If now one follows up the various transformations that have taken place in this domain within historical times, one finds that, as is also the case in the domain of natural science, they occur following the law of the _inversion of positions_. Does the one school, whether it be the materialistic or the idealistic, force its way into such a preponderating position as to become intolerable to sound common-sense, it is forced to give place to its opponent, which then for a season takes the lead, only, after a longer or shorter period, to undergo a like fate. It is like a game of see-saw. All the acuteness, all the profundity, all the mental florescence which the one school has manifested in the course of centuries of labour perhaps, in this period of decline are brought to destruction, and only by ardent collectors can be rescued and preserved as a palæontological form of mental life. At bottom, the whole of philosophy up to each new “now” is nothing but a more or less tastefully-arranged palæontological collection of thought-values. Above and alongside this play of inversions betwixt idealism and materialism--which I might call the inversion of the _lower order_--there takes place another inversion of a _higher order_. In certain intervals the human understanding begins to offer serious resistance to both the worldviews that base themselves on the concept of _substans_ in its two possible forms--that of substance and that of force--by hastening over to one that is the contrary of both, a world-conception from which _substans_ is wholly absent, a world consisting entirely of a mass of relation-values. This latter form of world-conception alone has the right to the designation of “scientific.” For there can be no science, properly so-called, where the subject dealt with is any shape or form of an imperceptible to sense. Now, the first inversion of the higher order with which we in our Western circles of culture are acquainted has, to be sure, a slight enough scientific cast. It is the inversion that set in with Protagoras the Sophist. With his thesis, “Man is the measure of all things--of those that are, that they are; of those that are not, that they are not”--he places himself in an attitude of opposition to both world-conceptions founded on the concept of _substans_; for in both these conceptions things, as existing in virtue of an unconditioned constant, must also be the measure of man. The appearance of Protagoras was a naturally-resulting protest against the absurdities to which materialism and idealism had mutually driven each other. The former found its culminating point in Democritus of Abdera, who left nothing in the world but matter in the shape of atoms. The latter reached its corresponding culmination in Plato, who left nothing in the world but the immaterial _substans_, ideas, to whom thereby matter became the non-existent.