All surviving sources, except Pliny the Elder, characterize Caligula as insane. However it is not known whether they are speaking figuratively or literally. Additionally, given Caligula's unpopularity among the surviving sources, it is difficult to separate fact from fiction. Recent sources are divided in attempting to ascribe a medical reason for Caligula's behavior, citing as possibilities encephalitis, epilepsy or meningitis. The question of whether or not Caligula was insane remains unanswered.
Philo of Alexandria, Josephus and Seneca also state Caligula was insane, but describe this madness as a personality trait that came through experience.[42][129][130] Seneca states that Caligula became arrogant, angry and insulting once becoming emperor and uses his personality flaws as examples his readers can learn from.[131] According to Josephus, power made Caligula incredibly conceited and led him to think he was a god.[42] Philo of Alexandria reports that Caligula became ruthless after nearly dying of his illness in 39.[132] Juvenal reports he was given a magic potion that drove him insane.
Suetonius said that Caligula suffered from "falling sickness," or Epilepsy, when he was young.[133] Modern historians have theorized that Caligula lived with a daily fear of seizures.[134] Despite swimming being a part of imperial education, Caligula could not swim.[135] Epileptics are encouraged not to swim because light reflecting off water can induce seizures.[136] Additionally, Caligula reportedly talked to the full moon.[69] Epilepsy was also long associated with the moon.[137]
Some modern historians think that Caligula suffered from hyperthyroidism.[138] This diagnosis is mainly attributed to Caligula's irritability and his "stare" as described by Pliny the Elder.