Today's pop culture is full of heroic and otherwise heroic characters with particularly resistant bodies composed of natural elements: Superman, the "Man of Steel," Johnny the "Human Torch," or Mr. Fantastic made of rubber; yet even medieval France could boast a king with similar physical strength... or at least that's what Charles VI of Valois (1368-1422) believed of himself.

Known to history as "the Mad King," Charles VI became king amid great hopes and expectations, later descending into an increasingly dark madness that would lead France to disaster in the Hundred Years' War. Among the numerous delusions associated with the French king, the most famous was certainly the belief that his body was made entirely of crystal and glass and that the slightest impact or too forceful contact would shatter it into a thousand pieces and cause him to die. This obsession was quite widespread in medieval Europe, where many people convinced themselves that their bodies were actually made of extremely fragile materials (glass represented both purity and vulnerability). This delusion led King Charles VI to isolate himself from his courtiers, forbid people from touching him, and wear padded clothing for protection. This delusion consequently worsened his already fragile mental instability (he is now thought to have suffered from bipolar disorder and schizophrenia), causing turmoil in his kingdom, as a king largely unfit to rule (power was divided among his relatives, such as the Burgundian family). In the last years of his life, Charles VI lived almost completely isolated from reality. His bouts of madness became increasingly frequent and prolonged.

The story of Charles VI of Valois represents one of the most fascinating examples of the intertwining of history, politics, and mental illness (in medieval times, mental illness was treated as one with physical pain and seen as a divine punishment or an abomination to be banished). His famous delusion that he was made of crystal became a symbol of human frailty and the psychological burden of power, which did not always find men capable of withstanding it, but who were crushed under the very crown they were called to bear. Behind the figure of the "mad king" lay a tormented man, living in a violent and unstable era, incapable of distinguishing reality from his own fears. His illness not only changed his life but also profoundly influenced the fate of medieval France (later revived by Joan of Arc).