"In Celtic folk-lore the belief in fabulous monsters has not yet ceased. Man was surrounded by dangers visible and invisible, and the time came when some prehistoric man of genius propounded the view that all the objects around him were no less living than himself. This animistic view of the world, once adopted, made great headway from the various centres where it originated, and man derived from it a new sense of kinship with his world, but also new terrors from it.
Knowing from the experience of dreams that he himself seemed able to wander away from himself, he thought in course of time that other living things were somehow double, and the world around him came to be occupied, not merely with things that were alive, but with other selves of these things, that could remain in them or leave them at will."
-Edward Anwyl 'Celtic Religion in Pre-Christian Times' pg. 22-23