516 THE SENSORIAL FUNCTIONS. 



upon as being always exactly correspondent with 

 the qualities of the external agent which excites 

 them. 



Evidence to the same effect may also be 

 gathered from the consideration of the narrow- 

 ness of those limits within which all our senses 

 are restricted. It requires a certain intensity in 

 the agent, whether it be light, or sound, or che- 

 mical substances applied to the senses of smell 

 or taste, in order to produce the very lowest 

 degree of sensation. On the other hand, when 

 their intensity exceeds a certain limit, the 

 nature of the sensation changes, and becomes 

 one of pain. Of the sensations commonly re- 

 ferred to the sense of touch, there are many 

 which convey no perception of the cause pro- 

 ducing them. Thus a slighter impression than 

 that which gives the feeling of resistance pro- 

 duces the sensation of itching, which is totally 

 different in its kind. The sensation of cold is 

 equally positive with that of warmth, and differs 

 from it, not in degree merely, but in species ; 

 although we know that it is only in its degree 

 that the external cause of each of these sensa- 

 tions differs. 



The only distinct notions we are capable of 

 forming respecting Matter, are that it consists of 

 certain powers of attraction and repulsion, occu- 

 pying certain portions of space, and capable of 

 moving in space ; and that its parts thereby 



