From New World Encyclopedia
The bust of Posidonius as an older man depicts his character as a Stoic philosopher. His brow is slightly creased to indicate the effort of thinking, but his expression is calm. His hair and philosopher’s beard are cut short and groomed plainly to indicate his concern with matters deeper than mere appearance.

Posidonius (or Poseidonus; Greek: Ποσειδώνιος) "of Rhodes" (ο Ρόδιος) or, alternatively, "of Apameia" (ο ΑπαμεϿς) (c. 135 B.C.E. - 51 B.C.E.), was a Greek Stoic philosopher, politician, astronomer, geographer, historian, and teacher. Born in Apamea, a Roman city in Syria, he settled in Rhodes around 95 B.C.E. and rose to such prominence that he served as a prytaneis (president) of Rhodes and was sent to Rome as an ambassador. He conducted research in numerous fields and traveled widely throughout the Roman Empire. His school in Rhodes attracted many Greek and Roman students, and his lectures were attended by Cicero during a visit to Rhodes.

Posidonius was the first Stoic to admit that passions were an inherent part of human nature, rather than errors in judgment based on an incorrect understanding of desire.

He was acclaimed as the greatest polymath of his age. Posidonius’ vast body of work has all been lost, but is referenced in the works of numerous later writers, and the titles and subjects of many of his works are known.


Posidonius, nicknamed "the Athlete," was born c. 135 B.C.E. to a Greek family in Apamea, a Roman city on the river Orontes in northern Syria. He completed his studies in Athens under Panaetius, head of the Stoic school. Around 95 B.C.E. he settled in Rhodes, a maritime state which had a reputation for scientific research, and became a citizen. He established a school in Rhodes; though little is known about its organization, it is clear that Posidonius taught a number of Greek and Roman students.

Posidonius took an active part in the political life of Rhodes, and attained the highest public office as one of the prytaneis (presidents, having a six months tenure) of Rhodes. He also served as ambassador to Rome in 87 - 86 B.C.E., during the Marian and Sullan era. In Rome he associated with some of the leading figures of late republican Rome, including Cicero and Pompey, both of whom visited him in Rhodes. Cicero attended Posidonius’s lectures in 77 B.C.E. and the two corresponded. In his work De Finibu, Cicero closely followed Posidonius's presentation of Panaetius's ethical teachings. Pompey visited Posidonius in Rhodes twice, once in 66 B.C.E. during his campaign against the pirates and again in 62 B.C.E. during his eastern campaigns, and asked Posidonius to write his biography. As a gesture of respect and great honor, Pompey lowered his fasces before Posidonius's door. The Romans Velleius, Cotta, and Lucilius also visited Posidonius in Rhodes.

After establishing himself in Rhodes, Posidonius traveled throughout the Roman world and even beyond its boundaries, and conducted scientific research. He traveled in Greece, Spain, Africa, Italy, Sicily, Dalmatia, Gaul, Liguria, North Africa, and on the eastern shores of the Adriatic Sea.

In Spain, on the Atlantic coast at Glades (the modern Cadiz), Posidonius observed that the daily tides were connected with the orbit and the monthly tides with the cycles of the moon, and he hypothesized about the connections of the yearly cycles of the tides with the equinoxes and solstices.

In Gaul, he studied the Celts. He left vivid descriptions of things he saw with his own eyes while among them: men who were paid to allow their throats to be slit for public amusement and the nailing of skulls as trophies to the doorways. Posidionis noted that the Celts honored the Druids, whom Posidonius saw as philosophers—and concluded that even among the barbaric, “pride and passion give way to wisdom, and Ares stands in awe of the Muses.” Posidonius wrote a geographic treatise on the lands of the Celts which has since been lost, but which has been assumed to be one of the sources for Tacitus Germania.

Posidonius conducted research in many areas of study, including astronomy, mathematics, history, political science and the art of war, always placing philosophy in the highest position as the master science, which gave direction to everything else.

Together with his teacher, Panaetius, Posidonius is credited with establishing Stoicism as a strong influence in the Roman world through his writing and his extensive personal contacts with influential Roman intellectuals. A century later, Seneca referred to Posidonius as one of those who had made the largest contribution to philosophy.

After Posidonius’ death in 51 B.C.E., his grandson Jason (who was the son of his daughter and Menekrates of Nysa) succeeded him as the head of his school in Rhodes.

Thought and Works


His writings on almost all the principal divisions of philosophy made Posidonius a renowned figure throughout the Graeco-Roman world and he was widely cited by writers of his era, including Cicero, Livy, Plutarch, Strabo (who called Posidonius "the most learned of all philosophers of my time"), Cleomedes, Seneca the Younger, Diodorus Siculus (who used Posidonius as a source for his Bibliotheca historia ("Historical Library"), and others. Although his ornate and rhetorical style of writing passed out of fashion soon after his death, Posidonius was acclaimed during his life for his literary ability and as a stylist. All of his original works have been lost, but scholars have been able to piece together substantial portions from the references and citations in the works of other writers.

Did you know?
The Greek polymath Posidonius attempted to create a unified worldview, showing the interconnectedness of the world and how all forces have an effect on each other and on human life

Posidonius also wrote on physics (including meteorology and physical geography), astronomy, astrology and divination, seismology, geology and mineralogy, hydrology, botany, ethics, logic, mathematics, history, natural history, anthropology, and tactics. His studies, though not without error, were serious attempts at scientific investigation.

At one time, scholars perceived Posidonius's influence in almost every subsequent writer, whether warranted or not. Today, Posidonius is recognized as having had an inquiring and wide-ranging mind, not entirely original, but with a breadth of view that connected, in accordance with his underlying Stoic philosophy, all things and their causes and all knowledge into an overarching, unified worldview. His work was an attempt to create a unified system for understanding the human intellect and the universe which would provide an explanation of, and a guide for, human behavior.


Although a firm Stoic, Posidonius was eclectic, like Panaetius and other Stoics of the middle period. He followed not only the older Stoics, but accepted some of the views of Plato and Aristotle. (It is thought that Posidonius may have written a commentary on Plato's Timaeus.)

He was the first Stoic to depart from the orthodox doctrine that passions were faulty judgments based on an incorrect understanding of desire, and to allow that passions were inherent in human nature. In addition to the rational faculties, Posidonius taught that the human soul had faculties that were spirited (anger, the desire for power, the desire for possessions) and desiderative (desires for sex and food). Ethics taught how to deal with these passions and restore reason as the dominant faculty.

Posidonius considered philosophy the dominant master art which alone could explain the cosmos, saying that fundamental principles depended on philosophers and individual problems on scientists. He accepted the Stoic categorization of philosophy into physics (natural philosophy, including metaphysics and theology), logic (including dialectic), and ethics. These three categories for him were, in Stoic fashion, inseparable and interdependent parts of an organic, natural whole. He compared them to a living being, with physics the meat and blood, logic the bones and tendons that held the organism together, and ethics – the most important part – the soul.

Like the early Stoics, Posidonius regarded the universe as a single interconnected corporeal entity. He upheld the Stoic concept of logos, a divine fire which imbued the entire universe and gave it form and direction. Posidonius also affirmed the Stoic doctrine that the universe passed through endless cycles, each one ending with a conflagration. His influence on philosophical thinking lasted until the Middle Ages, as is shown by references to his works in the Suda, the massive medieval lexicon.


Some fragments of Posidonius’ writings on astronomy survive in a treatise by Cleomedes, On the Circular Motions of the Celestial Bodies. The first chapter of the second book appears to have been mostly copied from Posidonius.

Posidonius advanced the theory that the sun emanated a vital force that permeated the world. He attempted to measure the distance and size of the sun. In about 90 B.C.E. Posidonius estimated the astronomical unit to be a0/rE = 9893, which was still too small by half. In measuring the size of the sun, however, he reached a figure larger and more accurate than those proposed by other Greek astronomers and Aristarchus of Samos. Posidonius also calculated the size and distance of the Moon. He constructed an orrery, possibly similar to the Antikythera mechanism. Posidonius's orrery, according to Cicero, exhibited the diurnal motions of the sun, moon, and the five known planets.

Posidonius measured the earth’s circumference by observing the position of the star Canopus. As explained by Cleomedes, Posidonius used the elevation of Canopus to determine the difference in latitude between Rhodes and Alexandria. His method was correct, but due to observational errors, his result was 240,000 stadia, or about a third smaller than the actual circumference of Earth. Ptolemy was impressed by the sophistication of Posidonius's methods, which included correcting for the refraction of light passing through denser air near the horizon. Ptolemy's approval of Posidonius's result, rather than Eratosthenes's earlier and more correct figure, caused it to become the accepted value for Earth's circumference for the next 1,500 years.

Geography, Ethnology, Meteorology, and Geology

About ten years after he arrived in Rhodes, Posidionus published a work "about the ocean and the adjacent areas." This work reviewed geographical questions according to scientific knowledge of the time. It also served to popularize his theories about the interconnectedness of the world, to show how all the forces had an effect on each other and on human life, political as well as personal. Posidonius put forth a detailed theory of the effect of climate on the character of a people, including a "geography of the races." This theory also had political implications—his readers were informed that the climatic central position of Italy was an essential condition of the Roman destiny to dominate the world. As a Stoic he did not make a fundamental distinction between the civilized Romans as masters of the world and the less civilized peoples. However, like other Greek intellectuals of that era, he favored Rome as a stabilizing power in a turbulent world.

Like Pytheas, Posidonius believed the tides are caused by the Moon. He was, however, wrong about the cause. Thinking that the Moon was a mixture of air and fire, he attributed the cause of the tides to the heat of the Moon, hot enough to cause the water to swell but not hot enough to evaporate it.

He recorded observations on earthquakes and volcanoes, including accounts of the eruptions of the volcanoes in the Aeolian Islands, north of Sicily.

Posidonius’ writings on meteorology followed the ideas of Aristotle. He theorized about the causes of clouds, mist, wind, and rain as well as frost, hail, lightning, and rainbows.

In addition to his writings on geometry, Posidonius is credited with creating some mathematical terms, including 'theorem' and 'problem.'

History and Tactics

In his Histories, Posidonius continued the World History of Polybius. His history of the period 146 - 88 B.C.E. is said to have filled 52 volumes, and continued the account of the rise and expansion of Roman dominance. Posidonius did not follow Polybius's more detached and factual style, for Posidonius saw human psychology as the cause of events; while he understood human passions and follies, he did not pardon or excuse them in his historical writing, using his narrative skill, in fact, to enlist the readers' approval or condemnation.

The Histories of Posidonius was not only the political history of individuals and peoples, but included discussions of all forces and factors (geographical factors, mineral resources, climate, nutrition), which let humans act and be a part of their environment. For example, Posidonius considered the climate of Arabia and the life-giving strength of the sun, tides, and climatic theory to explain people’s ethnic or national characters.

Of Posidonius's work on tactics, The Art of War, the Roman historian Arrian complained that it was written 'for experts,' which suggests that Posidonius may have had first hand experience of military leadership or, perhaps, utilized knowledge he gained from his acquaintance with Pompey.

A crater on the moon is named for Posidonius.

ISBN links support NWE through referral fees

  • Bevan, Edwyn. 1980. Stoics and Skeptics: Zeno of Citium and the Stoa, the Stoa, Posidonius of Apamea, the Sceptics, Pyrrho of Elis, Arcesilaus of Pitane, Carneades of C. Ares Publishers. ASIN B00070Y5SQ
  • Kidd, I. G. 1989. Posidonius: The Commentary (Cambridge Classical Texts and Commentaries vol. 14A). Cambridge: Cambridge University Press.
  • Tierney, J. J. 1960. The Celtic Ethnography of Posidonius. Royal Irish Academy. ASIN B0007BJK2G
  • Reeve, Michael and I. G. Kidd. 1972. Posidonius (Cambridge Classical Texts and Commentaries vol. 13). Paperback edition, 2004. Cambridge University Press. ISBN 0521604257
  • Sandbach, F. H. 1994. The Stoics. Indianapolis, IN: Hackett Publishing Co., Ltd. ISBN 0872202534

See also

External links

All links retrieved November 30, 2022.

  • Juergen Malitz, Poseidonios from Grosse Gestalten der griechischen Antike. 58 historische Portraits von Homer bis Kleopatra. Hrsg. von Kai Brodersen. München: Verlag C.H. Beck. S. 426-432.
  • Posidonius of Rhodes School of Mathematics and Statistics, University of St. Andrews, Scotland.

General Philosophy Sources


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