Suppose your friend has suffered a stroke⁠—a major one, which causes hemiplegia in which half of his body paralyzed. Despite being unable to move the left side of his body, your friend is in good spirits⁠—he just got through a stroke and is okay! He has no clue he is paralyzed. He denies paralysis. And when you ask him to move any paralyzed body part, he claims that he is doing it just fine.

Your friend is not just paralyzed; he also has a condition called anosognosia⁠—and he may never know that he has it.

The term anosognosia literally means “unaware of illness” and was coined by a French neurologist Joseph François Babinski in 1914. Anosognosia can be used to refer to any sort of ignorance of disease, as in hallucinations or delusions of schizophrenics and manic depressants, yet these are usually only partially impaired awareness. Full anosognosia usually accompanies blindness and especially hemiplegia – paralysis on one half of the body due to a stroke.

The cause of anosognosia is still largely unknown. Some psychologist claim that it is an extreme case of the denial defense mechanism, yet there is some physiological evidence pointing away from a purely psychological explanation. In the case of hemiplegics, anosognosia only accompanies damage to the right side of the brain, never on the left – if it was purely denial, there would be an equal chance of having anosognosia regardless of which hemisphere is damaged.

It is surprisingly difficult to get an anosognosiastic patient to recognize their own paralysis. If you ask them to do something with their paralyzed hand, for example, they will say that they can see and feel the action, despite their inactivity. You could ask them to clap and they would hopelessly wave one hand in the air⁠—but say that nothing is wrong with their technique. When pressed about their inactivity they may make up excuses, such as that they are tired, or arthritic. Also, anosognastiac patients seem unaware of other patients’ paralysis as well – as if paralysis was not something that happened to humans.

In the worst cases, one may even forget that certain appendages are their own – most commonly their paralyzed arm. They will vehemently deny that their arm is theirs, even when it is pointed out to them that it is attached to their body.

An anosgnostic patient can be made lucid of their problems by squirting cold water into the ear on the paralyzed side of the body. Suddenly, patients will become quite aware of their own disability. However, when asked about their previous denials of paralysis, they will tell you that they had been aware of their paralysis the entire time and had never denied this fact. Afterwards, when the effects of the water wears off, they will completely forget their episode of cognizance – in fact, they will know that they were asked whether they were paralyzed, but will say that they denied it.

In some cases anosognosia eventually wears off. In other cases the person is trained to function with their disability⁠—though they still may not be aware that they are disabled at all! Regardless, if the thought of getting anosognosia scares you, then take comfort in the fact that this condition is very rare. Besides… you will probably never know you have a problem at all.