Diocletianus moratus callide fuit, sagax praeterea et admodum subtilis ingenii, et qui severitatem suam aliena invidia vellet explere. diligentissimus tamen et sollertissimus princeps et qui imperio Romano primus regiae consuetudinis formam magis quam Romanae libertatis invexerit adorarique se iussit, cum ante eum cuncti salutarentur. ornamenta gemmarum vestibus calciamentisque indidit. nam prius imperii insigne in chlamyde purpurea tantum erat, reliqua communia. Herculius autem propalam ferus et incivilis ingenii, asperitatem suam etiam vultus horrore significans. hic naturae suae indulgens Diocletiano in omnibus est saevioribus consiliis obsecutus. cum tamen ingravescente aevo parum se idoneum Diocletianus moderando imperio esse sentiret, auctor Herculio fuit, ut in vitam privatam concederent et stationem tuendae rei publicae viridioribus iunioribusque mandarent. cui aegre collega obtemperavit. tamen uterque uno die privato habitu imperii insigne mutavit, Nicomediae Diocletianus, Herculius Mediolani, post triumphum inclitum, quem Romae ex numerosis gentibus egerant, pompa ferculorum inlustri, qua Narsei coniuges sororesque et liberi ante currum ducti sunt. Concesserunt tamen Salonas unus, alter in Lucaniam. Diocletianus privatus in villa, quae haud procul a Salonis est, praeclaro otio consenuit, inusitata virtute usus, ut solus omnium post conditum Romanum imperium ex tanto fastigio sponte ad privatae vitae statum civilitatemque remearet. contigit igitur ei, quod nulli post natos homines, ut cum privatus obisset, inter Divos tamen referretur.
(Eutropius, Brev. 9.26-28)
Diocletian was of a crafty disposition, with much sagacity, and keen penetration. He was willing to gratify his own disposition to cruelty in such a way as to throw the odium upon others; he was however a very active and able prince. He was the first that introduced into the Roman empire a ceremony suited rather to royal usages than to Roman liberty, giving orders that he should be adored, whereas all emperors before him were only saluted. He put ornaments of precious stones on his dress and shoes, when the imperial distinction had previously been only in the purple robe, the rest of the habit being the same as that of other men. But Herculius was undisguisedly cruel, and of a violent temper, and showed his severity of disposition in the sternness of his looks. Gratifying his own inclination, he joined with Diocletian in even the most cruel of his proceedings. But when Diocletian, as age bore heavily upon him, felt himself unable to sustain the government of the empire, he suggested to Herculius that they should both retire into private life, and commit the duty of upholding the state to more vigorous and youthful hands. With this suggestion his colleague reluctantly complied. Both of them, in the same day, exchanged the robe of empire for an ordinary dress, Diocletian at Nicomedia, Herculius at Milan, soon after a magnificent triumph which they celebrated at Rome over several nations, with a noble succession of pictures, and in which the wives, sisters, and children of Narseus were led before their chariots. The one then retired to Salonae, and the other into Lucania. Diocletian lived to an old age in a private station, at a villa which is not far from Salonae, in honourable retirement, exercising extraordinary philosophy, inasmuch as he alone of all men, since the foundation of the Roman empire, voluntarily returned from so high a dignity to the condition of private life, and to an equality with the other citizens. That happened to him, therefore, which had happened to no one since men were created, that, though he died in a private condition, he was enrolled among the gods. (tr. John Selby Watson)