Then there are encounters with "the third man" -- most often occurring to mountain climbers -- that seem to be the result of what happens to the brain's wiring when someone is in extreme physical distress. Griffith Pugh, a physiologist on the 1953 British Everest expedition, attributed the Third Man to "a decay of the brain functions . . . caused by extreme cold, exhaustion, and lack of oxygen."

At high altitudes, then, the brain doesn't get the oxygen it needs. Presto, along comes St. Hypoxia, not St. Michael.

When Charles Lindbergh wrote in his memoirs that, during his record-breaking 1927 transatlantic flight, he encountered ghostly visitors that helped him on his long lonely journey, a psychologist opined that "Lindberg's phantom passengers were a product of monotony" -- not angelic assistance.

The book's psychological explanations are very compelling, and it would serve the reader well to consider them. We learn how the mind struggles to overcome intensive periods of boredom -- in much the same way that an amputee feels a phantom limb or a child has an imaginary playmate when no one else will play with him. This theory of the psyche may take the idea of angels out of the picture, but it also inserts more of the angelic into ourselves.

And here's a question for Pugh: Why do all of these hallucinations have a helpful presence and loving feelings in common? Wouldn't the visions of someone with "brain decay" be more delirious and surreal? More nightmarish?

And do you mean to tell me that the visionary experiences of St. Teresa of Avila and the great Tibetan hermit Milarepa -- Geiger writes about both -- were just the result of hunger and boredom, because the visions were preceded by fasts and long stretches of solitary prayer?

You may come away from "The Third Man Factor" with a similar reaction, because Geiger deftly provokes such questions without arriving at definitive answers. He simply looks at this phenomenon from intriguing angles. Another example: He foresees a time in the future when incidents of Third Man sightings may continue -- even increase -- if we start sending astronauts to Mars and beyond. During the isolating conditions of a long space flight, "there is every reason to believe that more and more humans will experience the phenomenon," he writes.

"The Third Man Factor" ends on an optimistic, humanistic note. If scientists one day discover -- without any doubt -- that such visions originate in an "angel switch" in our brains, would this be a moment to grieve or celebrate? Geiger asks us: Wouldn't it be wonderful if we knew how to access this switch more often? Wouldn't it be helpful to us to flip that switch whenever we needed it?