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 the heterogeneity existing, or supposed to exist, between the nerve impression and the sensation. “However much we may follow the excitement through the whole length of the nerve,” writes Lotze, “or cause it to change its form a thousand times and to metamorphose itself into more and more delicate and subtle movements, we shall never succeed in showing that a movement thus produced can, by its very nature, cease to exist as movement and be reborn in the shape of sensation. . . .” It will be seen that it is on the opposition between molecular movement and sensation, that Lotze insists. In like manner Ferrier: “But how is it that the molecular modifications in the cerebral cells coincide with the modifications of the consciousness; how, for instance, do luminous vibrations falling upon the retina excite the modification of consciousness called visual sensation? These are problems we cannot solve. We may succeed in determining the exact nature of the molecular changes which take place in the cerebral cells when a sensation is felt; but this will not bring us an inch nearer to the explanation of the fundamental nature of sensation.” Finally, Du Bois Reymond, in his famous discussion in 1880, on the seven enigmas