228 RECENT PROGRESS OF THE THEORY OF VISION. 



similar colours, secondly because it represents a part of its re- 

 lations in space those, namely, -which belong to perspective* 

 by corresponding relations in space. 



Functional cerebral activity and the mental conceptions 

 which go with it may be ' images ' of actual occurrences in the 

 outer world, so far as the former represent the sequence in time 

 of the latter, so far as they represent likeness of objects by 

 likeness of signs that is, a regular arrangement by a regular 

 arrangement. 



This is obviously sufficient to enable the understanding to 

 deduce what is constant from the varied changes of the external 

 world and to formulate it as a notion or a law. That it is also 

 sufficient for all practical purposes we shall see in the next chap- 

 ter. But not only uneducated persons who are accustomed to 

 trust blindly to their senses, even the educated, who know that 

 their senses may be deceived, are inclined to demur to so com- 

 plete a want of any closer correspondence in kind between actual 

 objects and the sensations they produce than the law I have just 

 expounded. For instance, natural philosophers long hesitated 

 to admit the identity of the rays of light and of heat, and ex- 

 hausted all possible means of escaping a conclusion which seemed 

 to contradict the evidence of their senses. 



Another example is that of Goethe, as I have endeavoured 

 to show elsewhere. He was led to contradict Newton's theory of 

 colours, because he could not persuade himself that white, which 

 appears to our sensation as the purest manifestation of the 

 brightest light, could be composed of darker colours. It was 

 Newton's discovery of the composition of light that was the first 

 germ of the modern doctrine of the true functions of the senses ; 

 and in the writings of his contemporary, Locke, were correctly 

 laid down the most important principles on which the right in- 

 terpretation of sensible qualities depends. But, however clearly 

 we may feel that here lies the difficulty for a large number of 

 people, I have never found the opposite conviction of certainty 

 derived from the senses so distinctly expressed that it is possible 

 to lay hold of the point of error ; and the reason seems to me to 

 lie in the fact that beneath the popular notions on the subject lie 

 other and more fundamentally erroneous conceptions. 



