In the 1922 poem The Waste Land, T. S. Eliot writes, cryptically: Who is the third who always walks beside you?/When I count, there are only you and I together /But when I look ahead up the white road/There is always another one walking beside you.

In his footnotes to this verse, Eliot explained that the lines “were stimulated by the account of one of the Antarctic expeditions [Ernest Shackleton’s] ... that the party of explorers, at the extremity of their strength, had the constant delusion that there was one more member than could actually be counted.”

Third man, angel, alien or deity—all are sensed presences, so I call this the sensed-presence effect. In his gripping book, The Third Man Factor (Penguin, 2009), John Geiger documents the effect in mountain climbers, solo sailors and ultraendurance athletes. He lists conditions associated with it: monotony, darkness, barren landscapes, isolation, cold, injury, dehydration, hunger, fatigue and fear. I would add sleep deprivation; I have repeatedly experienced its effects and witnessed it in others during the 3,000-mile nonstop transcontinental bicycle Race Across America. Four-time winner Jure Robic, a Slovenian soldier, recounted to the New York Times that during one race he engaged in combat a gaggle of mailboxes he was convinced were enemy troops; another year he found himself being chased by a “howling band” of black-bearded horsemen: “Mujahedeen, shooting at me. So I ride faster.”

Sleep deprivation also accounts for Charles A. Lindbergh’s sensed presence during his transatlantic flight to Paris: “The fuselage behind me becomes filled with ghostly presences—vaguely outlined forms, transparent, moving, riding weightless with me in the plane ... conversing and advising on my flight, discussing problems of my navigation, reassuring me, giving me messages of importance unattainable in ordinary life.”

Whatever the immediate cause of the sensed-presence effect, the deeper cause is to be found in the brain. I suggest four explanations: 1) The hallucination may be an extension of the normal sensed presence we experience of real people around us, perhaps triggered by isolation. 2) During oxygen deprivation, sleep deprivation or exhaustion, the rational cortical control over emotions shuts down, as in the fight-or-flight response, enabling inner voices and imaginary companions to arise. 3) The body schema, or our physical sense of self—believed to be located primarily in the temporal lobe of the left hemisphere—is the image of the body that the brain has constructed. If for any reason your brain is tricked into thinking that there is another you, it constructs a plausible explanation that this other you is actually another person—a sensed presence—nearby. 4) The mind schema, or our psychological sense of self, coordinates the many independent neural networks that simultaneously work away at problems in daily living so that we feel like a single mind.

Neuroscientist Michael S. Gazzaniga of the University of California, Santa Barbara, calls this the left-hemisphere interpreter—the brain’s storyteller that pulls together countless inputs into a meaningful narrative story. In an experiment with a “split-brain” patient (whose brain hemispheres were surgically disconnected), Gazzaniga presented the word “walk” only to the right hemisphere. The patient got up and began walking. When he was asked why, his left-hemisphere interpreter made up a story to explain this behavior: “I wanted to go get a Coke.”

My brother-in-law Fred Ziel, who has twice climbed Mount Everest, tells me that both times he experienced a sensed presence: first when he was frostbitten and without oxygen at the limit of physical effort above the Hillary Step, and second on Everest’s north ridge after he collapsed from dehydration and hypoxia at 26,000 feet. Both times he was alone and feeling desirous of company. Tellingly, when I asked his opinion as a medical doctor on possible hemispheric differences to account for such phenomena, Fred noted, “Both times the sense was on my right side, perhaps related to my being left-handed.” The sensed presence may be the left-hemisphere interpreter’s explanation for right-hemisphere anomalies.

Whatever its cause, the fact that it happens under so many different conditions tells us that the presence is inside the head and not outside the body.