By the Celtic Fairy-Faith we mean that specialized form of belief in a spiritual realm inhabited by spiritual beings which has existed from prehistoric times until now in Ireland, Scotland, Isle of Man, Wales, Cornwall, Brittany, or other parts of the ancient empire of the Celts. In studying this belief, we are concerned directly with living Celtic folk-traditions, and with past Celtic folk-traditions as recorded in literature. And if fairies actually exist as invisible beings or intelligences, and our investigations lead us to the tentative hypothesis that they do, they are natural and not supernatural, for nothing which exists can be supernatural; and, therefore, it is our duty to examine the Celtic Fairy Races just as we examine any fact in the visible realm wherein we now live, whether it be a fact of chemistry, of physics, or of biology. However, as we proceed to make such an examination, we shall have to remember constantly that there is a new set of ideas to work with, entirely different from what we find in natural sciences, and often no adequate vocabulary based on common human experiences. An American who has travelled in Asia and an Englishman who has travelled in Australia may meet in Paris and exchange travelling experiences with mutual understanding, because both of them have experienced travel; and they will have an adequate vocabulary to describe each experience, because most men have also experienced travel. But a saint who has known the spiritual condition called ecstasy cannot explain ecstasy to a man who has never known it, and if he should try to do so would discover at once that no modern language is suitable for the purpose. His experience is rare and not universal, and men have developed no complete vocabulary to describe experiences not common to the majority of mankind, and this is especially true of psychical experiences. It is the same in dealing with fairies, as these are hypothetically conceived, for only a few men and women can assert that they have seen fairies, and hence there is no adequate vocabulary to describe fairies. Among the Ancients, who dealt so largely with psychical sciences, there seems to have been a common language which could be used to explain the invisible world and its inhabitants; but we of this age have not yet developed such a language. Consequently, men who deny human immortality, as well as men with religious faith who have not through personal psychical experiences transformed that faith into a fact, nowadays when they happen to read what Plato, Iamblichus, or any of the Neo-Platonists have written, or even what moderns have written in attempting to explain psychic facts, call it all mysticism. And to the great majority of Europeans and Americans, mysticism is a most convenient noun, applicable to anything which may seem reasonable yet wholly untranslatable in terms of their own individual experience; and mysticism usually means something quite the reverse of scientific