All such parallels, like their equivalents in Celtic belief, seem to rest on this psychological and physiological conception in the folk-mind. Human food is what keeps life going in a human body; fairy food is what keeps life going in a fairy body; and since what a man eats makes him what he is physically, so eating the food of Fairyland or of the land of the dead will make the eater partake of the bodily nature of the beings it nourishes. Hence when a man or woman has once entered into such relation or communion with the Otherworld of the dead, or of fairies, by eating their food, his or her physical body[210] by a subtle transformation adjusts itself to the new kind of nourishment, and becomes spiritual like a spirit's or fairy's body, so that the eater cannot re-enter the world of the living. A study of food taboos confirms this conclusion.[211] A third prominent taboo, the iron taboo, has been explained by exponents of the Pygmy Theory as pointing to a prehistoric race in Celtic lands who did not know iron familiarly, and hence venerated it so that in time it came to be religiously regarded as very efficacious against spirits and fairies. Undoubtedly there may be much reason in this explanation, which gives some ethnological support to the Pygmy Theory. Apparently, however, it is only a partial explanation of iron taboo in general, because, in many cases, iron in ancient religious rites certainly had magical properties attributed to it, which to us are quite unexplainable from this ethnological point of view;[212] and in Melanesia and in Africa, where iron is venerated now, the same explanation through ethnology seems far-fetched. But at present there seem to be no available data to explain adequately this iron taboo, though we have strong reasons for thinking that the philosophy underlying it is based on mystical conceptions of virtues attributed--reasonably or unreasonably--to various metals and precious stones, and that a careful examination of alchemical sciences would probably arrive at an explanation wholly psychological.