Posidonius (Ποσειδώνιος), a distinguished Greek Stoic philosopher, was a native of Apameia in Syria, but a citizen of Rhodes, where he resided the greater part of his life (Strabo, 14:655; Athen. 6:252 e). The dates of his birth and death are unknown; but he must have been born during the latter half of the 2nd century before the Christian era, as he was a disciple of Panuetius, who probably died about B.C. 100, and whom he succeeded as the head of the Stoic school. He removed to Rome in the consulship of Marcus Marcellus (Suidas, Posidon.), B.C. 51, and probably died soon after. He lived, according to Lucian (Macrob. c. 20), to the age of eighty-four, and was one of the most celebrated philosophers of his day. Cicero, who had received instruction from him (Cicero, De Plato, c. 3; De Nat. Deor. 1 3; De Fin. 1, 2), frequently speaks of him in the highest terms. Pompey also appears to have had a very high opinion of him, as we read of his visiting him at Rhodes shortly before the war against the pirates, B.C. 67 (Strabo, 11:492) and again in B.C. 62, after the termination of the Mithridatic war (Plutarch, Pomp. c. 42; Pliny, Hist. Nam. 7, 30). He must have been a man of very extensive and varied information in almost all the departments of human knowledge. Strabo calls him, ἀνὴρ τῶν καθ᾿ ἡμᾶς φιλοσόφων πολυμαθέστατος. Besides his philosophical treatises, he wrote works on geography, history, and astronomy; but none of them have come down to us, with the exception of their titles, and a few sentences quoted by Cicero, Diogenes Laertius, Strabo, and others. He seems to have traveled in different parts of the world for the purpose of collecting information. We learn incidentally from Strabo (13, 614; 3, 165; 4:197) that he had been in Spain, Liguria, and Gaul. Plutarch was also indebted to Posidonius, among others, for the materials of several of his lives. This is the case in the Lives of Marcellus, Paulus AEmilius, the Gracchi, and others; but particularly in the Life of Marius, with whom Posidonius had been personally acquainted (Plut. marius, c. 45). Posidonius wrote Meteorololoica. Cicero mentions (Natt. Deor. 2, 34) his artificial sphere, which represented the motions of the heavens. Posidonius was a much stricter Stoic than his master Pantius. He maintained that pain was not an evil, as we learn from an anecdote which Pompey frequently related respecting his visit to the philosopher at Rhodes (Cicero, Tusc. Disp. 2, 25). As a physical investigator he was greatly superior to the Stoics generally, attaching himself in this respect rather to Aristotle. Indeed, although attached to the Stoic system, he was far less dogmatical and obstinate than tile majority of that school, refusing to admit a dogma because it was one of the school if it did not commend itself to him for its intrinsic merits. His works on divination and the nature of the gods are referred to by Cicero, who probably made use of them in his works on the same subject (Cicero, De Div. 1, 3, 30, 64; De Nat. Deor. 1, 44). Strabo says (11, 492) that Posidonius wrote an account of the wars of Pompey, but did not pay much attention to accuracy. This account was, however, probably contained in his historical work, of which Athenmeus quotes (4, 168 d) the 49th book (comp. Athen. 4, 151 e). For further information respecting the opinions and writings of Posidonius, see Posidonii Reliquice Doctrinae; Collegit atque illustr-avit Janus Bake; Accedit D. Wyttenbachii Annotatio (Lugdugni Bat. 1810, 8vo). See also Smith, Dict. of Gr. and Ron. Biog. and Mythol. s.v.; Fabricius, Bibl. Graec. 3, 572; Vossius, De Hist. Graec. p. 193; Ritter, Gesch. der Philos. vol. 3, bk. 11, c. 6, p. 700; Ueberweg, Hist. of Philos. vol. 1.
There was another Posidonius of Alexandria, who was a pupil of Zeno, and consequently was prior to Polybius. Suidas, however, by mistake, ascribes to this Posidonius a continuation of Polybius in fifty-two books, which is evidently the work of the younger Posidonius.