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Poseidonius adhered to the division of philosophy .usual among the ancients, into physics, ethics, and dialectics (Diog. Laert. vii. 39), comparing the first to the blood and flesh of an animal, the second to the bones and nerves, the last to the soul. (Sextus Emp. adv. Math. vii. 19 ; Diog. Laert. vii. 40.) He recognised two principles (dp%cu) — passive (matter), and active (God). His physical doctrines were, in the main, those of the Stoics generally, though he differed from them in some particulars. He held that the vacuum beyond the universe was not infinite, but only large enough to allow of the dissolution of the universe (he discarded the doctrine of its destruction by fire, Phil. Jud. de Aet. Mmidi, ii. p. 497, ed. Mang.). He considered the heaven as the governing principle (to ^ye^oviKov] of the universe (Diog. Laert. vii. 139.) He cultivated astronomy with considerable diligence, and, unlike Panaetius, was a believer in astrology (Cic. de Div. ii. 42). Poseidonius also constructed a planetary machine, or revolving sphere, to exhibit the daily motions of the sun, moon and planets. (Cic. de Nat. Dear. ii. 34.) He inferred that the sun is larger than the earth, among other reasons because the shadow cast by the earth is conical. (Diog. Laert. vii. 144 ; Macrob. ad Konm. Scip. i. 20.) Its greater apparent magnitude as it sets he attributed to its being seen through dense and misty air, and supposed that if we could see it through a solid wall it would appear larger still. (Oleomedes, Cyd. Theor. ii: p. 430.) He calculated the diameter of the sun to be 4,000,000 stadia, on the assumption that the orbit of -the sun was 10,000 times the circumference of the earth, and that it is within a space of 400 stadia N. and S. that the sun casts no shadow. (Cleomcdes, /. c, p. 452.) The distance between the earth and the sun he set down at above 502,000,000 stadia. (Plin. //. N. ii. 21.) The moon also he considered to be larger than the earth, and composed of transparent elements, though on account of its great size the rays of the sun do not pass through it in eclipses. (Stob. Eel. Pays. i. p. 59; Cleom. /. c. ii. p. 500.) His view of the milky way, that it is of an igneous nature, not so dense as stars, but more so than light, and intended to warm those parts of the universe which the sun's heat does not reach, was extensively adopted. (Macrob. 1. c. i. 15.) Poseidonius's calculation of the circumference of the earth differed widely from that of Eratosthenes. He made it only 180,000 stadia, and his measurement was pretty generally adopted. His calculation was founded on observations of the star Canobus made in Spain., not, as Cleomedes says, in Rhodes. (Strab. ii. p. 119 ; Cleom. I. c. i. 8. ; comp. Mannert, Geogr. vol. i. p. 105, &c.) The shape of the habitable part of the earth he compared to that of a sling, the greatest extent being from E. to W. (Strab. ii. p. 267 ; Agathemerus, ap. Hudson. Geogr. Min. vol. ii. p. 2.) Of the connection between the moon and the tides he was well aware. (Strab. iii. p. 173.) Strabo frequently refers to Poseidonius as one of the most distinguished geographers. A great number of passages, containing the views of Poseidonius on various other geographical and astronomical points, has been collected by Bake.
As the basis of his ethical and mental philosophy Poseidonius took the Stoic system, though with considerable modifications, for he held it possible to amalgamate with it much of the systems of
Plato and Aristotle. In some respects his views approximated to the Pythagorean doctrines. (Sext. Empir. Adv. Math. vii. 93 ; Galen, de Hipp. et Plat. Plac. v. p. 171.) It seems to have been his object as far as possible to banish contradiction from philosophy, and bring all the systems which had been propounded into harmony with each other, and to infuse into the decaying vitality of philosophical thought something of the vigour of past times. But that he could suppose the doctrines of Zeno, Aristotle and Plato capable of reconciliation with each other, shows that he could not have seized very distinctly the spirit of each. To give anything like plausibility to this attempt, it was of course necessary to introduce considerable modifications into the Stoic doctrines. In some points however in which he differed from Panaetius he rather returned to the views of the earlier Stoic philosophers. His fourfold division of virtue is apparently that followed by Cicero in his De Offidis. He did not think virtue by itself sufficient for perfect happiness, unless accompanied by external, bodily good. (Diog. Laert. vii. 120.) The summum bonum he considered to be the living in the contemplation of the truth and order of all things, and the fashioning oneself, as far as possible, in accordance therewith, being led aside as little as possible by the irrational part of the soul. (Clem. Alex. Strom. ii. p. 416.) In the classification of the faculties of the soul he returned to the system of Plato, dividing them into reason, emotion, and appetite (dziKwaw SioiKov/n€Vovs ri^ds viro rpia/v eVi0u,u7jTt/c?js T6 ko.\ &vfj.oei$ovs Kal \o-, Galenus, /. c. viii. p. 319), with which division he considered questions of practical morality to be intimately connected (Galen. /. c. iv. p. 284, v. p. 291 ). It was apparently to keep up a bond of connection with the Stoic dogmas that he spoke of these Svvd/iieis as all belonging to one essence (Galen. /. c. vi. p. 298), though other features of his system are not easily reconcilable with that view. But instead of regarding the TrdO-rj of the soul as being, or ensuing upon, judgments (wpureisO of the reason, he deduced them from the irrational faculties of the soul, appealing to the fact that emotion and appetite manifest themselves in irrational 'beings. He connected affections and perturbations of the mind with external influences, the union cf the soul with the body, and the influence of the latter upon the former, some conditions of man being predominantly bodily, others spiritual ; some passing from the body to the soul, others from the soul to the body. This idea he carried out to the permanent modifications of character produced by particular bodily organisations, founding thereon a sort of physiognomical system. (Galen, i. c. v. p. 290.) He sometimes spoke of appetite as corresponding to vegetable life, emotion to animal life, reason to the properly human (/. c. p. 170).
None of the writings of Poseidonius has come down to us entire. We find mention of the following:—1. Tlepl &e£v, consisting of at least thirteen books (Diog. Laert. vii. 138). 2. Uspl /mz/rt/c^?, in five books. Poseidonius defended divination, and analysed its foundations. 3. Tlepl 4. Tl?pl 'Hpcocoi/ Kal Sai^ovwy. 5. &vo-lkos consisting of at least fifteen books (Diog. Laert. vii. 140). 6. TItpl /<roV,uoy. 7. 'E&yncris rov H\drwvos Tinaiov. 8. Ilepl Kevov. 9. Ilepi fjLerewpwv : Diogenes Laertius cites from the seventeenth book of