One day in the late 1840s, Princess Alexandra Amalie, the 23-year-old daughter of the recently abdicated King Ludwig I of Bavaria, was making her way through the corridors of the family palace. Her relatives noticed that the obsessive, highly intelligent young woman—who only wore the color white—was acting even stranger than usual. Alexandra Amelie was walking sideways through doorways and labyrinthine hallways, tiptoeing and carefully turning her body so that nothing would touch her.

When asked by her family what she was doing, the Princess explained that she had just discovered something remarkable. As a child, she had swallowed a full-sized grand piano made entirely of glass. It now resided inside her—wholly intact—and would shatter if faced with any sudden movement.

Surprisingly, Alexandra Amelie’s odd fixation was not an unheard-of disorder. The princess was, in fact, following in a long tradition of royals, nobles and scholars who believed that all or certain parts of their bodies were made of clear, fragile glass. Known as “the glass delusion,” this psychological malady, first recorded in the Middle Ages, would become quite common before virtually dying out in the late 19th century. It was so well known that it would be mentioned by Rene Descartes, Denis Diderot and in scholar Robert Burton’s 1621 medical compendium, Anatomy of Melancholy.

One of the first recorded patients to suffer from this delusion was probably its most famous victim. King Charles VI (1368–1422) had ascended the throne of France at the age of 11. Handsome, judicial and charismatic, he had spearheaded reform efforts after taking over from his corrupt regents in 1388—streamlining the royal bureaucracy and surrounding himself with enlightened advisors. These actions led him to be nicknamed Charles “the beloved.” But in 1392, he suffered a psychotic break (believed to be his first manifestation of schizophrenia), which would lead to sporadic violent episodes and periods of inertia and confusion for the rest of his life.

Charles “the beloved,” was now known as Charles “the mad.” Allegedly, the king had spells where he believed his body was made entirely of glass. To keep himself from “shattering,” Charles would stay motionless for hours, wrapped in piles of thick blankets. When he did have to move, he did so in a special garment, which included iron “ribs” to protect his glass organs.

Over the next few centuries, the delusion spread to the courts, monasteries and universities of Europe. According to researcher Gill Speak, who wrote the definitive paper on the glass delusion in 1990, two notable 16th-century doctors—Alfonso Ponce de Santa Cruz, the physician to Philip II of Spain, and Andre du Laurens, physician to Henry IV of France—told the story of an unnamed royal who believed he was not a human, but a glass vase. According to du Laurens, the nobleman was otherwise highly intelligent and well spoken.

The royal spent much of his time lying on a bed of straw to protect himself. Fed up, the man’s physician ordered that his bed of straw be set on fire and that the door to the man’s room be locked. When the man began to beat on the door begging for help, the doctor asked him why he wasn’t shattering despite the violent movements. The ploy worked. “Open, I am begging you, my friends and dearest servants,” the royal cried, according to PhD candidate Elena Fabietti, whose work focuses on the cultural history of transparent humans. “I don’t think I am a glass vase but just the most miserable of all men; especially if you will let this fire put an end to my life.”

There are recorded references throughout the Middle Ages and into the 17th century of people who believed they possessed glass hearts, feet and heads. Others thought they were actually glass flasks. Men seem to have had a certain predilection for glass buttocks, which would shatter if they sat down without a pillow strapped to their behinds. Nicole du Plessis, a relation of France’s all-powerful Cardinal Richelieu, suffered from this particular delusion. Another man believing he possessed a glass rear end was beaten by his doctor, in the hopes he would realize it was his flesh that was sore from the thrashing.

Many who suffered from a glass delusion, including Princess Alexandra Amelie and King Charles VI, were considered exceptional people of great intelligence and ingenuity. Depictions of unusually clever victims of the disorder popped up in popular plays and literature over the centuries, most notably in Miguel de Cervantes’ short story El licenciado Vidriera  (known variously in English as The Glass Graduate, Doctor Glass-Case and The Glass Lawyer), published in 1613. In it, a brilliant young lawyer called Tomas Rodaja is the victim of a love potion that causes him to believe he is made of glass. He renames himself Vidriera (window) and gives honest counsel to many, unencumbered by the bonds of flesh:

“He asked people to address him from a distance, and he said that they might ask what questions they liked, because he was a man of glass, not flesh, and since glass is of subtle and delicate matter, the soul works through it with more speed and efficiency than through the material of the normal body, which is heavy and earthy.”

So what exactly was the cause of this peculiar manifestation of mental illness? Scholars at the time, including Burton, attributed it to the now discredited diagnosis of melancholy—a kind of noble depression, often linked to aristocracy and genius. In the case of royals, contemporary psychologists speculate that believing one was glass could have been a way of expressing how vulnerable, fragile and exposed they felt in their public positions. It was a way of expressing humanity, sensitivity and perhaps a desire to be left alone.

“He shouted in the most terrible way,” Cervantes wrote in El licenciado Vidriera, “begging and pleading with predetermined words and expressions that no one come near because they would break him, that he really and truly was not like other men, that he was all glass from head to toe.”

Interestingly, at the time, glass—particularly clear glass—was a precious, novel commodity, mostly found in royal palaces, churches and government buildings. According to Professor Edward Shorter, a historian of psychiatry at the University of Toronto, fixations with innovative materials have been reported throughout history. Before the glass delusion, there were people who believed their bodies were composed of earthenware, and during the 19th century, people started to believe they were made of the dominant construction material of the day: concrete. Our modern-day delusions tend to involve technology: sufferers may believe the government has planted a microchip in their brain or that a computer is constantly monitoring them.

What people with these delusions have in common is that they all feel fragile. Indeed, when the author Giovanni Boccaccio was despairingly called a “man of glass” in 1393, he responded with a retort that could be understood by every human—from a high-born princess to a lowly pauper, notes Elena Fabietti in A Body of Glass, The case of El lienciaco Vidriera.

“We are all glass men, subjected to innumerable dangers,” he wrote in a written response to his critic. “The slightest touch would break us, and we would return to nothing.”