As the notion of a constant in itself becomes in the criticism of Hume a simple product of imagination, so for him does the concept of causality become the simple outcome of use and wont. Because in our representation of things we frequently observe two things to follow one upon the other, we assume that a necessary dependence exists between the two. Hume solves both these problems by declaring them, without a moment’s hesitation, to have no existence at all. After Hume, the see-saw game of the lower order went on for a time. Upon the intellectual materialism of the eighteenth century--especially as it prevailed in France, where it was represented by such men as La Mettrie and Von Holbach--there followed the idealism of Fichte, Schelling, and Hegel. After this had exploded of its own gaseousness, the scientific materialism of the nineteenth century set in, and up to our day has continued to hold the upper hand, though now it seems to be swinging back in a new idealistic movement. Alongside of this a new inversion of the higher order has managed to prepare itself, making its appearance in two distinct forms, of which one is the direct successor of the criticism of Hume, while the other derives from physics. The former is modern positivism, as developed in particular by Ernst Mach and R. Avenarius. The latter is the so-called world-theory of energetics, as represented more especially by Ostwald the physicist. Both schools partake of a purely scientific character in so far as they aim at furnishing world-theories from which a _substans_ is ruled out--seek to frame a world consisting solely of relation-values, a world in which the one thing constant is the constancy of the relations. A third school, modern monism, as represented especially by Haeckel, is not scientific at all. As already said, it is of the essence of every scientific view that it should apprehend the entire play of world-events purely as relation-values. Such a world-conception is bound always to set out from the midst of the play of events, with things already in full swing. Modern monism, with its teaching of primordial life in the form of a primordial cell or some other primordial form, is science only in outward appearance; at the core it is unmitigated superstition, and ought to be regarded as such by every thinking man, for it betrays itself such by its uncritical abuse of ecclesiastical dogma. After this historical review, given with the utmost possible brevity, we have to inquire:-- What is the reason then for this insufficiency of the _substans_-views, whether it refer to a material or to an ideal _substans_ in things? Why are materialism and idealism alike devoid of any kind of demonstrative ability? The answer to this is:--