Let us hear Ammianus himself.
Haec, ut miles quondam at Graecus, a principatu Caesaris Nervae exorsus, adusque Valentis interitum, pro virium explicavi mensura: opus veritatem professum nunquam, ut arbitror, sciens, silentio ausus corrumpere vel mendacio. Scribant reliqua potiores aetate, doctrinisque florentes. Quos id, si libuerit, aggressuros, procudere linguas ad majores moneo stilos. Ammian. xxxi. 16.
The first thirteen books, a superficial epitome of two hundred and fifty-seven years, are now lost; the last eighteen, which contain no more than twenty-five years, still preserve the copious and authentic history of his own times.