Others allowed that it was impossible that the earth should contain subterranean caverns large enough to hold Tartarus, the Elysian Fields and the infinite multitude of the dead. But they explained that the word subterranean (ὑπόγειος) had been misunderstood, that it denoted not the bowels of the earth but the lower half of the terrestrial globe, the southern hemisphere, which was unknown to the ancients, or even the whole celestial hemisphere, curved below this globe which hung motionless in the centre of the universe.[193] This hemisphere is always invisible,—so the ancients might say,—which is exactly the sense of the word Hades (= ἀειδής). The Axiochos, an apocryphal work attributed to Plato, was first to reveal this doctrine, claiming that it had been communicated to Socrates by the Mage Gobryes. It was in reality borrowed by the Greeks of the Alexandrian age from the astral theology of the Semitic peoples. According to this theology the world is divided into two halves by the line of the horizon; the upper hemisphere is the domain of the living and the higher gods, the lower that of the dead and the infernal gods. Descent to it and ascent from it are by way of two gates, situated west and east, where the sun appears and disappears. The marshes of the Acheron, the river Styx, and Charon and his boat are constellations which the souls cross when they have passed through the “gate of Hades.” The ancient Greeks had placed the Islands of the Blessed, whither the heroes were borne by the favour of the gods, somewhere far away in the ocean. These islands were now supposed to lie in the Antipodes, in the unknown half of the earth. All the poets’ stories of the fragrant and melodious gardens of this abode of delights were applied to these marvellous countries which no sailor had ever reached.[194] On the other hand, Tartarus was placed at the bottom of the celestial abysses, near the lowest point of the lower hemisphere, that is, diametrically opposite to Olympus, the dwelling of the gods, who were throned on the summit of the starry vault. It was into this sombre gulf that the wicked were flung; there yawned the bottomless pit in which the demons of the dusky world inflicted eternal torture on the guilty.