After Julian, open rationalism being already extinct, anti-Christian thought was simply tabooed; and though the leading historians for centuries were pagans, they only incidentally venture to betray the fact. It is told, indeed, that in the days of Valens and Valentinian an eminent physician named Posidonius, son of a great physician and brother of another, was wont to say, "that men do not grow fanatic by the agency of evil spirits, but merely by the superfluity of certain evil humours; and that there is no power in evil spirits to assail the human race"; [974] but though that opinion may be presumed to have been held by some other physicians, the special ascription of it to Posidonius is a proof that it was rarely avowed. With public lecturing forbidden, with the philosophic schools at Athens closed and plundered by imperial force, [975] with heresy ostracized, with pagan worship, including the strong rival cult of Mithraism, outwardly suppressed by the same power, [976] unbelief was naturally little heard of after the fifth century. About its beginning we find Chrysostom boasting [977] that the works of the anti-Christian writers had persuaded nobody, and had almost disappeared. As regarded open teaching, it was only too true, though the statement clashes with Chrysostom's own complaint that Porphyry had led many away from the faith. [978] Proclus was still to come (410-485), with his eighteen Arguments against the Christians, proceeding on the principle, still cherished from the old science, that the world was eternal. But such teaching could not reach even the majority of the more educated; and the Jewish dogma of creation ex nihilo became sacrosanct truth for the darkening world. In the east Eusebius, [979] and in the west Lactantius, [980] expressed for the whole Church a boundless contempt of everything in the nature of scientific research or discussion; and it was in fact at an end for the Christian world for well-nigh a thousand years. For Lactantius, the doctrine of a round earth and an antipodes was mere nonsense; he discusses the thesis with the horse-laughter of a self-satisfied savage. [981] Under the feet of arrogant and blatant ignorance we see trampled the first form of the doctrine of gravitation, not to be recovered for an æon. Proclus himself cherished some of the grossest pagan superstitions; and the few Christians who had in them something of the spirit of reason, as Cosmas "Indicopleustes," "the Indian navigator," who belongs to the sixth century, were turned away from what light they had by their sacred books. Cosmas was a Nestorian, denying the divinity of Mary, and a rational critic as regards the orthodox fashion of applying Old Testament prophecies to Jesus. [982] But whereas pagan science had inferred that the earth is a sphere, his Bible taught him that it is an oblong plain; and the great aim of his Topographia Christiana, sive Christianorum opinio de mundo,