He is known principally as a character who appears in Lucian's The Death of Peregrinus (Latin: De Morte Peregrini), where he is introduced as praising Peregrinus' desire to kill himself by self-immolation: Proteus," he cried, "Proteus vain-glorious?
that he was cast out of Rome, — he whose brilliance exceeds the Sun, fit rival of the Lord of Olympus?
[2] Theagenes compared Peregrinus' suicide with the similar practices of the Gymnosophists in India.
[4] Theagenes is also mentioned by Galen, who recalls how he lectured daily to large crowds in Trajan's Forum in Rome.
[5] Galen describes how Theagenes died when he was given an inappropriate treatment by Statilius Attalus (physician to Marcus Aurelius) for inflammation of the liver.