972 Dynamic TJieory. 



also ends when it gives rise to an efferent current or shock to another 

 organ, or entejs into some combination in the formation of a will ; and 

 that same sensation will never be experienced again ; but another very 

 much like it may be experienced if another pulse of stimulation should 

 be sent from the same quarter and strike the same organ. Or, if the 

 receiving organ be again stimulated by the overflow of excitement from 

 an organ with which it is related, other than the one from which it re- 

 ceived its first differentiation, the effect is recollection, the physiology 

 of it being the revibration of the ether of the organ in the same pitch, 

 but in reduced amplitude. It is these revived sensations, that, entering 

 into the combinations forming the will, give purpose to most of our 

 voluntary actions. In order to have a continuous sensation or con- 

 sciousness of one object, we must have a continuous flow of stimula- 

 tions from such object. Thus, as long as we keep our eyes fixed upon 

 an object, vibrations of the external ether reflected from the object, 

 flash against the retina at the rate of five or six hundred trillions per 

 second, and the current up the optic nerve is kept going continuously. 

 But the moment we take our eyes off, the current stops, and if we con- 

 tinue to have any sensation of the object it is only restimulated memory. 



It seems, then, that the motion of the sensory organ, which appears 

 to the thought as simply a quiver of a subtile substance, appears to the 

 feeling as a sensation. This is, however, after all only such contrast 

 as appears between the different senses, such as a touch and a taste of 

 the same object, both perhaps effected through the same tongue, or 

 sight and sound, or even between two tones, as red and blue, &c. It is 

 as impossible to tell in what way these differ from each other, as in 

 what way they differ from thought. All of them considered as objects 

 are only various modes of molecular motion of a material substance. 



The only knowledge we can get of a sensation is to experience it. 

 There is no possible waj r in which we could convey to a blind man an 

 idea of green. This is one of our sensations, and is a sequel to waves 

 of ether g^-Wo of an inch in length. But if we should tell that to the 

 blind man, what idea of green would be conveyed to him? Even if we 

 knew exactly the sort of movements which take place in the cell itself 

 when such movements are sensation, we could not thereby get the 

 slightest idea of the nature of the sensation. If two persons each 

 look at a green leaf, and each says that he experiences the sensation 

 green, neither one knows that his sensation is like that of the other 

 person. If A cannot be X, he cannot experience sensations that are 

 the same as those of X. Sensations can be described only in terms of 

 sensation, and if X appears to be like A, the latter may describe his 

 own sensations by telling X they are like his. They may be supposed 

 to be similar, but they are necessarily not precisely alike. Every sen- 



