In early 1988, the British neuropsychiatrist Peter Fenwick found himself drowning in letters from people who believed they had survived an encounter with death.

“I slowly floated down a tunnel, not afraid in any way but looking forward to something,” one man wrote to him. “When it came I was absolutely at peace and going towards the most wonderful light. Believe me, it was great. No worries, problems or anything, just wonderful.”

In another letter, a woman described walking down a country lane and coming upon golden gates.

“Inside was the most beautiful garden, no lawn, path or anything else, but flowers of every kind,” she wrote. “Those that attracted me most were Madonna lilies, delphiniums and roses, but there were many, many more.”

The letters were among more than 2,000 that Dr. Fenwick received shortly after he appeared in a BBC documentary, “Glimpses of Death,” in which he commented on the near-death visions of people who had apparently briefly died, or nearly died, and then come back to life.

“These letters were written by people who had never, ever before told anyone about their experiences,” Dr. Fenwick said in a 2012 lecture at TEDxBerlin. “Why? Because they’re too frightened. They told it to their wives or their husbands; they said they weren’t interested. They told it to their friends; they said, ‘You’re mad.’”

But Dr. Fenwick, an expert on consciousness, was keenly interested. Possessing a more scientifically open mind than many of his peers, he had begun studying near-death experiences — a contentious subject in neuroscience — in the mid-1970s. He believed that consciousness existed beyond physical death, and he thought the letters would help strengthen his position.

Dr. Fenwick sent the letter writers a lengthy questionnaire to categorize their experiences. He presented his findings, alongside excerpts from the letters, in “The Truth in the Light: An Investigation of Over 300 Near-Death Experiences” (1995), which he wrote with his wife, Elizabeth Fenwick. The book established him as a leading authority in near-death studies.

Dr. Fenwick died on Nov. 22 at his home in London, his daughter Annabelle Fenwick said. He was 89.

“The Truth in the Light” revealed startling similarities among the letter writers. More than 50 percent of them reported traveling in a tunnel. Seventy-two percent saw a bright light. Nearly 40 percent met someone they knew, including deceased relatives. Strikingly, 72 percent reported that they had made the decision to return.

A woman who had been in a horrific car accident recalled being “encouraged by a strong feeling to enter the light” through a tunnel.

“I was peaceful, totally content, and I understood I was born on earth and knew the answer to every mystery — I was not told, I just knew, the light held all the answers,” she wrote. “Then there was sudden confusion. I had to go back to the tunnel quickly; something was wrong.”

Suddenly, she continued, “I regained my body and all emotions. I panicked and felt pain, tremendous pain, all over my body. I believe I died for a short time.”

Neuroscientists have for decades dismissed near-death experiences, or N.D.E.s, as symptoms of anoxia — a lack of oxygen flowing to the brain. Dr. Fenwick rebutted that assessment in “The Truth in the Light,” pointing to the instruction of pilots.

“Pilots in training regularly undergo acute anoxia in simulators to make sure they can get their oxygen masks on in time,” Dr. Fenwick wrote. “Those who fail to do so do not have N.D.E.s; they either go unconscious or become so confused that they try to land their planes on clouds.”

He also dismissed another common critique of near-death experiences: that they are run-of-the-mill hallucinations, like those experienced by people with high fevers.

“But describing it as a hallucination does nothing to explain the underlying mechanism and leaves many of the same old questions unanswered,” Dr. Fenwick wrote. “Why should everyone have more or less the same hallucination in the same circumstances? And why should it seem so real?”

Peter Brooke Cadogan Fenwick was born on May 25, 1935, in Nairobi, Kenya, where his father, Anthony Fenwick, had been sent by his family in northern England to farm coffee. His mother, Betty (Darling) Fenwick, was an Australian-born physician and director of surgery at Nairobi Hospital.

Peter was a curious and mischievous boy. He liked to build things, including the occasional small bomb. One evening, while his parents were preparing to host dinner guests, Peter quietly laid a trail of gunpowder around the table in hopes of lighting it for entertainment. His father disrupted the plot.

“I think he was clearly one of these kids who is incredibly bright but maybe not always so brilliant at reading the room,” his daughter Annabelle said in an interview. She added, “He did things because he could.”

After graduating from Stowe School, a prestigious boarding institution in the English countryside, Dr. Fenwick studied natural sciences at the University of Cambridge. He graduated in 1957 and then continued his studies there, receiving his medical degree in 1960.

Dr. Fenwick aspired to become a brain surgeon, but he changed his mind after observing a brain surgery.

“I suddenly realized that if you were a brain surgeon you looked down a deep, dark hole into the brain, and I could see there was no fun in that,” he told the British newspaper The Telegraph last year. “I realized I didn’t want to be a neurosurgeon, I wanted to be a neuropsychiatrist so I could talk to people and not have them unconscious while I looked into that deep, dark hole.”

He joined Maudsley Hospital in London, the largest psychiatric teaching hospital in Britain, where he at first specialized in epilepsy. He also studied sleepwalking, dreams and meditation. (One of his first research subjects in meditation was George Harrison of the Beatles.)

In 1975, the American philosopher and psychiatrist Raymond A. Moody Jr., published “Life After Life,” one of the first books by a physician about near-death experiences. It was an international best seller, but Dr. Fenwick, like many other readers, was skeptical about the deathbed visions recounted in the book.

Then, the next year, a patient of his told him that he had seen a bright light through a tunnel while experiencing near-fatal complications during heart surgery.

“I was able to look at him, discuss it with him and see in fact that this was no psychobabble — it was a real experience,” Dr. Fenwick told The Telegraph. “This was enormously important.”

Dr. Fenwick was a founder of the International Association for Near-Death Studies UK. He was also president of the Scientific and Medical Network, an organization that supports research into the connections between science, philosophy and spirituality.

In addition to his daughter Annabelle, Dr. Fenwick is survived by his wife, Elizabeth (Roberts) Fenwick, with whom he wrote four books in addition to “The Truth in the Light,” including “The Art of Dying” (2008), about the process of death; another daughter, Natasha Lowe; a son, Tristam; and nine grandchildren.

In “The Truth in the Light,” Dr. Fenwick revealed that 82 percent of the people he surveyed were less afraid of dying as a result of their near-death experiences, and that 42 percent reported being more spiritual. Forty-eight percent, he wrote, were “convinced” there was “survival after death.”

“Once you’ve had this experience you are changed, whether you like it or not,” he told The Telegraph.

His belief that there was death of the body, but not the individual person, erased any fear he had about dying.

“Actually,” he said, “I’m looking forward to it.”