SENSATION. 335 



being apprized, by the increasing loudness of the 

 sound of falling waters, as he advances in a par- 

 ticular direction, that he is coming nearer and 

 nearer to the cataract. Yet how much is really 

 implied in all these apparently simple phenomena! 

 Science has taught us that these perceptions of 

 external objects, far from being direct or intuitive, 

 are only the tinal results of a long series of opera- 

 tions, produced by agents of a most subtle nature, 

 which act by curious and complicated laws, upon 

 a refined organization, disposed in particular situa- 

 tions in our bodies, and adjusted with admirable 

 art to receive their impressions, to modify and 

 combine them in a certain order, and to convey 

 them in regular succession, and without confusion, 

 to the immediate seat of sensation. 



Yet this process, complicated as it may appear^ 

 constitutes but the first stage of the entire function 

 oi perception : for before the mind can arrive at a 

 distinct knowledge of the presence and peculiar 

 qualities of the external object which gives rise to 

 the sensation, a long series of mental changes must 

 intervene, and many intellectual operations must 

 be performed. All these take place in such rapid 

 succession, that even when we include the move- 

 ment of the limb, which is consequent upon the 

 perception, and which we naturally consider as 

 part of the same continuous action, the whole 

 appears to occupy but a single instant. On a 

 careful analysis of the phenomena, however, as I 

 shall afterwards attempt to show, we find that no 

 less than twelve distinguishable kinds of changes, 

 or rather processes, some of which imply many 

 changes, must always intervene, in regular succes- 



