"Let me ask whether persons so confined could have seen anything of themselves or of each other, beyond the shadows thrown by the fire upon the part of the cavern facing them? And is not their knowledge of the things carried past them equally limited? And if they were able to converse with one another, would they not be in the habit of giving names to the objects which they saw before them? If their prison house returned an echo from the part facing them, whenever one of the passers-by opened his lips, to what could they refer the voice, if not to the shadow which was passing? Surely such person would hold the shadows of those manufactured articles to be the only realities." With reference to our absolute knowledge of the phenomena of nature, this splendid comparison is as correct today as it was in the days of Plato, about 400 B. C.; we are only as prisoners in a great cave, watching shadows of passing objects thrown upon the cavern wall, and reflecting upon the real natures of the things whose shadows we see. We know things only by their effects; the essential nature of matter, ether and energy is far from our understanding. [Sidenote: The progress of science rests on the law of cause and effect.] In early and mediaeval times, the recognition of the fact that nature in its ultimate form is unknowable, led to many harmful superstitions. Chief among the fallacies of the early ages was the belief that God at will could, and did, cause various phenomena to appear in nature, which were contrary to all human experience. As observed in chapter 4, a class of men arose who claimed to be in possession of knowledge which made them also able, at will, to cause various supernatural manifestations. Thus arose the occult sciences, so called,--alchemy, astrology, magic, witchcraft, and all other similar abominations of the intellect. Such beliefs made the logical study of nature superfluous, for any apparent regularity or law in nature might at any time be overturned by a person in possession of a formula of the black art or a properly treated broomstick.