Posidonius (pronounced /ˌpɒsɪˈdoʊniəs/; Ancient Greek: Ποσειδώνιος Poseidṓnios, "of Poseidon") was a Greek scholar born in Apamea, Syria. He was known as "of Apameia" (ὁ Ἀπαμεύς) or "of Rhodes" (ὁ Ῥόδιος). He lived around 135–51 BC and was a politician, astronomer, astrologer, geographer, historian, mathematician, and teacher. He was considered the most knowledgeable person of his time and possibly the most learned member of the Stoic school.

After studying Stoic philosophy with Panaetius in Athens, Posidonius traveled widely for many years. He conducted scientific research in Spain, Africa, Italy, Gaul, Liguria, Sicily, and along the eastern coast of the Adriatic Sea. Later, he settled in Rhodes, where he taught and gained fame among scholars.

Alongside Panaetius, Posidonius played a major role in spreading Stoicism to the Roman world through his writings and lectures. He became well known to important figures such as Pompey and Cicero.

Although his works are now lost, they provided valuable information to later writers. More than twenty of his writings are known, and their subjects include topics studied by Stoic philosophers. Like other Stoics of his time, he combined ideas from different schools of thought, including those of Plato and Aristotle.

Posidonius was an expert in many areas, including natural science, geography, natural history, mathematics, and astronomy. He worked to measure the distance and size of the Sun, calculate the Earth’s diameter, and study how the Moon affects ocean tides.

Life

Posidonius, nicknamed "the Athlete" (Ἀθλητής), was born around 135 BC. He was born into a Greek family in Apamea, a Greek-influenced city on the Orontes River in northern Syria. Historian Philip Freeman described Posidonius as "Greek to the core." However, he did not praise his hometown, Apamea, in his writings and even made fun of its people.

As a young man, Posidonius moved to Athens, where he studied under Panaetius, the leading Stoic philosopher of the time and the head of the Stoic school in Athens. Panaetius died in 110 BC, when Posidonius was about 25 years old. Instead of staying in Athens, Posidonius settled in Rhodes and became a citizen there. In Rhodes, he started his own school, which became the most important Stoic institution of the time.

Around the 90s BC, Posidonius traveled widely across the Mediterranean, collecting scientific information and observing the customs of the people he visited. His journeys took him to Greece, Spain, Italy, Sicily, Dalmatia, Gaul, Liguria, North Africa, and the eastern coast of the Adriatic.

In Spain, at Gades (modern-day Cádiz), Posidonius noticed much higher tides than in the Mediterranean. He wrote that daily tides are connected to the Moon’s movement, and that tidal heights change with the Moon’s cycles. He also suggested that yearly tidal patterns might be linked to the equinoxes and solstices.

In Gaul, Posidonius studied the Celts. He recorded customs such as hanging skulls on doorways as trophies and heard stories about people being paid to have their throats cut for entertainment. He noted that the Celts respected the Druids, whom he compared to philosophers. He wrote that even among people seen as "barbaric," wisdom could rise above pride and passion. A lost geographic work on the Celts is mentioned in writings by Diodorus of Sicily, Strabo, Caesar, and Tacitus.

In Rhodes, Posidonius held a high political position as one of the Prytaneis, a role that combined leadership and executive duties. Five or six men shared this position for six months at a time.

He was chosen to represent Rhodes in an embassy to Rome in 87/86 BC, during the First Mithridatic War. At this time, Roman control over Greek cities was being challenged by Mithridates VI of Pontus, and the political situation was tense.

Under Posidonius, Rhodes became the leading center for Stoic philosophy in the 1st century BC. This shift may have started earlier, during Panaetius’ time, as Panaetius was from Rhodes and may have helped establish a Stoic school there. Rhodes was appealing to scholars because it was independent, wealthy, and known for scientific research, like that of Hipparchus.

Although details about Posidonius’ school are unclear, many Greek and Roman students studied under him. Notable figures, such as Pompey, attended his lectures. In 66 and 62 BC, Pompey listened to lectures on the topic "There is no good but moral good." At this time, Posidonius was likely in his 70s and suffered from gout. He illustrated his lecture by pointing to his painful leg and saying, "It is no good, pain; you may bother me, but you will never convince me you are evil."

When Cicero was in his late 20s, he studied under Posidonius. Later, Cicero invited Posidonius to write about his own time as a consul, but Posidonius politely declined. Cicero often called Posidonius "my teacher" and "my dear friend" in his writings. Posidonius died in his 80s in 51 BC. His grandson, Jason of Nysa, later became the head of the Stoic school in Rhodes.

Partial scope of writings

Posidonius was well-known as an expert in many fields during the Graeco-Roman world. He almost mastered all the knowledge of his time, much like Aristotle and Eratosthenes. He tried to create a way to connect ideas about the human mind and the universe, which would explain and guide how people should act.

Posidonius wrote about many subjects, including physics (like weather and geography), astronomy, astrology, seismology, geology, hydrology, botany, ethics, logic, mathematics, history, natural history, anthropology, and tactics. His studies were important research on these topics, even though he made some mistakes.

None of Posidonius's writings have survived completely. Only pieces of his works remain, although we know the titles and subjects of many of his books. Writers like Strabo and Seneca give most of the information about his life and writings.

Philosophy

Posidonius believed that philosophy was the most important area of study, and all other subjects, like science and history, were connected to philosophy. He thought philosophy alone could explain the universe. Every work he wrote, whether about science or history, always included ideas from philosophy.

He agreed with the Stoics, a group of philosophers, that philosophy had three main parts: physics (which included natural science and ideas about the universe), logic (which included reasoning and debate), and ethics (which focused on how people should live). Posidonius saw these parts as connected, like different parts of a living body. He compared physics to the flesh and blood, logic to the bones and muscles that hold the body together, and ethics to the soul, which he considered the most important part.

Although Posidonius followed Stoic ideas, he also used ideas from other philosophers, like Plato and Aristotle. He studied Plato’s work called Timaeus and wrote about its ideas related to numbers and math. As a creative thinker, Posidonius added new ideas to Stoic philosophy.

Some people once thought Posidonius had different ideas from earlier Stoics about the mind. Earlier Stoics, like Chrysippus, believed emotions and reason were not separate. Posidonius, however, wrote about a different view, suggesting the human mind had three parts: reason, spirited feelings (like anger), and desires (like hunger). Some scholars now think Posidonius may have been trying to explain, not oppose, earlier ideas. Writers like Cicero and Seneca did not see a difference between Posidonius and Chrysippus.

Posidonius believed the universe was one connected whole, like a living being. He supported ideas that earlier Stoics had doubted, like the belief that the future could be predicted through signs in nature, such as dreams or the movement of stars.

Posidonius worked on geometry, trying to prove a rule called the fifth postulate. He changed how parallel lines were defined to help prove this rule. He also helped define terms like "theorem" and "problem" in math.

Some of Posidonius’s ideas about astronomy are still known today. His work on the stars and planets was recorded by another writer named Cleomedes. Posidonius believed the Sun had a special power that influenced the world. He tried to measure how far the Sun and Moon were from Earth. His estimate for the Sun’s distance was not perfect, but his guess for the Sun’s size was more accurate than others.

He also studied how the Moon affects the tides. He thought the Moon’s heat caused the water to rise and fall, even though this was incorrect. Posidonius wrote about natural events like earthquakes and volcanic eruptions, including those in the Aeolian Islands.

To measure Earth’s size, Posidonius used the position of a star called Canopus. He thought the distance between two cities, Rhodes and Alexandria, was 5,000 stadia. Using the angle of the star above the horizon, he calculated Earth’s circumference as 240,000 stadia. However, his calculations had errors because the cities were not exactly on the same line.

Posidonius also wrote about how climate affects people’s lives. He believed where people lived influenced their character and culture. His ideas helped explain why Italy’s location was important for Rome’s power. He wrote about the Jewish people, and some of his ideas may have influenced other writers like Strabo.

Like another ancient thinker named Pytheas, Posidonius thought the Moon caused tides. He was wrong, though, because he believed the Moon was made of fire and air, and its heat made the water rise.

Posidonius’s work was widely read and respected. His book about the ocean and nearby lands explained how the world was connected and helped people understand how natural forces affect life.

Reputation and influence

Posidonius was a well-known philosopher in the ancient Greek and Roman world. His writings covered many important areas of philosophy, and many famous writers of his time, such as Cicero, Livy, Plutarch, Strabo, Cleomedes, Seneca the Younger, and Diodorus Siculus, often referred to his work. Strabo called Posidonius "the most learned of all philosophers of my time," and Diodorus Siculus used Posidonius as a source for his book, the "Historical Library." Although his writing style, which was flowery and persuasive, became less popular after his death, people during his lifetime praised his skill with words and his ability to write clearly.

Posidonius was a major source of information about the Celts of Gaul. His work was frequently quoted by writers like Timagenes, Julius Caesar, Diodorus Siculus, and Strabo.

As an ambassador from the city of Rhodes, Posidonius moved easily among important people in Roman society. He met with influential figures in late republican Rome, including Cicero and Pompey. Cicero attended Posidonius's lectures when he was in his twenties (77 BC) and continued to write to him. In his book "De Finibus," Cicero closely followed Posidonius's explanation of Panaetius's ethical teachings.

Posidonius met Pompey when he was an ambassador in Rome. Pompey visited Posidonius in Rhodes twice—once in 66 BC during a campaign against pirates and again in 62 BC during his eastern campaigns. Pompey asked Posidonius to write his biography. As a sign of respect, Pompey lowered his fasces (symbols of authority) before Posidonius's door. Other Romans who visited Posidonius in Rhodes included Velleius, Cotta, and Lucilius.

Ptolemy admired Posidonius's methods, including his way of calculating how light bends when passing through thick air near the horizon. Ptolemy accepted Posidonius's estimate of Earth's size instead of a more accurate earlier number by Eratosthenes. This incorrect number remained the accepted value for Earth's circumference for 1,500 years.

Posidonius helped strengthen Stoicism, a school of philosophy, by combining it with new ideas. Along with his teacher Panaetius, he worked to spread Stoicism throughout the Roman world. A century later, Seneca called Posidonius one of the most important contributors to philosophy.

Posidonius's influence on Greek philosophy lasted until the Middle Ages, as shown by how often he is mentioned in the "Suda," a 10th-century encyclopedia from the Byzantine Empire.

Wilhelm Capelle found that many ideas in the philosophical work "De Mundo" came from Posidonius. Today, Posidonius is seen as a thoughtful and broad-minded person who connected many ideas and knowledge into a single, unified view of the world, based on his belief in Stoicism.

A crater on the Moon is named after Posidonius.

Editions and translations

• Kidd, I. G., and Edelstein, Ludwig. (1972). Posidonius. Volume I: The Fragments. Cambridge University Press. ISBN 0521080460.
• Kidd, I. G. (1988). Posidonius. Volume II: The Commentary. Cambridge University Press. ISBN 0521200628.
• Kidd, I. G. (1999). Posidonius. Volume III: The Translation of the Fragments. Cambridge University Press. ISBN 0521622581.