a distinguished Stoic philosopher, was a native of Apameia in Syria, and was the pupil of Panatus and contemporary of Pompey and Cicero. He was probably born about 135 B.C.; but the date of his birth is not known with exactness. On the death of Panatus at Athens, Posidonius set out on his travels, and first visited Spain. Having collected a variety of information on points of geography and natural history, he next visited Italy, Sicily, Dalmaia, Illyricum, Massilia, Gallia Narbonensis, and Liguria, and fixed his abode at Rhodes. Here he became president of the Stoic school, and took a prominent part in the political affairs of the republic. He was sent on an embassy to Rome, B.C. 86, when he became personally acquainted with Marius. Cicero and Pompey both visited him at Rhodes; and he is reported to have gained much geographical and historical knowledge from the latter. In B.C. 51 Posidonius removed to Rome, where he soon after died. As a physical investigator he was greatly superior to the generality of the Stoics, attaching himself in this respect more to Aristotle. He was a man of very extensive information on almost all departments of human knowledge, and particularly of astronomy and geography. He calculated the diameter of the earth, the distance of the sun and its magnitude, and the influence of the moon on the tides.

Of the writings of Posidonius there were in all about twenty-five, none of which has come down to us entire. The relics of his writings have been carefully collected and illustrated by Janus Bake, in a work called Posidoni Rodii Reliquiae Doctrinae, Lugd. Batav. 1810. (Fabric. Bibl. Graec., vol. iii.; Ritter, Gesch. der Philosophie, vol. iii.; Smith's Dictionary of Greek and Roman Biog. and Myth.)