THE SENSATION OF SIGHT. 261 



they proceed from one body upon another, as in the case 

 of chemical reactions ; or they produce their effect on our 

 organs of special sense, and are there recognised as sensa- 

 tions^ as those of sight, with which we have now to do. 

 Any of these actions is called a ' property,' when its 

 object is understood without being expressly mentioned. 

 Thus, when we speak of the ' solubility ' of a substance, 

 we mean its behaviour toward water ; when we speak of 

 its 4 weight,' we mean its attraction to the earth ; and in 

 the same way we may correctly call a substance ' blue,' 

 understanding, as a tacit assumption, that we are only 

 speaking of its action upon a normal eye. 



But if what we call a property always implies an action 

 of one thing on another, then a property or quality can 

 never depend upon the nature of one agent alone, but 

 exists only in relation to, and dependent on, the nature 

 of some second object, which is acted upon. Hence, 

 there is really no meaning in talking of properties of 

 light which belong to it absolutely, independent of all 

 other objects, and which we may expect to find repre- 

 sented in the sensations of the human eye. The notion 

 of such properties is a contradiction in itself. They 

 cannot possibly exist, and therefore we cannot expect to 

 find any coincidence of our sensations of colour with 

 qualities of light. 



These considerations have naturally long ago sug- 

 gested themselves to thoughtful minds ; they may be 

 found clearly expressed in the writings of Locke and 

 Herbart, 1 and they are completely in accordance with 

 Kant's philosophy. But in former times, they demanded 

 a more than usual power of abstraction, in order that 

 their truth should be understood ; whereas now the facts 



1 Johann Friedrich Herbart, born 1776, died 1841, professor of philo- 

 sophy at Konigsberg and Gottingen, author of Psychologic als Wissen* 

 ichaft, nevgegrundet auf Erfahrung, MctaphysiJc und Mathematik. TK. 



