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Contemporary historiography

Most facts and circumstances of Caligula's reign are lost to history. The two most important literary sources on Caligula and his reign are Suetonius, a government official of equestrian rank, born around 70 AD; and Cassius Dio, a Bithynian senator who held consulships in AD 205 and 229. Suetonius tends to arrange his material thematically, with little or no chronological framework, more biographer than historian. Dio provides a somewhat inconsistent chronology of Caligula's reign. He dedicates 13–21 chapters to positive features of Caligula's reign but nearly 40 to Caligula as "monster".

Philo 's works On the Embassy to Gaius and Flaccus give some details on Caligula's early reign, but more on events involving Jews in Judea and Egypt, whose political and religious interests conflicted with those of the ethnically Greek, pro-Roman population. Philo saw Caligula as responsible for the suffering of the Jews, whom he invariably portrays in a morally positive light. Seneca's various works give mostly scattered anecdotes on Caligula's personality, probably written in the reign of Claudius, who had a vested interest in the portrayal of his predecessor as "cruel and despotic, even mad". Seneca was prone to "grovelling flattery" of whoever reigned at the time. His experience under Caligula "could have clouded his judgment". He narrowly avoided a death sentence in AD 39, probably imposed for his association with known conspirators. Caligula had a low opinion of his literary style.

Further contemporaneous histories of Caligula's reign are attested by Tacitus, who describes them as biased for or against Caligula; of Tacitus' own work, little of relevance to Caligula survives but Tacitus' works testify to his general hostility to the imperial system. Among the known losses of his works is a substantial portion of the Annals. Fabius Rusticus and Cluvius Rufus wrote histories, now lost, condemning Caligula. Tacitus describes Fabius Rusticus as a friend of Seneca, prone to embellishments and misrepresentations. Cluvius Rufus was a senator involved in Caligula's assassination; his original works are lost, but he was a competent historian, used as a primary source by Josephus, Tacitus, Suetonius and Plutarch.

Caligula's sister, Agrippina the Younger, wrote an autobiography that included a detailed account of Caligula's reign, but it too is lost. Agrippina was banished by Caligula for her connection to Marcus Lepidus, who conspired against him. Caligula also seized the inheritance of Agrippina's son, the future emperor Nero. Gaetulicus flattered Caligula in writings now lost. Suetonius wrote his biography of Caligula 80 years after his assassination, and Cassius Dio over 180 years after; the latter offers a loose chronology. Josephus gives a detailed account of Caligula's assassination and its aftermath, published around 93 AD, but it is thought to draw upon a "richly embroidered and historically imaginative" anonymous biography of Herod Agrippa, presented as a Jewish "national hero". Pliny the Elder 's Natural History has a few brief references to Caligula, possibly based these on the accounts by his friend Suetonius, or an unnamed, shared source. Of the few surviving sources on Caligula, none paints Caligula in a favourable light. Little has survived on the first two years of his reign, and only limited details on later significant events, such as the annexation of Mauretania, Caligula's military actions in Britannia, and the basis of his feud with the Senate.