VISION. 73 



parts of the surface of the body is counted as a 

 part of common sensation, though a very different 

 thing from touch ; and it is from this that the first 

 conceptions of magnitudes, including distance, are 

 derived ; while we afterwards learn by experience 

 to translate certain phenomena of sight and sound 

 as effects of distance. As to tastes and smells, they 

 differ from objects of sight and hearing in being 

 referred to the situation of their respective organs ; 

 yet they may, when accompanied with movement, 

 and the comparison of differing degrees of the 

 sensation taking place one after the other, become 

 associated with the appreciation of locality, as 

 occurs remarkably in dogs following the scent. 

 And not only is the exercise of the senses thus 

 mixed with the idea of space, but every sensation 

 involves consciousness of its duration and repeti- 

 tion or non-repetition. Thus the ideas of time and 

 space become the means of unifying the results 

 of sensations incomparable in their own nature ; 

 so that the hand, the eye, and the ear, combine to 

 increase the common stock of information. 



It follows that extension, configuration and move- 

 ment are properties of external objects which fall 

 under a different head from colour and sound as we 

 see and hear them. A sense of redness has no objec- 

 tive reality, but the vibration which causes it, the re- 



