The "Res Gestae" ("Rerum gestarum Libri XXXI") was originally composed of thirty-one books, but the first thirteen have been lost. The surviving eighteen books cover the period from 353 to 378. It constitutes the foundation of modern understanding of the history of the fourth century Roman Empire. It is lauded as a clear, comprehensive, and generally impartial account of events by a contemporary; like many ancient historians, however, Ammianus was in fact not impartial, although he expresses an intention to be so, and had strong moral and religious prejudices. Although criticised as lacking literary merit by his early biographers, he was in fact quite skilled in rhetoric, which significantly has brought the veracity of some of the "Res Gestae" into question.