218 PSYCHOLOGY. 



see by means of them are equally sucli conclusions ? Ought 

 we not, in short, to say unhesitatingly that distance must be 

 an intellectual and not a sensible content of consciousness ? 

 and that each of these eye-feelings serves as a mere signal 

 to awaken this content, our intellect being so framed that 

 sometimes it notices one signal more readily and sometimes 

 another ? 



Reid long ago (Inquiry, c. vi. sec. 17) said : 



" It may be taken for a general rule that things which are produced 

 by custom may be undone or changed by disuse or by contrary custom. 

 On the other hand, it is a strong argument that an effect is not owing 

 to custom, but to the constitution of nature, when a contrary custom is 

 found neither to change nor to weaken it." 



More briefly, a way of seeing things that can be un- 

 learned was presumably learned, and only what we cannot 

 unlearn is instinctive. 



This seems to be Helmholtz's view, for he confirms 

 Reid's maxim by saying in emphatic print : 



" No elements in our perception can be sensational which may be 

 overcome or reversed by factors of demonstrably experimental origin. 

 Whatever can be overcome by suggestions of experience must be re- 

 garded as itself a product of experience and custom. If we follow this 

 rule it will appear that only qualities are sensational, whilst almost all 

 spatial attributes are results of habit and experience."* 



This passage of Helmholtz's has obtained, it seems to- 

 me, an almost deplorable celebrity. The reader will please 

 observe its very radical import. Not only would he, and 

 does he, for the reasons we have just been ourselves con- 

 sidering, deny distance to be an optical sensation ; but, 

 extending the same method of criticism to judgments of 

 size, shape, and direction, and finding no single retinal or 

 muscular process in the eyes to be indissolubly linked with 

 any one of these, he goes so far as to say that all optical 

 space-perceptions whatsoever must have an intellectual 



* Physiol. Optik, p. 438. Helmholtz's reservation of ' qualities ' is in- 

 consistent. Our judgments of light and color vary as much as our judg- 

 ments of size, shape, and place, and ought by parity of reasoning to be 

 called intellectual products and not sensations. In other places he does 

 treat color as if it were an intellectual product. 



