554 POPULAR SCIENCE MONTHLY 



I suppose that between the instants a and /? I have moved neither 

 my body nor my eye, as I know from my muscular sense. Nor have 

 I moved either my head, my arm or my hand. I ascertain that at the 

 instant a impressions that I attributed to the object A were trans- 

 mitted to me, some by one of the fibers of my optic nerve, the others 

 by one of the sensitive tactile nerves of my finger; I ascertain that at 

 the instant (3 other impressions which I attribute to the object B are 

 transmitted to me, some by this same fiber of the optic nerve, the others 

 by this same tactile nerve. 



Here I must pause for an explanation; how am I told that this 

 impression which I attribute to A, and that which I attribute to B, 

 impressions which are qualitatively different, are transmitted to me 

 by the same nerve? Must we suppose, to take for example the visual 

 sensations, that A produces two simultaneous sensations, a sensation 

 purely luminous a and a colored sensation a', that B produces in the 

 same way simultaneously a luminous sensation b and a colored sensa- 

 tion V , that if these different sensations are transmitted to me by the 

 same retinal fiber, a is identical with b, but that in general the colored 

 sensations a' and b' produced by different bodies are different? In 

 that case it would be the identity of the sensation a which accompanies 

 a' with the sensation b which accompanies b', which would tell that all 

 these sensations are transmitted to me by the same fiber. 



However it may be with this hypothesis and although I am led to 

 prefer to it others considerably more complicated, it is certain that 

 we are told in some way that there is something in common between 

 these sensations a-\- a' and b -\-b', without which we should have no 

 means of recognizing that the object B has taken the place of the 

 object A. 



Therefore I do not further insist and I recall the hypothesis I have 

 just made : I suppose that I have ascertained that the impressions 

 which I attribute to B are transmitted to me at the instant /? by the 

 same fibers, optic as well as tactile, which, at the instant a, had trans- 

 mitted to me the impressions that I attributed to A. If it is so, we 

 shall not hesitate to declare that the point occupied by B at the instant 

 /? is identical with the point occupied by A at the instant a. 



I have just enunciated two conditions for these points being iden- 

 tical ; one is relative to sight, the other to touch. Let us consider them 

 separately. The first is necessary, but is not sufficient. The second 

 is at once necessary and sufficient. A person knowing geometry could 

 easily explain this in the following manner: Let be the point of the 

 retina where is formed at the instant a the image of the body A; let 

 M be the point of space occupied at the instant a by this body A; let 

 W be the point of space occupied at the instant (3 by the body B. For 

 this body B to form its image in 0, it is not necessary that the points 

 M and W coincide; since vision acts at a distance, it suffices for the 



