The theory of world-cycles was so widely current that it may almost be described as the orthodox theory of cosmic time among the Greeks, and it passed from them to the Romans. [Footnote: Plato's world-cycle. I have omitted details not essential; e.g. that in the first period men were born from the earth and only in the second propagated themselves. The period of 36,000 years, known as the Great Platonic Year, was probably a Babylonian astronomical period, and was in any case based on the Babylonian sexagesimal system and connected with the solar year conceived as consisting of 360 days. Heraclitus seems to have accepted it as the duration of the world between his periodic universal conflagrations. Plato derived the number from predecessors, but based it on operations with the numbers 3, 4, 5, the length of the sides of the Pythagorean right-angled triangle. The Great Year of the Pythagorean Philolaus seems to have been different, and that of the Stoics was much longer (6,570,000 years). I may refer here to Tacitus, Dialogus c. 16, as an appreciation of historical perspective unusual in ancient writers: "The four hundred years which separate us from the ancients are almost a vanishing quantity if you compare them with the duration of the ages." See the whole passage, where the Magnus Annus of 12,954 years is referred to.] According to some of the Pythagoreans [Footnote: See Simplicius, Phys. 732, 26.] each cycle repeated to the minutest particular the course and events of the preceding. If the universe dissolves into the original chaos, there appeared to them to be no reason why the second chaos should produce a world differing in the least respect from its predecessor. The nth cycle would be indeed numerically distinct from the first, but otherwise would be identical with it, and no man could possibly discover the number of the cycle in which he was living. As no end seems to have been assigned to the whole process, the course of the world's history would contain an endless number of Trojan Wars, for instance; an endless number of Platos would write an endless number of Republics. Virgil uses this idea in his Fourth Eclogue, where he meditates a return of the Golden Age: Alter erit tum Tiphys, et altera quae uehat Argo Delectos heroas; erunt etiam altera bella, Atque iterum ad Troiam magnus mittetur Achilles.