This wasn't the first time that David had tried to amputate his leg. When he was just out of college, he'd tried to do it using a tourniquet fashioned out of an old sock and strong baling twine. David (not his real name) locked himself in his bedroom at his parents' house, his bound leg propped up against the wall to prevent blood from flowing into it. After two hours the pain was unbearable, and fear sapped his will.

Undoing a tourniquet that has starved a limb of blood can be fatal: injured muscles downstream of the blockage flood the body with toxins, causing the kidneys to fail. Even so, David released the tourniquet himself; it was just as well that he hadn't mastered the art of tying one.

Failure did not lessen David's desire to be rid of the leg. It began to consume him, to dominate his awareness. The leg was always there as a foreign body, an impostor, an intrusion.

He spent every waking moment imagining freedom from the leg. He'd stand on his "good" leg, trying not to put any weight on the bad one. At home, he'd hop around. While sitting, he'd often push the leg to one side. The leg just wasn't his. He began to blame it for keeping him single; but living alone in a small suburban townhouse, afraid to socialise and struggling to form relationships, David was unwilling to let anyone know of his singular fixation.

One night about a year ago, when he could bear it no longer, David called his best friend. There was something he had been wanting to reveal his whole life, David told him. His friend's response was empathetic – exactly what David needed. Even as David was speaking he began searching online for material. "He told me that there was something in my eyes the whole time I was growing up," David said. "It looked like I had pain in my eyes, like there was something I wasn't telling him."

Once David opened up, he discovered that he was not alone. He found a community on the internet of others who were also desperate to excise some part of their body – usually a limb, sometimes two. These people were suffering from what is now called "body integrity identity disorder" (BIID). With a handful of websites and a few thousand members, the community even has its internal subdivisions: "devotees" are fascinated by or attracted to amputees, often sexually, but don't want amputations themselves; "wannabes" strongly desire an amputation of their own. A further delineation, "need-to-be", describes someone whose desire for amputation is particularly fierce. It was a wannabe who told David about a former BIID patient, a "gatekeeper", who had been connecting other sufferers to a surgeon in Asia. For a fee, this doctor would perform off-the-book amputations.

Scientists are only just beginning to understand BIID. The first modern account of the condition dates from 1977, when The Journal of Sex Research published a paper on "apotemnophilia" – the desire to be an amputee. The paper categorised the desire for amputation as a paraphilia, a catch-all term used for deviant sexual desires. Although it's true that most people who desire such amputations are sexually attracted to amputees, the term paraphilia has long been a convenient label for misunderstandings: after all, at one time homosexuality was also labelled as paraphilia.

But there do appear to be a small number of people who feel incomplete with all four limbs. It's difficult for most of us to relate to a notion like this. Your sense of self, like mine, is probably tied to a body that has its entire complement of limbs. When asked to describe his leg, David said, "It feels like my soul doesn't extend into it."

Neuroscience has shown us over the past decade or so that this sense of ownership over our body parts is strangely malleable, even among normal healthy people. In 1998, cognitive scientists at Carnegie Mellon University in Pittsburgh performed a deceptively simple experiment. They sat subjects down at a table with their left hands resting on a table. A screen prevented the subjects from seeing their hands: instead, a rubber hand was placed in front of the screen. The researchers then used two small paint brushes to stroke both the real hand and the rubber hand at the same time. When questioned later, the subjects said that they eventually felt the brush not on their real hand, but on the rubber hand. More significantly, many said they felt as if the rubber hand was their own.

The rubber-hand illusion illustrates how the way we experience our body parts is a dynamic process, one that involves constant integration of various senses. And if we can feel as if we own something as inanimate as a rubber hand, can we own something that doesn't exist? Seemingly, yes. Patients who have lost a limb can sometimes sense its presence, often immediately after surgery and at times even years after the amputation - so-called "phantom limbs".

David contacted the gatekeeper about his desire for an off-the-book operation, but more than a month passed and he did not hear back. His hopes of surgery began to fade and his depression deepened. The leg intruded more insistently into his thoughts. David decided to try again to get rid of it himself.

This time he settled for dry ice, one of the preferred methods of self-amputation among the BIID community. The idea is to freeze the offending limb and damage it to the point that doctors have no choice but to amputate. David drove to his local Walmart and bought two large trashcans. The plan was brutal, but simple. First, he would submerge the leg in a can full of cold water to numb it. Then he would pack it in a can full of dry ice until it was injured beyond repair. He bought rolls of bandages, but he couldn't find the dry ice or the prescription painkillers he needed if he was going to keep the leg in dry ice for eight hours. David went home despondent, with just two trashcans and bandages, preparing himself mentally to go out the next day to find the other ingredients. The painkillers were essential; he knew that without them he would never succeed. Then, before going to bed that night, he checked his computer.

There it was: a message. The gatekeeper wanted to talk.

This is an exclusive extract from Do No Harm, the debut article from MATTER, a new online publisher focused on long-form science and technology writing. Visit readmatter.com to purchase the full article ($0.99), or buy it through Amazon's Kindle Singles store

