Posidonius was born in 135 BC, in what is now Syria. After completing his early education, he moved to Athens to pursue philosophical study at the Stoic school under Panaetius

Bust of Posidonius

Posidonius

Similar to his teacher, Posidonius found himself in disagreement with many of the foundational principles of Stoicism. He often found himself in debates with fellow scholars, siding frequently with Panaetius who was working to usher in the new era of Middle Stoicism.

Unlike Panaetius, however, reforming Stoicism did not prove satisfying. Posidonius was unable to reconcile his issues with the Stoic tenets and eventually left the school entirely. His departure was also motivated by his desire to explore fields such as geography and astronomy with the same rigorous attention to detail that the Stoic school afforded philosophy. Moving forward, he began to study both Plato and Aristotle in-depth, rejecting the rigidity of his educational background.

Following his departure from the Stoic school around 95 B.C.E., Posidonius moved to Rhodes. While the region was famous for its scientific advancements, Posidonius found himself immersed in its political realm. He eventually became a Prytaneis of Rhodes, serving as its primary leader for half a year (the maximum term). In addition, he served as an ambassador to Rome for the region.

It is particularly interesting to note that Posidonius, while straying from Stoicism, was following in the footsteps of his predecessor, Panaetius. Panaetius was born in Rhodes and frequently visited Rome, spreading Stoicism throughout the region. It is unclear if Posidonius consciously decided to imitate his teacher, but it is clear that the teachings of Panaetius emphasized the importance of interacting with his international counterparts.

Similar to Panaetius, Posidonius came to favor Rome as a philosophical and political hotspot. He developed strong relationships with the upper class in Rome, benefiting his political endeavors as well as his scientific work. These connections provided him with opportunities to travel throughout the known world, expanding his knowledge. Specifically, Posidonius had the opportunity to travel through the majority of the Mediterranean, including North Africa.

Posidonius’s travels inspired many of his great works of writing. Specifically, he had the chance to absorb Celtic practices in Gaul. He came to interpret Celtic practices concerning Druids to be philosophical. He wrote extensively on the region and his treatise on the Celts is frequently referenced in other works.

Through his political and philosophical endeavors, Posidonius developed a reputation throughout both Greece and Rome. Over time, students began flocking to Rhodes to study under him. He became viewed as a philosophical authority, effectively serving as Panaetius’s successor despite being located away from the Stoic school in Athens. Following his death in 51 B.C.E., his children maintained his school in Rhodes.

Today, we understand Posidonius to be one of the greatest polymaths of his time. In other words, he mastered a wide array of subjects, contributing greatly to each. He is known for his work as a philosopher primarily but excelled in astronomy, geography, mathematics, war practices, history, and politics.

Notable Works & Suggested Readings
While Posidonius was considered a polymath and excelled in a wide variety of fields, his work in philosophy remains the most influential. He even believed that all sciences should defer to philosophy, as nothing else could explain the universe adequately. As a result, many of his works outside of the field take on a philosophical tone.

Well-versed in the Stoic tradition, Posidonius agreed with his predecessors that philosophy was divided into physics, logic, and ethics. His work, however, begins to depart from that of earlier Stoics when we get beyond the basics. Like his immediate predecessor Panaetius, he took an eclectic approach to philosophy. He introduced Platonic and Aristotelian ideas to Stoicism, effectively eliminating many of the discrepancies among the schools of thought.

He stands out from Panaetius, however, in that he was the first Stoic to suggest that Plato’s conception of the human soul was correct. This meant that passions and desires, rather than false perceptions to be overcome, were an integral part of human identity that must be reasoned with.

While Posidonius’s works have been lost, quotes and citations remain in the works of Cicero, Livy, Plutarch, Strabo, Cleomedes, and Seneca the Younger, among others. He was often praised for his writing style, which was described as extremely stylistic and verging on poetic. Using these sources, we have been able to piece together some of what Posidonius may have written.

THE HALL OF STOIC PHILOSOPHERS

Junius Rusticus

Seneca

Marcus Aurelius

Epictetus

Statue of Thrasea Paetus

Thrasea Paetus

Lucius Annaeus

Diotimus

Publius Rutilius Rufus

Porcia of Cato

Cicero

Posidonius

Panaetius of Rhodes