What was the glass delusion?

One such story is recorded in the casebooks of the French royal physician André du Laurens (1558–1609), of “a great Lord, which thought himself to be glass”. In every other respect the lord was quite rational and enjoyed the company of his friends, “but so as that he would desire them, that they would not come near unto him”. Other patients believed that they had a glass bottom and could not sit down; one became convinced that his chest was transparent and others could see straight into him. Perhaps the most famous sufferer was Charles VI of France (1368–1422). The king refused to let anyone come near him or touch him, and wore specially reinforced clothing so that his fragile body would not crack.

We might link the glass delusion to fear of assassination or contagious disease. For medieval and Renaissance physicians, the glass delusion exposed the profound correspondence between our inner dispositions and the world around us. It was seen as a symptom of melancholy, the disease of body and mind arising from an excess of the cold and dry humour, black bile. Du Laurens thought that the dry qualities of melancholy imprinted themselves on the sufferer’s imagination: looking at a brittle, fragile glass could lead to someone not just identifying with it, but believing that they glass themselves.

Answered by Mary Ann Lund, author of A User's Guide to Melancholy (Cambridge University Press, 2021)