Cassius Longinus (c. AD 213–273) was a Greek rhetorician and philosophical critic. The origin of his gentile name Cassius is unknown; it can only be conjectured that he adopted it from a Roman patron. He was perhaps a native of Ernesa (Horns) in Syria, the birthplace of his uncle Fronto the rhetorician. He studied at Alexandria under Origen the heathen, and taught for thirty years in Athens, one of his pupils being the Neoplatonist Porphyry.

Longinus did not embrace the new speculations then being developed by Plotinus, but continued as a Platonist of the old type. In opposition to Plotinus, he upheld the doctrine that the Platonic ideas existed outside the divine Nous. Plotinus, after reading his treatise On First Principles, remarked that Longinus might be a scholar, but that he was no philosopher.

The reputation which Longinus acquired by his learning was immense; he is described by Porphyry as the first of critics, and by Eunapius as a living library and a walking museum or encyclopaedia. During a visit to the east, he became teacher of Greek, and subsequently chief counsellor in state affairs to Zenobia, queen of Palmyra. It was by his advice that she endeavoured to regain her independence; Aurelian, however, crushed the attempt, and while Zenobia was led captive to Rome to grace Aurelian's triumph, Longinus paid with the forfeit of his life.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.