And finally on the matter of his insanity: Ancient scholars discussed the phenomena and causes of psychopathology from the fifth century B.C. on. During the reign of the emperor Tiberius, Caligula’s predecessor, the Roman author Cornelius Celsus wrote on this subject in his books on medicine (De Medicina). Celsus characterizes insanity (insania) as a disease that manifests itself in senseless behavior or incomprehensible speech. He describes two categories of patients: those who have delusions but whose intellectual reasoning is otherwise unimpaired, and those whose reason is itself disturbed. Later medical authors who employ the same distinction give as an example of the former a man named Theophilus, who — although he could speak and reason properly otherwise — believed that he was constantly surrounded by people playing flutes, making noises, and observing him. He often upset the household by shouting orders that these interlopers should be thrown out. As an example of the latter ailment the authors describe a patient who suffered from the delusion that he had no head. He believed that he was a beheaded tyrant.

The subject was covered in Roman law as well. A series of texts on homicide, treason (maiestas), libel, and property damage declare that “the insane” (furiosi, insani) are not legally responsible for their actions. “What delinquency… can there be in a person who is not in his right mind?” asks the legal scholar Pegasus (Dig. 9.2.5.2). It is even noted specifically that in the case of crimes committed by someone of unsound mind, it is not the perpetrator himself who deserves punishment, but rather those who failed to keep watch over him.