From childhood, we are taught that the human body has five senses. I’m sure we can all recite them: sight, hearing, touch, taste, and smell. This list has remained unchanged since the time of Aristotle. To most people, a “sixth sense” refers either to one outside the realm of the scientific, or one that simply does not exist in most humans.

However, ask a neurologist how many senses the human body has, and you might get a surprising answer. Many identify nine or more senses- some listing as many as twenty-one. The first category of senses is the “special” senses, including the familiar sight, hearing, taste, and smell. The second category is made up of the somatic senses, which we usually lump under “touch”- including our perception of pressure, heat, and pain. The third category, however, is not nearly as well-known. These are the interoceptive senses- those that deal with data originating in the body itself.

It is fairly obvious what happens to a person when a sense fails. Many members of society are missing one or more senses. It is common knowledge that blindness is the absence of sight. Deafness, of hearing. Everyone knows what it’s like to lose taste and smell as well; this loss accompanies every head cold. But what happens when the body loses knowledge of itself is a far stranger occurrence.

The interoceptive senses are lumped together in various configurations, but there are basically three. The first⁠— balance⁠— is the sense of the body’s alignment. This is the sense that keeps an animal upright; the famous ability of cats to always land on its feet, for example, is due to this sense. The organic sense is what alerts the body to its internal condition; this is how you know that you are hungry or thirsty. The third sense is known as proprioception. This, put simply, is the brain’s knowledge of the relative positions of the body’s parts.

To visualize this sense, close your eyes and extend your hand in a random direction. Now identify in your mind its exact position and open your eyes. Note that your brain was well aware of your hand’s position, even though none of the “classic” five senses were currently detecting it. This is proprioception. If you want another example of this sense, try driving erratically enough to attract official attention. The familiar walk-the-line and finger-to-nose sobriety tests that you will be subjected to are yet more examples of what your body can do- or at least should be able to do- thanks to proprioception.

The loss of this ability is known by several names. Proprioception Deficit Disorder, Sacks’ Syndrome, and Descartes’ Disease are all titles for the same illness, which is a complete and total failure of the body’s knowledge of itself. Since it is a rare disorder, it is difficult to say what the premonitory symptoms are. However, there have been reports that the first symptoms are extremely vivid dreams of lost motor function or physical control. These are followed by an increasing lack of coordination, culminating in full-blown proprioceptive failure. At this point, the human mind is completely unaware of its own body. Sufferers report a “disembodied” feeling, as if the mind and body have completely separated.

Oliver Sacks, one of the disease’s namesakes, reported perhaps the first recognized case of the disorder. In The Disembodied Woman, he describes the process by which a patient- whom he calls “Christina”- succumbed to PDD. In a matter of days, she transitioned from a healthy and active mother of two to a helpless physical wreck, with no sensation of her own body. The preliminary testing showed that her nerves were in perfect working order; she could feel physical sensations nearly as well as she always had. However, somewhere between mind and body a roadblock had developed, preventing her brain from forming an accurate body image- or indeed a body image at all.

The results of this disorder are logical once one understands the concept of proprioception. Think of all the activities in a typical day that require the body’s knowledge of its own position. If you carry your briefcase to the car while fumbling for the keys, your legs do not buckle because they are currently unsupervised. Your hand does not drop its load because you neglected for a moment to think, hold on to the briefcase. Your jaw does not hang slack because you weren’t specifically concentrating on keeping your mouth closed. But for someone with PDD, these are exactly the type of things that happen.

Currently there is no known cure for PDD. Sometimes the body self-corrects, but in Christina’s case it never did. Like any other sensory-deprived person, victims of PDD begin to naturally replace the missing sense with those that remain. Christina replaced it with sight. Since she no longer possessed an intuitive knowledge of where her respective body parts were located, she built up that knowledge herself using her eyes. If she wanted to pick up a cup of coffee, she watched her hand carefully until she had successfully put it down again. If she wanted to walk across the room, she had to supervise her feet until she was safely sitting on the other side. Sitting itself is difficult for a sufferer of PDD; if the hands are not watched carefully they will either grip the chair with white-knuckle intensity or release their hold, allowing the startled individual to slide to the floor.

In many respects, the body of a PDD victim becomes their puppet. Each movement must be carefully analyzed and put into motion; no longer can one trust the body to “just walk” or “just sit.” Thus, the movement does not look at all natural; opening a door becomes a laborious process of extend hand, tighten each finger, rotate knob, extend arm, raise foot, lower foot, and so on. Meanwhile one must not become distracted from the other foot, and who knows what is happening with the body parts not directly involved in the current process.

Fortunately, PDD is an extremely rare disorder. It is neither contagious nor genetic, and no one knows what causes the permanent variety. There is a type that is brought on by massive overdoses of vitamin B6 (pyridoxine), but it is for the most part temporary with few lingering effects. A rather eerie set of statistics suggests that, unlike most disorders, the more education one has the more likely one is to develop the affliction. This factor, as well as the initial dream symptoms, suggests that the disease could have psychophysiologic roots. However, until we know for sure, this disorder represents one of the parts of the human mind that remains a mystery. It also proves with tragic clarity the old axiom: you never know what you have until you lose it.