WHY OUR SENSATIONS DIFFER, 469 



giving rise to visual sensations. But then, since light and 

 pressure, electricity and cutting, all cause visual sensations, 

 we have no valid reason for supposing that light, more than 

 either of the others, is really in any way like our sensation 

 of light: or that sight-feeling differs from sound-feeling 

 because objectively light differs from sound. The eye is an 

 organ specially set apart to be excited by light, and accord- 

 ingly so fixed as to have its nerve-fibres far more often ex- 

 cited by that form of force than by any other; but the fact 

 that light sensations can be otherwise aroused shows plainly 

 that their kind or character has nothing directly to do with 

 any property of light. Just as by pinching or heating or 

 galvanizing a motor nerve we can make the muscles attached 

 to it contract, and the contraction has nothing in common 

 with the excitant, so the visual sftrsa.i-,iQn J as such, is inde- 

 pendent of the stimulus arousing it and, of itself, tells 

 us nothing concerning the kind of stimulus which has 

 operated. 



Differences in kind between external forces being thus 

 eliminated as possible causes of the modalities of our sen- 

 sations, ^^'Q next naturally fall back upon differences in the 

 sense-organs themselves. They do undoubtedly differ both 

 in gross and microscopic structure, and the fact that pres- 

 sure on the closed eye arouses a touch-feeling where the 

 skin is compressed, and a sight-feeling where the optic nerve 

 is, might well be due to the fact that a peripheral touch- 

 organ was different from a peripheral sight-organ, and the 

 same force might therefore produce totally different effects 

 on them and so cause different kinds of feelings. However, 

 here also closer examination shows that we must seek far- 

 ther. Sensation is not produced in a sense-organ, but far 

 away from it in the brain; the organ is merely an apparatus 

 for generating nervous impulses. If the optic nerves be 

 divided, no matter how perfect the eyeballs, no amount of 

 light will arouse visual sensations; if the spinal cord be cut 

 in the middle of the back no pressure on the feet will cause 

 a tactile or other feeling; though the skin, and its nerves 

 and the lower half of the spinal cord be all intact. In all 

 cases we find that if the nerve-paths between a sense-organ 



