(Ποσειδώνιος), distinguished Stoic philosopher, was a native of Apameia in Syria (Strab. xiv. p.968, xvi. p. 1093; Suidas, s. v. Ποσειδ.). He was called sometimes the Apamean, from his birthplace, sometimes the from the place where he taught (Lucian, Macrob. vol. iii. p. 223; Athen. 6.252e.) He was also known by the surname Ἀθλήτης (Suid. l.c.). The date of his birth is not known with any exactness; but he was a disciple of Panaetius and a contemporary of Pompeius and Cicero. Athenaeus (xii p. 549e.), by a great mistake, mentions Poseidonius instead of Panaetius as the companion of Scipio Africanus on his embassy to Egypt. Elsewhere (xiv. p. 657) he talks of him as a contemporary of Strabo, misunderstanding a passage of the latter (xvi. p. 1093), where the expression καθʼ ἡμᾶς, in an author who quotes from so many writers of different ages, may very well be understood of one who preceded him but a short time. Vossius supposes that the old age of Poseidonius may have coincided with the childhood of Strabo. The supposition is not necessary. As Panaetius died in B. C. 112, and Poseidonius came to Rome in the consulship of M. Marcellus (B. C. 51), and according to Lucian (l.c.) reached the age of 84 years, B. C. 135 is probably not far from the date of the birth of Poseidonius.

Poseidonius, leaving Syria, betook himself to Athens, and became the disciple of Panaetius, and never returned to his native country. (Suid. l.c. ; Cic. de Off. 3.2, Tusc. Disp. 5.37.) On the death of Panaetius he set out on his travels, and first visited Spain. At Gades he staid thirty days, observing the setting of the sun, and by his observations confuting the ignorant story of the hissing sound made by the sun as it descended into the ocean. Having collected a variety of information on points of geography and natural history, he set out for Italy. Nor was he idle on the voyage, paying attention to the course of the winds, and examining the peculiarities of the coasts along which he passed. He visited Sicily and the neighbouring islands, and then proceeded to Dalmatia and Illyricum (Strab. iii. p.165, iv. p. 197, xiii. p. 614; Vitruv. de Archit. 8.4). After visiting Massilia, Gallia Narbonensis, and Liguria, he returned to the East, and fixed his abode at Rhodes, where he became the president of the Stoic school. He also took a prominent part in the political affairs of the republic, influencing the course of legislation, and among other offices filling that of Prytanis (Strab. iv. p.655, vii. p. 316). He was sent as ambassador to Rome in B. C. 86. With Marius he became personally acquainted, and Plutarch in his life of Marius was considerably indebted to information derived from him (Plut. Mar. 45). Cicero, when he visited Rhodes, received instruction both from Molo and from Poseidonius (Cic. de Nat. Deor. 1.3, de Fin. 1.2 ; Plut. Cic. 4). Pompey also had a great admiration for Poseidonius, and visited him twice, in B. C. 67 and 62. (Strab. xi. p.492; Plut. Pomp. 42 ; Plin. Nat. 7.31.) To the occasion of his first visit probably belongs the story that Poseidonius, to prevent the disappointment of his distinguished visitor, though severely afflicted with the gout, held a long discourse on the topic that pain is not an evil (Cic. Tusc. Disp. 2.25). He seems to have availed himself of his acquaintance with Pompey to gain such additions as he could to his geographical and historical knowledge (Strab. xi. p.492). In B. C. 51 Poseidonius removed to Rome, and appears to have died soon after. He was succeeded in his school by his disciple and grandson Jason. [JASON, p. 556.] Among his disciples were Phanias (D. L. 7.41), and Asclepiodotus (Senec. Qu. Nat. 2.26, 6.17). Besides Cicero, he seems to have had among his hearers C. Velleius, C. Cotta, Q. Lucilius Balbus, and probably Brutus. (Cic. de Nat. Deor. 1.44; Plut. Brut. p. 984.) Of Pompey we have already spoken.

Poseidonius was a man of extensive and varied acquirements in almost all departments of human knowledge. Strabo (xvi. p.753) calls him ἀνὴρ τῶν καθʼ ἡμᾶς φιλοσόφων πολυμαθέστατος. Cicero thought so highly of his powers, that he requested him to write an account of his consulship (ad Att. 2.1). As a physical investigator he was greatly superior to the Stoics generally, attaching himself in this respect rather to Aristotle. His geographical and historical knowledge was very extensive. Though attached to the Stoic system, he was far less dogmatical and obstinate than the 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. This scientific cast of his mind Galen attributes to his accurate acquaintance with geometry (De Plac. Hipp. et Plat. iv. p. 279, viii. p. 319). His style of composition also seems to have been far removed from the ungraceful stiffness which was frequently affected by Stoic writers. (Strab. v. p.147; comp. Galen, l.c. iv. p. 281, v. p. 296.)

Poseidonius adhered to the division of philosophy usual among the ancients, into physics, ethics, and dialectics (D. L. 7.39), comparing the first to the blood and flesh of an animal, the second to the bones and nerves, the last to the soul. (Sextus Emp. ad v. Math. 7.19; D. L. 7.40.) He recognised two principles (ἀρχαί) - passive (matter), and active (God). His physical doctrines were, in the main, those of the Stoics generally, though he differed from them in some particulars. He held that the vacuum beyond the universe was not infinite, but only large enough to allow of the dissolution of the universe (he discarded the doctrine of its destruction by fire, Phil. Jud. de Act. Mundi, ii. p. 497, ed. Mang.). He considered the heaven as the governing principle (τὸ ἡγεμονικόν) of the universe (D. L. 7.139.) He cultivated astronomy with considerable diligence, and, unlike Panaetius, was a believer in astrology (Cic. de Div. 2.42). Poseidonius also constructed a planetary machine, or revolving sphere, to exhibit the daily motions of the sun, moon and planets. (Cic. de Nat. Deor. 2.34.) He inferred that the sun is larger than the earth, among other reasons because the shadow cast by the earth is conical. (D. L. 7.144; Macrob. ad Somn. Scip 1.20.) Its greater apparent magnitude as it sets he attributed to its being seen through dense and misty air, and supposed that if we could see it through a solid wall it would appear larger still. (Cleomedes, Cycl. Theor. ii. p. 430.) He calculated the diameter of the sun to be 4,000,000 stadia, on the assumption that the orbit of the sun was 10,000 times the circumference of the earth, and that it is within a space of 400 stadia N. and S. that the sun casts no shadow. (Cleomedes, l.c. p. 452.) The distance between the earth and the sun he set down at above 502,000,000 stadia. (Plin. Nat. 2.21.) The moon also he considered to be larger than the earth, and composed of transparent elements, though on account of its great size the rays of the sun do not pass through it in eclipses. (Stob. Eel. Phys. i. p. 59; Cleom. l.c. ii. p. 500.) His view of the milky way, that it is of an igneous nature, not so dense as stars, but more so than light, and intended to warm those parts of the universe which the sun's heat does not reach, was extensively adopted. (Macrob. l.c. 1.15.) Poseidonius's calculation of the circumference of the earth differed widely from that of Eratosthenes. He made it only 180,000 stadia, and his measurement was pretty generally adopted. His calculation was founded on observations of the star Canobus made in Spain, not, as Cleomedes says, in Rhodes. (Strab. ii. p.119; Cleom. l.c. 1.8. ; comp. Mannert, Geogr. vol. i. p. 105, &c.) The shape of the habitable part of the earth he compared to that of a sling, the greatest extent being from E. to W. (Strab. ii. p.267; Agathemerus, ap. Hudson. Geogr. Min. vol. ii. p. 2.) Of the connection between the moon and the tides he was well aware. (Strab. iii. p.173.) Strabo frequently refers to Poseidonius as one of the most distinguished geographers. A great number of passages, containing the views of Poseidonius on various other geographical and astronomical points, has been collected by Bake.

As the basis of his ethical and mental philosophy Poseidonius took the Stoic system, though with considerable modifications, for he held it possible to amalgamate with it much of the systems of Plato and Aristotle. In some respects his views approximated to the Pythagorean doctrines. (Sext. Empir. Adv. Math. 7.93; Galen. de Hipp. et Plat. Plac. v. p. 171.) It seems to have been his object as far as possible to banish contradiction from philosophy, and bring all the systems which had been propounded into harmony with each other, and to infuse into the decaying vitality of philosophical thought something ot the vigour of past times. But that he could suppose the doctrines of Zeno, Aristotle and Plato capable of reconciliation with each other, shows that he could not have seized very distinctly the spirit of each. To give anything like plausibility to this attempt, it was of course necessary to introduce considerable modifications into the Stoic doctrines. In some points however in which he differed from Panaetius he rather returned to the views of the earlier Stoic philosophers. His fourfold division of virtue is apparently that followed by Cicero in his He did not think virtue by itself sufficient for perfect happiness, unless accompanied by external, bodily good. (D. L. 7.128.) The summum bonum he considered to be the living in the contemplation of the truth and order of all things, and the fashioning oneself, as far as possible, in accordance therewith, being led aside as little as possible by the irrational part of the soul. (Clem. Alex. Strom.ii. p. 416.) In the classification of the faculties of the soul he returned to the system of Plato, dividing them into reason, emotion, and appetite (δείκνυσιν διοικουμένους ἡμᾶσὑπὸ τριῶν δυνάμεων, ἐπιθυμητικῆς τε καὶ θυμοειδοῦς καὶ λογιστικῆς, Galenus, l.c. viii. p. 319), with which division he considered questions of practical morality to be intimately connected (Galen. l.c. iv. p. 284, v. p. 291). It was apparently to keep up a bond of connection with the Stoic dogmas that he spoke of these Δυνάμεις as all belonging to one essence (Galen. l.c. vi. p. 298), though other features of his system are not easily reconcilable with that view. But instead of regarding the πάθη of the soul as being, or ensuing upon, judgments (κρίσεις) of the reason, he deduced them from the irrational faculties of the soul, appealing to the fact that emotion and appetite manifest themselves in irrational beings. He connected affections and perturbations of the mind with external influences, the union of the soul with the body, and the influence of the latter upon the former, some conditions of man being predominantly bodily, others spiritual; some passing from the body to the soul, others from the soul to the body. This idea he carried out to the permanent modifications of character produced by particular bodily organisations, founding thereon a sort of physiognomical system. (Galen. l.c. v. p. 290.) He sometimes spoke of appetite as corresponding to vegetable life, emotion to animal life, reason to the properly human (l.c.. p. 170).