THE RECEPTOR SYSTEM 185 



This irresistible mental tendency to refer certain of our states 

 of feeling to causes outside of our Bodies, whether in contact with 

 them or separated from them by a certain space, is known as the 

 phenomenon of the extrinsic reference of our sensations. It seems 

 largely to depend on the fact that the sensations cxtrinsically 

 referred can be modified by movements of our Bodies. Hunger, 

 thirst, and toothache all remain the same whether we turn to the 

 right or left, or move away from the place we are standing in. 

 But a sound is altered. We may find that in a certain position 

 of the head it is heard more by the right ear than the left ; but on 

 turning round the reverse is the case; and half-way round the 

 loudness in each ear is the same. Hence we are led, by mental 

 laws outside of the physiological domain, to suspect that its 

 cause is not in our Body, but outside of it; and depends not on a 

 condition of the Body but on something else. 



Sensory Illusions. "I must believe my own eyes" and "we 

 can't always believe our senses" are two expressions frequently 

 heard, and each expressing a truth. No doubt a sensation in 

 itself is an absolute incontrovertible fact : if I feel redness or hot- 

 ness I do feel it, and that is an end of the matter: but if I go be- 

 yond the fact of my having a certain sensation and conclude from 

 it as to properties of something else if I form a judgment from 

 my sensation I may be totally wrong; and in so far be unable to 

 believe my eyes or skin. Such judgments are almost inextricably 

 woven up with many of our sensations, and so closely that we 

 cannot readily separate the two; not even when we know that 

 the judgment is erroneous. 



For example, the moon when rising or setting appears bigger 

 than when high in the heavens we seem to feel directly that it 

 arouses more sensation, and yet we know certainly that it does 

 not. With a body of a given brightness the amount of change 

 produced in the end organs of the eye will depend on the size of 

 the image formed in the eye, provided the same part of its sensory 

 surface is acted upon. Now the size of this image depends on the 

 distance of the object; it is smaller the farther off it is and greater 

 the nearer, and measurements show that the area of the sensitive 

 surface affected by the image of the rising moon is no larger than 

 that affected by it when overhead. W T hy then do we, even after 

 we know this, see it bigger? The reason is that when the moon is 



