The limitations of human vision are such that the eye only sees things between narrow bounds, and beyond these limits, on this side and on that, it sees nothing. The body may be compared to a harp of two chords,--the optic nerve and the auditory nerve. One kind of vibrations excites the first and another kind the second. That is the whole story of human sensation, which is even inferior to that of many of the lower animals; certain insects, for example, in whom the nerves of vision and of hearing are more delicate than in man. Now there are in nature, not two, but ten, a hundred, a thousand kinds of movement or vibration. We learn, then, from physical science, that we are living in the midst of a world invisible to us, and that it is not impossible that there may be living upon the earth a class of beings, also invisible to us, endowed with a wholly different kind of senses, so that there is no way by which they can make themselves known to us, unless they can manifest themselves in acts and ways that can come within the range of our own order of sensations. In the presence of such truths as these, which have as yet only been barely announced, how absurd and worthless seems mere blind denial! When we compare the little that we know and the narrow limits of our range of perception with the vast extent of the field of knowledge, we can scarcely refrain from the conclusion that we know nothing and that everything yet remains to be known. With what right do we pronounce the word "impossible" in the presence of facts which we prove to be genuine without yet being able to discover their causes? It is by the scientific study of effects that we arrive at the determination of causes. In the class of investigations which we group under the general head "Spiritualism," FACTS EXIST. But no one understands the method of their production. Their existence, nevertheless, is just as true as the phenomena of electricity. But, as for understanding them--why, gentlemen, nobody understands biology, physiology, psychology. What is the human body? What is the brain? What is the absolute action of the soul or mind? We do not know. And, neither do we know anything whatever of the essence of electricity or the essence of light. It is prudent, then, to observe with unbiased judgment all such matters as these, and to try to determine their causes, which are perhaps of different kinds and more numerous than has ever been supposed up to the present time.[7] It will be seen that what I publicly uttered as I stood on the hillock above the grave into which Allan Kardec's coffin had just been lowered differs not at all from the purely scientific program of the present work.