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- 
