As Luke Robertson slogged further into his solo South Pole trek, he had more episodes of "felt presence".
Ben Alderson-Day, an associate professor of psychology at Durham University in the UK, is the author of a new book called Presence: The Strange Science and True Stories of the Unseen Other.
He has found that these experiences are not limited to people in extreme situations. You may well have had the sense yourself at some point that someone is right there in the room with you, even though you can't see them. It's not uncommon after a bereavement or in people who have psychosis.
As many as a quarter of those with Parkinson's report experiencing it. It can also happen when you're on the cusp of waking or falling asleep.
For some the experience can occur as part of Sleep Paralysis, where you wake up, but can't move.
People can have the strong sense that someone is in the room with them, or even sitting on their chest, pinning them down. Alderson-Day has found that half the time these experiences involving sleep paralysis involve a very frightening presence.
A felt presence feels as though it's there with you in your personal space. It's hard to pin down exactly what a felt presence consists of. It's not experienced via the five physical senses of touch, sight, hearing, smell or taste, so it's not an hallucination. Objectively, in reality, there is nothing there at all. Yet they're not quite delusions either, which involve thoughts. Nor is it the same as imagining someone is there.
People sometimes talk of something as nebulous as "a thickness in the air". It's almost like a sixth sense, which feels very real at the time. As Alderson-Day puts it: "It's too empty to be a hallucination, but too tangible to be a delusion."
In his search for explanations, Alderson-Day turns to a combination of the physical and the psychological. With mountaineers and explorers, a lack of oxygen to the brain may play a part, something which is also known to induce hallucinations.
But there's also the survival aspect. Is the mind somehow conjuring up a presence that helps us through?
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