Trajan
Sources

As an emperor, Trajan's reputation has endured – he is one of the few rulers whose reputation has survived 19 centuries. Every new emperor after him was honoured by the Senate with the wish felicior Augusto, melior Traiano (that he be "luckier than Augustus and better than Trajan"). Among medieval Christian theologians, Trajan was considered a virtuous pagan. In the Renaissance, Machiavelli, speaking on the advantages of adoptive succession over heredity, mentioned the five successive good emperors "from Nerva to Marcus " – a trope out of which the 18th-century historian Edward Gibbon popularized the notion of the Five Good Emperors, of whom Trajan was the second.

An account of the Dacian Wars, the Commentarii de bellis Dacicis, written by Trajan himself or a ghostwriter and modelled after Caesar 's Commentarii de Bello Gallico, is lost with the exception of one sentence. Only fragments remain of the Getica, a book by Trajan's personal physician Titus Statilius Criton. The Parthica, a 17-volume account of the Parthian Wars written by Arrian, has met a similar fate. Book 68 in Greek author Cassius Dio 's Roman History, which survives mostly as Byzantine abridgements and epitomes, is the main source for the political history of Trajan's rule. Besides this, Pliny the Younger 's Panegyricus and Dio Chrysostom 's orations are the best surviving contemporary sources. Both are adulatory perorations, typical of the High Imperial period, that describe an idealized monarch and an equally idealized view of Trajan's rule, and concern themselves more with ideology than with fact.

The 10th volume of Pliny's letters contains his correspondence with Trajan, which deals with various aspects of imperial Roman government. It is generally agreed that Pliny, being part of the emperor's inner circle, provides a unique and valuable source of information through his letters with Trajan, the only surviving correspondence between a governor and his emperor. However, it has been argued that Pliny's correspondence with Trajan is neither intimate nor candid, but rather an exchange of official mail in which Pliny's stance borders on the servile. Some authors have even proposed that much of the text was written and/or edited by Trajan's Imperial secretary, his ab epistulis. Given the scarcity of literary sources, discussion of Trajan and his rule in modern historiography cannot avoid speculation. Non-literary sources such as archaeology, epigraphy, and numismatics are also useful for reconstructing his reign.
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