_In abstracto_, we might think of two attempts at bridging over this gulf: the first one is that we try to transform sensation itself into something material, and the other is that we attribute sensation also to that which, according to our observation, seems to be without sensation; namely, to matter and its elements, the atoms. Both of these attempts have been made--the former by D. F. Strauss in his "The Old Faith and the New," and by the English philosopher, Herbert Spencer, in his "First Principles of Philosophy;" the latter, first pointed out by Schopenhauer, was taken up and farther developed by Zoellner in his work, "Ueber die Natur der Kometen" ("Nature of the Comets"), Leipzig, Engelmann, 1872, and with special acuteness by an "Anonymus" in the work: "Das Unbewusste von Standpunkt der Physiologie und Descendenztheorie" ("The Unconscious from the Standpoint of Physiology and Descent Theory"), Berlin, Duncker, 1872. Strauss says, in the previously-mentioned work: "If, under certain conditions, motion is transformed into heat, why may it not, under other conditions, be transformed into sensation?" And Herbert Spencer says, in his "First Principles of Philosophy," (page 217): "Various classes of facts thus unite to prove that the law of metamorphosis, which holds among the physical forces, holds equally between them and the mental forces. Those modes of the unknowable which we call motion, heat, light, chemical affinity, etc., are alike transformable into each other, and into those modes of the unknowable which we distinguish as sensation, {129} emotion, thought: these, in their turns, being directly or indirectly retransformable into the original shapes." But motion--even the finest material motion, that of ether, (which, in consequence of the very important discovery of the conservation of force and of the mechanical equivalent of heat, made by Robert von Mayer, at present is taken to be heat)--is so decidedly a material process, the sensation of motion is so decidedly a reflex of the material in something immaterial, that the assertion of a transformation of motion into sensation seems to us only to change the point of view, and not to explain the difference, but to efface it. And we think that the appeal of Strauss from his contemporaries, who do not understand him, to posterity, who would understand him better and esteem him, has but little prospect of being operative.