Charles VI, French King during a portion of the Hundred Years War, is known to history in large part due to his periodic bouts of mental illness. During one prolonged episode, he became convinced that he was comprised of glass, and refused to allow others to touch him for fear of shattering. He had his clothes fortified with iron rods to further protect his body during the height of this insanity.

This behavior was not terribly surprising for the king, who sometimes forgot who he was and no longer knew he had a kingdom. What is more surprising is that his behavior was more common than one would think. The so-called “glass delusion” was often found among the upper class of Medieval Europe, and, unlike Charles VI, many individuals manifesting this disorder were otherwise well-adjusted. Princess Alexandra Amelie of Bavaria, for instance, was known for her intelligence.

The disorder was so well-known for centuries that works from Cervantes and Descartes make reference to it. However, by the nineteenth century, it appears to have largely disappeared. Largely, of course, is the operative word. Even in contemporary times, there are occasional reports of individuals with the delusion that they are constructed of glass. Many psychoanalysts believe that the glass delusion is connected with social anxiety and diminished personal space. Perhaps this is part of the cause, but the glass delusion shows little sign of ever being fully understood.