Demetrius in Harpers Dictionary of Classical Antiquities

Of Sunium; a Cynic philosopher, who flourished at Corinth in the first century. During the reign of Caligula he taught philosophy at Rome, where he obtained the highest reputation for wisdom and virtue. He was banished from Rome in the time of Nero for his free censure of public manners. After the death of this emperor he returned to Rome, but the boldness of his language soon offended Vespasian and again subjected him to the punishment of exile. Apollonius, with whom he had formed a friendship, prevailed on Titus to recall him; but under Domitian he withdrew to Puteoli. Seneca, who was acquainted with him, speaks in the highest terms of his masculine eloquence, sound judgment, intrepid fortitude, and inflexible integrity (De Vit. Beat. 25).