Posidonius (pohs-ih-DOH-nee-uhs), a Stoic philosopher, studied under Panaetius of Rhodes before the latter’s death in 104 b.c.e. He then became a citizen of Rhodes. Probably in the 90’s b.c.e., he toured the Mediterranean world to collect material for his studies. Returning to Rhodes, Posidonius was elected to the office of the prytany and was sent on an embassy to Rome in 87/86 b.c.e. Eminent Romans, such as Pompey the Great and Cicero, came to hear him. He died shortly after a second embassy to Rome in 51 b.c.e.
Posidonius’s writings show a wide range of interests. For example, in his analysis of natural phenomena, he was most well known for his explanation of the relation between tides and the Moon. In ethics, his most profound contribution was in the field of psychology and the examination of the emotions. His Histories (now lost) continued Polybius’s work, extending it from 146 to 86 b.c.e. An obsession with etiology, the examination of causes, underlies his exploration of all these subjects.
The writings of Posidonius survive only in citations in later writers’ works. His investigation of natural phenomena and history drew most interest in antiquity. Unfortunately, the fragmentary remains of his work do not adequately indicate his interest in etiology, which links the various parts of his once vast corpus.
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