Ammianus Marcellinus Res Gestae 29.2.18

[18] O noble system of wisdom, by heaven’s gift bestowed upon the fortunate, thou who hast often ennobled even sinful natures! How much wouldst thou have corrected in those dark days, if it had been permitted Valens to learn through you that royal power—as the philosophers declare—is nothing else than the care for others’ welfare; ¹ that it is the duty of a good ruler to restrain his power, to resist unbounded desire and implacable anger, and to know—as the dictator Caesar used to say—that the recollection of cruelty is a wretched support ² for old age. And therefore, if he is going to pass judgment affecting the life and breath of a human being, who forms a part of the world and completes the number of living things, he ought to hesitate long and greatly and not be carried away by headlong passion to a point where what is done cannot be undone; ³ of which we have a very well-known instance in olden times.