The glass delusion is an external manifestation of a psychiatric disorder recorded in Europe mainly in the late Middle Ages and early modern period (15th to 17th centuries). People feared that they were made of glass "and therefore likely to shatter into pieces". One famous early sufferer was King Charles VI of France who refused to allow people to touch him, and wore reinforced clothing to protect himself from accidental "shattering".
Concentration of the glass delusion among the wealthy and educated classes allowed modern scholars to associate it with a wider and better described disorder of scholar's melancholy.
Robert Burton's "The Anatomy of Melancholy" (1621) touches on the subject in the commentary as one of many related manifestations of the same anxiety: "Fear of devils, death, that they shall be so sick, of some such or such disease, ready to tremble at every object, they shall die themselves forthwith, or that some of their dear friends or near allies are certainly dead; imminent danger, loss, disgrace still torment others; that they are all glass, and therefore will suffer no man to come near them; that they are all cork, as light as feathers; others as heavy as lead; some are afraid their heads will fall off their shoulders, that they have frogs in their bellies, Etc."
Miguel de Cervantes based one of his short "Exemplary Novels", "The Glass Graduate" (, 1613), on the delusion of the title subject, an aspiring young lawyer. Thomas Rodaja fell into a grave depression after being bedridden for six months after being poisoned with a purportedly aphrodisiac potion.
He claimed that, being of glass, his perceptions are clearer than those of men of flesh and demonstrated by offering witty comments.
After two years of illness, Rodaja was cured by a monk; no details of the cure are provided except that the monk was allegedly a miracle-maker.
Dutch poet Constantijn Huygens wrote a "Costly Folly" (1622) centered on a subject who "fears everything that moves in his vicinity... the chair will be the death for him, he trembles at the bed, fearful that one will break his bum, the other smash his head".
French philosopher René Descartes wrote "Meditations on First Philosophy" (1641), using the glass delusion as an example of an insane person whose perceived knowledge of the world differs from the majority.
In modern times, the glass delusion has not completely disappeared. There are still isolated cases today. "Surveys of modern psychiatric institutions have only revealed two specific (uncorroborated) cases of the glass delusion. Foulché-Delbosc reports finding one Glass Man in a Paris asylum, and a woman who thought she was a potsherd was recorded at an asylum in Merenberg." Andy Lameijn, a psychiatrist from the Netherlands, reports that he has a male patient suffering from the delusion in Leiden.
The neurotic behavior of the 19th-century Russian composer Peter Ilyich Tchaikovsky seems reminiscent of the Glass Delusion, centering as it did around his difficulties caused by his belief that his head would fall off while conducting if he didn't hold his chin. While the legend may be exaggerated, it seems to have some basis in fact:
"In March 1868 in his first attempt [at conducting], Tchaikovsky conducted dances from "The Voevoda", 'and had felt that his head would fall sideways unless he fought to keep it upright'...(per David Brown, 'The Final Years' page 97).. and so he avoided conducting...In October 1886 Tchaikovsky pointed out to his patroness 'all my life I have been tormented by awareness of my inability to conduct. It has seemed to me there is something shameful and disreputable in not being able to stop myself trembling with fear and horror at the very thought of going out in front of the public with a baton'...However on Jan. 31, 1887 Tchaikovsky in his third attempt overcame his fear and conducted the premier of "The Enchantress"...as a further inducement 'he was not unaware that a conductor could enjoy more celebrity in his own time than a composer.'"