546 POPULAR SCIENCE MONTHLY 



each other in the same order, and yet these two changes are regarded 

 by us as very different; the first is a displacement, the second a change 

 of state. Why? Because in the first case it is sufficient for me to go 

 around the sphere to place myself opposite the red hemisphere and 

 reestablish the original red sensation. 



Still more; if the two hemispheres, in place of being red and blue, 

 had been yellow and green, how should I have interpreted the revolu- 

 tion of the sphere ? Before, the red succeeded the blue, now the green 

 succeeds the yellow; and yet I say that the two spheres have undergone 

 the same revolution, that each has turned about its axis; yet I can not 

 say that the green is to yellow as the red is to blue; how then am I led 

 to decide that the two spheres have undergone the same displacement? 

 Evidently because, in one case as in the other, I am able to reestablish 

 the original sensation by going around the sphere, by making the same 

 movements, and I know that I have made the same movements because 

 I have felt the same muscular sensations; to know it, I do not need, 

 therefore, to know geometry in advance and to represent to myself the 

 movements of my body in geometric space. 



Another example: An object is displaced before my eye; its image 

 was first formed at the center of the retina; then it is formed at the 

 border; the old sensation was carried to me by a nerve fiber ending at 

 the center of the retina ; the new sensation is carried to me by another 

 nerve fiber starting from the border of the retina ; these two sensations 

 are qualitatively different; otherwise, how could I distinguish them? 



Why then am I led to decide that these two sensations, qualitatively 

 different, represent the same image, which has been displaced? It is 

 because I can follow the object with the eye and by a displacement of 

 the eye, voluntary and accompanied by muscular sensations, bring back 

 the image to the center of the retina and reestablish the primitive 

 sensation. 



I suppose that the image of a red object has gone from the center 

 A to the border B of the retina, then that the image of a blue object 

 goes in its turn from the center A to the border B of the retina; I 

 shall decide that these two objects have undergone the same displace- 

 ment. Why? Because in both cases I shall have been able to rees- 

 tablish the primitive sensation, and that to do it I shall have had to 

 execute the same movement of the eye, and I shall know that my eye 

 has executed the same movement because I shall have felt the same 

 muscular sensations. 



If I could not move my eye, should I have any reason to suppose 

 that the sensation of red at the center of the retina is to the sensation 

 of red at the border of the retina as that of blue at the center is to 

 that of blue at the border ? I should only have four sensations quali- 

 tatively different, and if I were asked if they are connected by the pro- 

 portion I have just stated, the question would seem to me ridiculous, 



