Posidonius was a Stoic philosopher, historian, and scientist of the Hellenistic period. He was born around 135 BCE in Apamea, Syria, and died around 51 BCE. He studied in Athens before settling on the island of Rhodes, where he founded an influential school and served as a leading citizen. His school attracted prominent Roman figures, including Cicero.

He was an extraordinarily wide-ranging thinker, but none of his works survive complete. Modern scholars know his ideas only through fragments and summaries in later writers. His writings formed a vast collection intended to unify all knowledge. They included major works on physics and the cosmos, theology, ethics, and psychology. He also wrote a 52-volume history continuing the work of Polybius, and specialized studies on geography, astronomy, and anthropology.

According to modern scholars, Posidonius is considered one of the last great polymaths of the ancient Greek world. He sought to explain everything—from the movements of the stars to human emotions—within a single, rational Stoic system. His scientific observations, such as his estimate of the Earth's circumference and his theories on tides, were influential for centuries. Through his teaching and his Roman connections, he played a key role in transmitting Greek philosophy and science to the Roman world.