De officio praefecti praetorio liber singularis
Aurelius Arcadius Charisius, Master of Requests, On the Duties of Prætorian Prefect. It is necessary to state briefly whence the origin of the office of Prætorian Prefect was derived. It has been asserted by some writers that Prætorian Prefects were formerly created instead of Masters of Cavalry; for, as in the time of the ancients the supreme power was occasionally conferred upon dictators, they were accustomed to choose their Masters of Cavalry, who were associated with them in the discharge of their military duties, and held the next rank after them. The government of the republic having been permanently transferred to the Emperors, Prætorian Prefects were chosen by those princes, just as had been done in the case of the Masters of Cavalry, and upon them was conferred greater power for the purpose of promoting public discipline. 1The authority of the Prefects having originated in this manner, it was subsequently increased to such an extent that no appeal can be taken from the decision of a Prætorian Prefect; for when formerly a question arose as to whether an appeal could be taken from the decision of a Prætorian Prefect, which, in fact, was allowed by law, and examples of those who did so are extant; afterwards, by an Imperial Decree publicly promulgated, the right of appeal was forbidden. For the Emperor thought that those who were appointed to this high office on account of their eminent industry, after their discernment and integrity had been established, would render judgment not otherwise than he himself would do, the wisdom and enlightenment attaching to their rank being taken into consideration. 2Prætorian Prefects also enjoyed an additional privilege; for minors could not obtain restitution after condemnation, from any other magistrates than from the Prætorian Prefects themselves.