These mediaeval metaphysicians, inheritors of pre-Platonic, Platonic, and neo-Platonic teachings, purposely obscured their doctrines under a covering of alchemical terms, so as to safeguard themselves against persecution, open discussion of occultism not being safe during the Middle Ages, as it was among the ancients and happily is now again in our own generation. But they were quite scientific in their methods, for they divided all invisible beings into four distinct classes: the Angels, who in character and function are parallel to the gods of the ancients, and equal to the Tuatha De Danann of the Irish, are the highest; below them are the Devils or Demons, who correspond to the fallen angels of Christianity; the third class includes all Elementals, sub-human Nature-Spirits, who are generally regarded as having pygmy stature, like the Greek daemons; and the fourth division comprises the Souls of the Dead, and the shades or ghosts of the dead. For us, the third class, which includes spirits of pygmy-like form, is the most important in this present discussion. All its members are of four kinds, according as they inhabit one of the four chief elements of nature.[146] Those inhabiting the earth are called Gnomes. They are definitely of pygmy stature, and friendly to man, and in fairy-lore ordinarily correspond to mine-haunting fairies or goblins, to pixies, _corrigans_, leprechauns, and to such elves as live in rocks, caverns, or earth--an important consideration entirely overlooked by champions of the Pygmy Theory. Those inhabiting the air are called Sylphs. These Sylphs, commonly described as little spirits like pygmies in form, correspond to most of the fairies who are not of the Tuatha De Danann or 'gentry' type, and who as a race are beautiful and graceful. They are quite like the fairies in Shakespeare's _Midsummer-Night's Dream_; and especially like the aerials in _The Tempest_, which, according to Mr. Morton Luce, a commentator on the drama, seem to have been shaped by Shakespeare from his knowledge of Rosicrucian occultism, in which such spirits hold an important place. Those inhabiting the water are called Undines, and correspond exactly to the fairies who live in sacred fountains, lakes, or rivers. And the fourth kind, those inhabiting the fire, are called Salamanders, and seldom appear in the Celtic Fairy-Faith: they are supreme in the elementary hierarchies. All these Elementals, who procreate after the manner of men, are said to have bodies of an elastic half-material essence, which is sufficiently ethereal not to be visible to the physical sight, and probably comparable to matter in the form of invisible gases. Mr. W. B. Yeats has given this explanation:--'Many poets, and all mystic and occult writers, in all ages and countries, have declared that behind the visible are chains on chains of conscious beings, who are not of heaven but of the earth, who have no inherent form, but change according to their whim, or