The harmony which man is striving to establish between his reason and his other faculties can obtain only during comparatively short intervals of time. Our reason grows in power and keenness; new observations and discoveries are almost constantly made; old ideas and opinions do not, upon closer investigation, satisfy the more developed demands of our thinking; doubts arise, and this is a necessary condition for all theoretical progress. Such a doubt, not of the immaterial experience which we all have, but of the way in which this experience is to be explained, has been expressed in the theory called materialism, which is a widely spread doctrine in our time. Natural science in itself is never materialistic in the sense in which this word is here used, because natural science does not concern itself with anything immaterial. But if this be the case, how is it possible that science can have anything in common with materialism which, strictly speaking, is a doctrine about spiritual things? We answer that life in this world is joined to and revealed through the material world. A more complete knowledge of the nature of matter ought, therefore, to bring about a decision by and by as to whether the soul is a bodily function or a substance differing from matter. In other words, natural science must sooner or later arrive at a stage when it either verifies materialism or gives us tangible and obvious evidence for the truth of idealism. It was to such a point that science arrived in the last century when Büchner presented his well known “Force and Matter,” in which he endeavors to prove that the soul is an attribute of the body, religion, immortality and so on being only illusions.