Page:Observations on Man 1834.djvu/88

 same association may also be shewn to contribute to introduce pure ultimate spiritual happiness, in all, by a direct argument, as well as by the just-mentioned indirect one.

first, Sensation, the perception of ideas, and a locomotive faculty, i.e. muscular motion, are the three most eminent marks of distinction between the animal and vegetable world: therefore, since it is already found that the two first are performed by the same means, i.e. vibrations, there is some presumption that the last will not require a different one.

Secondly, Of the two sorts of motion, viz. automatic and voluntary, the first depends upon sensation, the last upon ideas, as I shall shew particularly hereafter, and may appear, in general, to any one, upon a slight attention; whence it follows, that sensation, and automatic motion, must be performed in the same general manner, also the perception of ideas, and voluntary motion: and therefore, since sensation and perception, the two antecedents, agree in their causes, automatic and voluntary motion, the two consequents, i.e. all the four, must likewise.

Thirdly, It appears from the first and second propositions, that the white medullary substance is the common instrument of sensation, ideas, and motion; and by the fifth, that this substance is uniform and continuous every where. Hence it follows, that the subtle motions excited in the sensory nerves and medullary substance of the brain, during sensation and intellectual perception, must, of whatever kind they be, pass into the motory nerves; and when they are arrived there, it is probable that they must cause the contraction of the muscles, both because otherwise their arrival at the motory nerves would be superfluous, and because some such subtle motions are required for this purpose.

. I. All arguments, therefore, which prove the performance of sensation and intellectual perception, by means of vibrations of the small medullary particles, must infer, that muscular motion