PERCEPTION. 363 



and making them communicate, when a pungent and disa- 

 greeable metallic taste is instantly perceived: this happens 

 because the nerves of the tongue, being acted upon by the 

 galvanism thus excited, communicate the same sensation as 

 that which would be occasioned by the actual application of 

 sapid bodies to that organ. Thus, it appears that causes 

 which are very different in their nature, may, by acting on 

 the same nerves, produce the very same sensation; and it 

 follows, therefore, that our sensations cannot be depended 

 upon as being always exactly correspondent with the quali- 

 ties of the external agent which excites them. 



Evidence to the same effect may also be gathered from 

 the consideration of the narrowness of those limits within 

 which all our senses are restricted. It requires a certain in- 

 tensity 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 referred to the sense of 

 touch, there are many which convey no perception of the 

 cause producing them. Thus, a slighter impression than 

 that which gives the feeling of resistance produces the sensa- 

 tion 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; al- 

 though we know that it is only in its degree that the exter- 

 nal cause of each of these sensations differs. 



The only distinct notions we are capable of forming re- 

 specting Matter, are that it consists of certain powers of 

 attraction and repulsion, occupying certain portions of space, 

 and capable of moving in space; and that its parts thereby 

 assume different relative positions or configurations. But of 

 mind, our knowledge is more extensive and more precise, 

 because we are conscious of its existence, and of many of 

 its operations, which are comprised in the general term 

 thought. To assert that thought can be a property of mat 



