For Posidonius, philosophy was the dominant master art and all the individual sciences were subordinate to philosophy, which alone could explain the cosmos. All his works, from scientific to historical, were inseparably philosophical. 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 flesh and blood, logic the bones and tendons holding the organism together, and finally ethics—the most important part—corresponding to the soul. Although a firm Stoic, Posidonius was syncretic like Panaetius and other Stoics of the middle period. He followed not only the earlier Stoics, but made use of the writings of
Plato and
Aristotle. As a creative philosopher, Posidonius would however be expected to create innovations within the tradition of the philosophical school to which he belonged.
David Sedley remarks: It involves knowledge of both the human and the divine, and a knowledge of the universe to which human reason is related. Other writers who knew the ethical works of Posidonius, including Cicero and
Seneca, grouped Chrysippus and Posidonius together and saw no opposition between them. 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. 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. He believed valid predictions could be made from signs in nature—whether through astrology or prophetic dreams—as a kind of scientific prediction.
Mathematics Posidonius was one of the first to attempt to prove
Euclid's fifth postulate of geometry. He suggested changing the definition of parallel straight lines to an equivalent statement that would allow him to prove the fifth postulate. From there, Euclidean geometry could be restructured, placing the fifth postulate among the theorems instead. 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'.
Astronomy and meteorology 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. 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 BC, Posidonius estimated the distance from the Earth to the Sun (see
astronomical unit) to be 9,893 times the Earth's radius. This 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. Posidonius 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 in his writings on meteorology followed Aristotle. He theorized on the causes of clouds, mist, wind, and rain as well as frost, hail, lightning, and rainbows. He also estimated that the boundary between the clouds and the heavens lies about 40 stadia above the Earth.
Geography, ethnology, and geology and
Melchior Tavernier. Many of the details could not have been known to Posidonius; rather, Bertius and Tavernier show Posidonius's ideas about the positions of the continents. Posidonius's fame beyond specialized philosophical circles had begun, at the latest, in the eighties with the publication of the work "''''". 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". This theory was not solely scientific, but also had political implications—his Roman 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, however, make a fundamental distinction between the civilized Romans as masters of the world and the less civilized peoples. Posidonius's writings on the Jews were probably the source of
Diodorus Siculus's account of the siege of
Jerusalem and possibly also for
Strabo's. Some of Posidonius's arguments are contested by
Josephus in
Against Apion. Like
Pytheas, Posidonius believed the
tide is caused by the Moon. Posidonius 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 both earthquakes and volcanoes, including accounts of the eruptions of the volcanoes in the
Aeolian Islands, north of
Sicily.
Earth's circumference Posidonius calculated the
Earth's circumference by the
arc measurement method, by reference to the position of the star
Canopus. As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at
Alexandria he saw it ascend as far as 7½ degrees above the horizon (the
meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes). Since he thought Rhodes was 5,000
stadia due north of Alexandria, and the difference in the star's elevation indicated the distance between the two locales was 1/48 of the circle, he multiplied 5,000 stadia by 48 to arrive at a figure of 240,000 stadia for the circumference of the Earth. His estimate of the latitude difference of these two points, 360 degrees/48=7.5 degrees, is rather erroneous. (The modern value is approximately 5 degrees.) In addition, they are not quite on the same meridian as they were assumed to be. The longitude difference of the points, slightly less than 2 degrees, is not negligible compared with the latitude difference. Translating stadia into modern units of distance can be problematic, but it is generally thought that the stadion used by Posidonius was almost exactly 1/10 of a modern statute mile. Thus Posidonius's measure of 240,000 stadia translates to compared to the actual circumference of .
Pliny the Elder mentions Posidonius among his sources and without naming him reported his method for estimating the Earth's circumference. He noted, however, that
Hipparchus had added some 26,000 stadia to Eratosthenes's estimate. The smaller value offered by Strabo and the different lengths of Greek and Roman stadia have created a persistent confusion around Posidonius's result.
Ptolemy used Posidonius's lower value of 180,000 stades (about 16% too low) for the Earth's circumference in his
Geography. This was the number used by
Christopher Columbus to underestimate the distance to India as 70,000 stades.
History and tactics In his
Histories, Posidonius continued the
World History of
Polybius. His history of the period 146–88 BC is said to have filled 52 volumes. His
Histories continue the account of the rise and expansion of Roman dominance, which he appears to have supported. Posidonius did not follow Polybius's more detached and factual style, for Posidonius saw events as caused by human psychology; 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. For Posidonius "history" extended beyond the earth into the sky; humanity was not isolated each in its own political history, but was a part of the cosmos. His
Histories were not, therefore, concerned with isolated political history of peoples and individuals, but they 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 (taken from his book on the oceans), and climatic theory to explain people's ethnic or national characters. Of Posidonius's work on tactics,
The Art of War, the Greek historian
Arrian complained that it was written 'for experts', which suggests that Posidonius may have had first hand experience of military leadership or, perhaps, used knowledge he gained from his acquaintance with
Pompey. ==Reputation and influence==