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56 -theory have shown a tendency to liken thought to a 'force' which the brain exerts, or to a 'state' into which it passes. Herbert Spencer, for instance, writes:—

"The law of metamorphosis, which holds among the physical forces, holds equally between them and the mental forces. . . . How this metamorphosis takes place; how a force existing as motion, heat, or light can become a mode of consciousness; how it is possible for aerial vibrations to generate the sensation we call sound, or for the forces liberated by chemical changes in the brain to give rise to emotion,—these are mysteries which it is impossible to fathom. But they are not profounder mysteries than the transformations of the physical forces into each other." [First Principles, 2nd Edition, p. 217.]

So Büchner says: "Thinking must be regarded as a special mode of general natural motion, which is as characteristic of the substance of the central nervous elements as the motion of contraction is of the nerve-substance, or the motion of light is of the universal-ether. . . . That thinking is and must be a mode of motion is not merely a postulate of logic, but a proposition which has of late been demonstrated experimentally. . . . Various ingenious experiments have proved that the swiftest thought that we are able to evolve occupies at least