In 1717, during imprisonment in the Bastille, Voltaire learned about a man called the "Man in the Iron Mask" (… – 1703), named so for he always wore a mask secured with metal straps. The mystery surrounding the prisoner’s identity prompted Voltaire, his contemporary Dumas, and many others over time to try to uncover the truth. To this day, the mystery remains unsolved. 

From prison archives and accounts of jailers, Voltaire discovered that this prisoner received special treatment compared to others: fine clothing, access to books, excellent food, the ability to play the lute, and permission to walk in the prison’s open spaces — provided he was alone. However, he was forbidden to speak to anyone except his confessor and the chief jailer (and only about matters related to his detention). The prisoner was required to wear the iron mask at all times, except when eating or sleeping. 

Piecing together these clues, Voltaire—and later Dumas—concluded that the prisoner must have been someone whose face was highly recognizable, someone whose identity could spark unrest. Moreover, the strict prohibition on communication suggested that the prisoner possessed dangerous knowledge. His existence was considered too significant to end with a simple execution like that of an ordinary inmate. 

Since the prisoner died suddenly in 1703 and was buried secretly under a false name, neither Voltaire nor Dumas could locate the body. They instead speculated on his identity, proposing several theories. The most popular theory posited that the Man in the Iron Mask was either the twin brother or half-brother of King Louis XIV, imprisoned to prevent a challenge to the throne. This would explain the mask and the communication ban. Another theory suggested he was the illegitimate firstborn son of the king, incarcerated to protect the position of the Dauphin Louis (who died young). 

The most intriguing hypothesis proposed that the Man in the Iron Mask was the father of Louis XIV. Since the Sun King was born many years after the marriage of Louis XIII, rumors swirled that the king was either sterile or impotent, necessitating the use of another man to produce an heir. The mystery remains unsolved.