Posidonius 135 BC – 51 BC

Posidonius of Apamea was a Greek Stoic philosopher, polymath, and one of the most learned men of antiquity. Settling in Rhodes, where he taught the young Cicero, he produced encyclopedic works on natural philosophy, ethics, history, geography, and astronomy, almost all of which are lost. He revised Stoic moral psychology to allow a richer role for the irrational passions and developed a sympathetic cosmology in which all parts of the world are joined in a single living organism. His reach is preserved indirectly through Cicero, Strabo, Galen, and Seneca, on whom his influence was profound.

Posidonius was born around 135 BC at Apamea on the Orontes in Syria, in the Hellenistic kingdom of the Seleucids. He studied at Athens under Panaetius, the leading Stoic of the preceding generation, and around 95 BC settled at Rhodes, where he opened his own school and was elected to the city's chief magistracy. From Rhodes he undertook a long scientific and ethnographic journey through the western Mediterranean, including Spain, Gaul, and the Italian coast.

Posidonius wrote on physics, metaphysics, ethics, psychology, theology, history, geography, oceanography, astronomy, mathematics, and rhetoric. None of his works survives, but they are quoted, summarized, or argued against by Cicero, Strabo, Galen, Sextus Empiricus, Seneca, and Diogenes Laertius. His Histories continued Polybius from 145 BC down to his own day, and his geographical and oceanographic studies — including the first careful estimate of the size of the earth after Eratosthenes and his theory of the tides — were standard references for centuries.

He was the most learned of the middle Stoics, partial to a more Platonizing psychology of three soul-parts, and the principal philosophical influence on Cicero, who came to Rhodes around 78 BC to study with him. He visited Rome on embassy in 87 BC, where Pompey paid him visits in his old age. He died at Rhodes around 51 BC.