THE SENSATION OF SIGHT. 229 



We must not be led astray by confounding the notions of a 

 phenomenon and an appearance,. The colours of objects are 

 phenomena caused by certain real differences in their consti- 

 tution. They are, according to the scientific as well as to the 

 uninstructed view, no mere appearance, even though the way 

 in which they appear depends chiefly upon the constitution of 

 our nervous system. A ' deceptive appearance ' is the result of 

 the normal phenomena of one object being confounded with those 

 of another. But the sensation of colour is by no means a decep- 

 tive appearance. There is no other way in which colour can 

 appear ; so that there is nothing which we could describe as 

 the normal phenomenon, in distinction from the impressions of 

 colour received through the eye. 



Here the principal difficulty seems to me to lie in the notion 

 of quality. All difficulty vanishes as soon as we clearly under- 

 stand that each quality or property of a thing is, in reality, 

 nothing else but its capability of exercising certain effects upon 

 other things. These actions either go on between similar parts 

 of the same body, and so produce the differences of its aggregate 

 condition ; or 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 sensations, 

 as those of sight, with which we have now to do. Any of these ac- 

 tions is called a ' property, 'when its object is understood without 

 being expressly mentioned. Thus, when we speak of the ' solu- 

 bility ' of a substance, we mean its behaviour towards water ; 

 when we speak of its ' 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 de- 

 pend upon the nature of one agent alone, but exists only in re- 

 lation 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, in- 

 dependent of all other objects, and which we may expect to find 



