[7] After a period learning Stoic philosophy from Panaetius in Athens, he spent many years in travel and scientific researches in Spain, Africa, Italy, Gaul, Liguria, Sicily and on the eastern shores of the Adriatic.

Next to Panaetius he did most, by writings and personal lectures, to spread Stoicism to the Roman world, and he became well known to many leading men, including Pompey and Cicero.

[15][16] Around the 90s BC Posidonius embarked on a series of voyages around the Mediterranean gathering scientific data and observing the customs and people of the places he visited.

[10] He traveled in Greece, Hispania, Italy, Sicily, Dalmatia, Gaul, Liguria, North Africa, and on the eastern shores of the Adriatic.

[2] In Hispania, on the Atlantic coast at Gades (the modern Cadiz), Posidonius could observe tides much higher than in his native Mediterranean.

[15] He left descriptions of customs such as nailing skulls to doorways as trophies, which he witnessed,[19] and vivid legends told to him by the Celts, such as a story that in the past, men were paid to allow their throats to be slit for public amusement.

[20] But he 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 is referred to extensively (both directly and otherwise) in the works of Diodorus of Sicily, Strabo, Caesar and Tacitus' Germania.

[15] Although the purpose of the embassy is unknown, this was at the time of the First Mithridatic War when Roman rule over the Greek cities was being challenged by Mithridates VI of Pontus and the political situation was delicate.

[24] Ian Kidd remarks that Rhodes "was attractive, not only as an independent city, commercially prosperous, go-ahead and with easy links of movement in all directions, but because it was welcoming to intellectuals, for it already had a strong reputation particularly for scientific research from men like Hipparchus.

He compared them to a living being, with physics the flesh and blood, logic the bones and tendons holding the organism together, and finally ethics—the most important part—corresponding to the soul.

[32] David Sedley remarks:[33] On the vast majority of philosophical issues, what we know of both Panaetius and Posidonius places them firmly within the main current of Stoic debate.

Their innovatively hospitable attitude to Plato and Aristotle enables them to enrich and, to a limited extent, reorientate their inherited Stoicism, but, for all that, they remain palpably Stoics, working within the established tradition.Ethics, Posidonius taught, is about practice not just theory.

[34][35] The philosophical grand vision of Posidonius was that the universe itself was interconnected as an organic whole, providential and organised in all respects, from the development of the physical world to the behaviour of living creatures.

[36] Panaetius had doubted both the reality of divination and the Stoic doctrine of the future conflagration (ekpyrosis), but Posidonius wrote in favour of these ideas.

[33] As a Stoic, Posidonius was an advocate of cosmic "sympathy" (συμπάθεια, sympatheia)—the organic interrelation of all appearances in the world, from the sky to the Earth, as part of a rational design uniting humanity and all things in the universe.

[38] In addition to his writings on geometry, Posidonius was credited for creating some mathematical definitions, or for articulating views on technical terms, for example 'theorem' and 'problem'.

Some fragments of his writings on astronomy survive through the treatise by Cleomedes, On the Circular Motions of the Celestial Bodies, the first chapter of the second book appearing to have been mostly copied from Posidonius.

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's fame beyond specialized philosophical circles had begun, at the latest, in the eighties with the publication of the work "About the ocean and the adjacent areas".

This work was not only an overall representation of geographical questions according to current scientific knowledge, but it served to popularize his theories about the internal connections of the world, to show how all the forces had an effect on each other and how the interconnectedness applied also to human life, to the political just as to the personal spheres.

In this work, Posidonius detailed his theory of the effect on a people's character by the climate, which included his representation of the "geography of the races".

Posidonius calculated the Earth's circumference by the arc measurement method, by reference to the position of the star Canopus.

The smaller value offered by Strabo and the different lengths of Greek and Roman stadia have created a persistent confusion around Posidonius's result.

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.

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 the Earth's circumference for the next 1,500 years.

His influence on Greek philosophical thinking lasted until the Middle Ages, as is demonstrated by the large number of times he is cited as a source in the Suda (a 10th-century Byzantine encyclopedia).