364 Relation between external [Boo 



produce fight ; delations between certain vibrations of 

 the air and the fenforium of the ear produce hearing, 

 and fo of the other fenfes. We by no means, how- 

 ever, can pretend, in. any of thefe cafes, to deter- 

 mine all the intermediate caufes and effects between 

 the quality in a body, which renders it an object of 

 fenfation, and the perception in ourfelves ; nor are we 

 by any means authorized to 'conclude, that our per- 

 ceptions are juft emblems of the objects which occa- 

 fion them. But this circumftance, upon which fo 

 much has been faid, can be to us of little im- 

 portance, fince it ftill remains equally true, that our 

 fenfations are regulated by fixed laws eftablifhed by the 

 Deity himfelf, and fince we mull fuppofe that the 

 Creator of the nniverfe has ordered all things in wif- 

 dom and goodnefs. 



We are ignorant of the means by which the objects 

 of fenfation affeft the body ; but the moft obvious 

 and fimple idea that we can form on this fubject is, 

 that they act by impulfe. Thus the rays of light are 

 known to travel with aftonifhing velocity, and to 

 pofiefs a power of moving light bodies. Sound is 

 a tremulous motion of the air, capable of being com- 

 municated to bodies in harmonic proportion with it. 

 Odorous particles require the affiftance and motion of 

 air to affect the organs of fcent. The objects of tafte 

 are more perfectly perceived by being prefled between 

 the tongue and palate. In order to feel any thing, it is 

 neceflary that the fkin mould be prefled againfl it 

 with a certain degree of force, or, what is Hill more, 

 effectual, rubbed over its furface. 



The inilruments, which are defigned to convey the 

 effects produced by material objects on the organs of 

 lenfation to the brain, are the nerves, which have been 

 already defcribed as diftributed to the feveral parts 



of 



